National Security Agency
Cybersecurity Technical Report
Network Infrastructure
Security Guide
October 2023
U/OO/118623-22
PP-22-0293
Version 1.2
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Notices and history
Document change history
Date
Version
Description
October 2023
1.2
Updated links to references
June 2022
1.1
Minor clarifications and additional vendor links
March 2022
1.0
Report released
Disclaimer of warranties and endorsement
The information and opinions contained in this document are provided “as is” and without any warranties
or guarantees. Reference herein to any specific commercial products, process, or service by trade name,
trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement,
recommendation, or favoring by the United States Government, and this guide shall not be used for
advertising or product endorsement purposes.
Trademark recognition
Cisco
®
and Cisco IOS
®
are registered trademarks of Cisco Systems, Inc.
Publication information
Author(s)
National Security Agency
Cybersecurity Directorate
Contact information
Client Requirements / General Cybersecurity Inquiries:
Cybersecurity Requirements Center, 410-854-4200, Cybersecurit[email protected]
Media inquiries / Press Desk:
Media Relations, 443-634-0721, MediaRelations@nsa.gov
Defense Industrial Base Inquiries for Cybersecurity Services:
DIB Cybersecurity Program, DIB_Def[email protected]
Purpose
This document was developed in furtherance of NSA’s cybersecurity missions. This includes its
responsibilities to identify and disseminate threats to National Security Systems, Department of Defense
information systems, and the Defense Industrial Base, and to develop and issue cybersecurity
specifications and mitigations. This information may be shared broadly to reach all appropriate
stakeholders.
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Contents
Network Infrastructure Security Guide..............................................................................i
Contents ......................................................................................................................................iii
1. Introduction ............................................................................................................................... 1
1.1 Regarding Zero Trust ........................................................................................................................................ 1
2. Network architecture and design ............................................................................................ 2
2.1 Install perimeter and internal defense devices ....................................................................................... 2
2.2 Group similar network systems ..................................................................................................................... 3
2.3 Remove backdoor connections .................................................................................................................... 4
2.4 Utilize strict perimeter access controls ...................................................................................................... 4
2.5 Implement a network access control (NAC) solution ........................................................................... 5
2.6 Limit virtual private networks (VPNs) ......................................................................................................... 5
3. Security maintenance............................................................................................................... 8
3.1 Verify software and configuration integrity ............................................................................................... 8
3.2 Maintain proper file system and boot management ............................................................................. 9
3.3 Maintain up-to-date software and operating systems ........................................................................ 10
3.4 Stay current with vendor-supported hardware ...................................................................................... 11
4. Authentication, authorization, and accounting (AAA) ....................................................... 12
4.1 Implement centralized servers .................................................................................................................... 12
4.2 Configure authentication ................................................................................................................................ 13
4.3 Configure authorization .................................................................................................................................. 14
4.4 Configure accounting ...................................................................................................................................... 15
4.5 Apply principle of least privilege ................................................................................................................. 15
4.6 Limit authentication attempts ....................................................................................................................... 17
5. Local administrator accounts and passwords .................................................................... 17
5.1 Use unique usernames and account settings ....................................................................................... 18
5.2 Change default passwords ........................................................................................................................... 19
5.3 Remove unnecessary accounts ................................................................................................................. 19
5.4 Store passwords with secure algorithms ................................................................................................ 19
5.5 Create strong passwords .............................................................................................................................. 21
5.6 Utilize unique passwords ............................................................................................................................... 23
5.7 Change passwords as needed ................................................................................................................... 23
6. Remote logging and monitoring ........................................................................................... 24
6.1 Enable logging ................................................................................................................................................... 25
6.2 Establish centralized remote log servers ................................................................................................ 25
6.3 Capture necessary log information ............................................................................................................ 26
6.4 Synchronize clocks .......................................................................................................................................... 27
7. Remote administration and network services .................................................................... 28
7.1 Disable clear text administration services .............................................................................................. 28
7.2 Ensure adequate encryption strength ...................................................................................................... 30
7.3 Utilize secure protocols .................................................................................................................................. 31
7.4 Limit access to services ................................................................................................................................. 31
7.5 Set an acceptable timeout period .............................................................................................................. 32
7.6 Enable Transmission Control Protocol (TCP) keep-alive ................................................................. 33
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7.7 Disable outbound connections .................................................................................................................... 33
7.8 Remove SNMP read-write community strings ...................................................................................... 34
7.9 Disable unnecessary network services ................................................................................................... 35
7.10 Disable discovery protocols on specific interfaces ........................................................................... 36
7.11 Configure remote network administration services .......................................................................... 36
7.11.1 Configuring SSH for remote administration ................................................................................ 36
7.11.2 Configuring HTTP for remote administration .............................................................................. 39
7.11.3 Configuring SNMP for remote administration ............................................................................ 40
8. Routing ..................................................................................................................................... 40
8.1 Disable IP source routing .............................................................................................................................. 41
8.2 Enable unicast reverse-path forwarding (uRPF).................................................................................. 41
8.3 Enable routing authentication ...................................................................................................................... 42
9. Interface ports ......................................................................................................................... 43
9.1 Disable dynamic trunking .............................................................................................................................. 43
9.2 Enable port security ......................................................................................................................................... 44
9.3 Disable default VLAN ...................................................................................................................................... 45
9.4 Disable unused ports ...................................................................................................................................... 47
9.5 Disable port monitoring .................................................................................................................................. 48
9.6 Disable proxy Address Resolution Protocol (ARP) ............................................................................. 49
10. Notification and consent banners ...................................................................................... 50
10.1 Present a notification banner .................................................................................................................... 50
11. Conclusion ............................................................................................................................ 51
Abbreviations .............................................................................................................................. 52
References ................................................................................................................................... 54
Works cited ................................................................................................................................................................. 54
Related guidance ..................................................................................................................................................... 56
Figure 1: Network perimeter with firewalls and a DMZ.................................................................................... 3
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1. Introduction
Guidance for securing networks continues to evolve as adversaries exploit new
vulnerabilities, new security features are implemented, and new methods of securing
devices are identified. Improper configurations,
incorrect handling of configurations, and weak
encryption keys can expose vulnerabilities in the
entire network. All networks are at risk of
compromise, especially if devices are not
properly configured and maintained. An
administrator’s role is critical to securing the
network against adversarial techniques and requires dedicated people to secure the
devices, applications, and information on the network.
This report presents best practices for overall network security and protection of
individual network devices. It will assist administrators in preventing an adversary from
exploiting their network. While the guidance presented here can be applied to many
types of network devices, the National Security Agency (NSA) has provided sample
commands for Cisco Internetwork Operating System (IOS) devices. These commands
can be executed to implement recommended mitigations.
1.1 Regarding Zero Trust
Zero Trust is a security model, a set of system design principles, and a coordinated
cybersecurity and system management strategy based on an acknowledgement that
threats exist both inside and outside traditional network boundaries. NSA fully supports
the Zero Trust security model, and much of the guidance in this report can be applied at
different boundaries as recommended in Zero Trust guidance. However, this report
provides guidance to mitigate common vulnerabilities and weaknesses on existing
networks. As system owners introduce new network designs intended to achieve more
mature Zero Trust principles, this guide may need to be modified.
An administrator’s
role is critical in
securing networks.
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2. Network architecture and design
A secure network design that implements
multiple defensive layers is critical to defend
against threats and protect resources within
the network. The design should follow
security best practices and model Zero Trust
principles, both for network perimeter and
internal devices.
2.1 Install perimeter and internal defense devices
A network requires a substantial defensive strategy to protect individual components
and the information they contain. Multiple layers of defense should be implemented at
the network’s perimeter to protect against external threats, and to monitor and restrict
inbound and outbound traffic.
NSA recommends configuring and installing security devices at the perimeter of the
network according to security best practices:
Install a border router to facilitate a connection to the external network, such as
an Internet service provider (ISP).
Implement multiple layers of next-generation firewalls throughout the network to
restrict inbound traffic, restrict outbound traffic, and examine all internal activity
between disparate network regions. Each layer should utilize different vendors to
protect against an adversary exploiting the same unpatched vulnerability in an
attempt to access the internal network.
Place publicly accessible systems and outbound proxies in between the firewall
layers in one or more demilitarized zone (DMZ) subnets, where access can be
appropriately controlled between external devices, DMZ devices, and internal
systems.
Implement a network monitoring solution to log and track inbound and outbound
traffic, such as a network intrusion detection system (NIDS), a traffic inspector, or
a full-packet capture device.
Deploy multiple dedicated remote log servers to enable activity correlation
among devices and detection of lateral movement.
Implement redundant devices in core areas to ensure availability, which can be
load-balanced to increase network throughput and decrease latency.
Apply multiple layers
of defense for a more
secure network design.
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Figure 1: Network perimeter with firewalls and a DMZ
2.2 Group similar network systems
Similar systems within a network should be logically grouped together to protect against
adversarial lateral movement from other types of systems. Adversaries will target
systems that are easier to exploit, such as printers, and use that initial access to further
propagate to other systems on the network. Proper network segmentation significantly
reduces the ability for an adversary to reach and exploit these other systems (see
Cybersecurity and Infrastructure Security Agency’s (CISA’s) “Layering Network Security
Through Segmentation” and NSA’s “Segment Networks and Deploy Application-aware
Defenses”) [1], [2]. Additionally, access restrictions between different types of systems
are easier to manage, control, and monitor if they are logically grouped together.
NSA recommends isolating similar systems into different subnets or virtual local area
networks (VLANs), or physically separating the different subnets via firewalls or filtering
routers. Workstations, servers, printers, telecommunication systems, and other network
peripherals should be separate from each other. Operational technology, such as
industrial control systems, typically need to be isolated from other information
technology and high-risk networks like the Internet. This physical separation provides
stronger protection because the intermediate device between subnets must be
compromised for an adversary to bypass access restrictions. Implement access
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restrictions on the internal routers, switches, or firewalls to allow only those ports and
protocols that are required for network operations or valid mission need. Access control
lists (ACLs) may need to be duplicated and applied directly to the switches to restrict
access between VLANs, or they can be applied to core routers where routing is
performed between internal subnets.
2.3 Remove backdoor connections
A backdoor network connection is between two or more devices located in different
network areas, generally with different types of data and security requirements. If one
device is compromised, an adversary can use this connection to bypass access
restrictions and gain access to other areas of the network. An example of a backdoor
network connection is an external border router connected to an ISP that is also directly
connected to the internal or management subnets. An adversary that can compromise
this external border router would likely have access to the internal network, bypassing
all firewalls.
NSA recommends removing all backdoor network connections and using caution when
connecting devices with more than one network interface. Verify that all network
interfaces of a device are at similar security levels, or that an intermediate device
provides both logical and physical separation between different network areas.
2.4 Utilize strict perimeter access controls
Network perimeter devices are essential elements in a security model and should be
configured to complement each other by implementing ACLs to regulate ingress and
egress of network traffic. These access control rulesets should be configured to
explicitly allow only services and systems that are required to support the mission of the
network.
NSA recommends a deny-by-default, permit-by-exception approach achieved by
carefully considering which connections to allow, and then creating rulesets that focus
on permitting only the allowed connections. This method allows a single rule to deny
several types of connections, instead of needing to create a separate rule for each
blocked connection. Failure to use a deny-by-default, permit-by-exception approach can
permit unnecessary access and increase the risk of compromise and information
gathering. If it is necessary to dynamically apply additional perimeter rulesets to prevent
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an adversary from completing or continuing an exploit, NSA recommends the use of an
intrusion prevention system (IPS).
NSA also recommends enabling logging, at a minimum, on all rulesets that deny or drop
network traffic. Logging should also be enabled on successful
and
unsuccessful
administrator access to critical devices.
2.5 Implement a network access control (NAC) solution
An adversary who wishes to gain internal access to a network must either find a way
through the external perimeter of the network or obtain access from inside the network.
A NAC solution prevents unauthorized physical connections and monitors authorized
physical connections on a network.
NSA recommends implementing a NAC solution that identifies and authenticates unique
devices connected to the network. Port security is a mechanism that can be
implemented on switches to detect when unauthorized devices are connected to the
network via a device’s media access control (MAC) address.
However, port security can be difficult to manage. For example, it increases the number
of support tickets due to valid blocked network ports (e.g., connected devices that
change often, such as conference rooms). In addition, adversaries who can spoof a
MAC address can bypass it as well. A more robust solution utilizes 802.1X, which
authenticates devices based on a trusted digital certificate installed on the device. While
it is more complex to implement, due to the use of certificates, it is easier to manage
than port security and offers a higher level of assurance.
2.6 Limit virtual private networks (VPNs)
A VPN tunnel can be established between two endpoints to provide an encrypted
communication channel over a network. It should only be used when the confidentiality
and integrity of the traffic cannot be maintained through other methods. VPN gateways
are typically accessible from the Internet and are prone to network scanning, brute force
attempts, and zero-day vulnerabilities. To mitigate many of these vulnerabilities, disable
all unnecessary features on the VPN gateways and implement strict traffic filtering rules
[2].
NSA recommends limiting VPN gateway access to User Datagram Protocol (UDP)
port 500, UDP port 4500, Encapsulating Security Payload (ESP), and other appropriate
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ports as needed. When possible, limit accepted traffic to known VPN peer Internet
Protocol (IP) addresses. Remote access VPNs cannot be added to a static filtering rule
if the remote peer IP address is unknown. If traffic cannot be filtered to specific IP
addresses, use an IPS in front of the VPN gateway to monitor for malformed IP Security
(IPsec) traffic and inspect IPsec session negotiations [3].
All IPsec VPN configurations require an IPsec policy and an Internet Key Exchange
(IKE) policy. These policies determine how it will negotiate each phase when
establishing the IPsec tunnel. If either phase is configured to allow weak cryptography,
the entire VPN may be at risk and data confidentiality will be lost. Each IKE policy
includes at least three key components:
1. Diffie-Hellman algorithm/group
2. Encryption algorithm
3. Hashing algorithm
The following are the minimum recommended settings per Committee on National
Security Systems Policy (CNSSP) 15:
Diffie-Hellman Group: 16 with 4096 bit Modular Exponent (MODP)
Diffie-Hellman Group: 20 with 384 bit elliptic curve group (ECP)
Encryption: Advanced Encryption Standard (AES)-256
Hash: Secure Hash Algorithm (SHA)-384
Diffie-Hellman Group 15 is also acceptable based on the minimum requirements of
CNSSP 15, but Group 15 is not recommended due to interoperability issues that have
been observed.
When establishing a VPN proposal, policy, or transform-set, ensure it follows CNSSP
15 recommendations [4]. The CNSSP 15 requirements are explained in the draft
Internet Engineering Task Force (IETF) document on “Commercial National Security
Algorithm (CNSA) Suite Cryptography for Internet Protocol Security (IPsec)” [5].
To ensure they are not inadvertently used, disable the default polices and proposals for
Internet Security Association and Key Management Protocol (ISAKMP) and IKEv2 with
the following configuration commands:
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no crypto isakmp default policy
no crypto ikev2 policy default
no crypto ikev2 proposal default
no crypto ipsec transform-set default
Note:
If the default policies are disabled, only the explicitly configured policies will be
used.
Establish an IKEv2 proposal, policy, and profile with the following example configuration
commands:
crypto ikev2 proposal <IKEV2_PROPOSAL_NAME>
encryption aes-gcm-256
group [16|20]
crypto ikev2 policy <IKEV2_POLICY_NAME>
proposal <IKEV2_PROPOSAL_NAME>
crypto ikev2 profile <IKEV2_PROFILE_NAME>
match identity remote ...
authentication remote ...
authentication local ...
The configuration of the profile will depend on the network it is configured for, and must
have local and remote authentication methods and a
match
statement. A separate
keyring can also be established and applied to the profile for multiple pre-shared keys.
Establish an IPsec transform-set with the following example configuration commands:
crypto ipsec transform-set <IPSEC_TRANSFORM_NAME> esp-gcm 256
mode tunnel
Establish an IPsec profile, which utilizes the IKEv2 profile and IPsec transform-set
defined above, with the following example configuration commands:
crypto ipsec profile <IPSEC_PROFILE_NAME>
set transform-set <IPSEC_TRANSFORM_NAME>
set pfs group16
set ikev2-profile <IKEV2_PROFILE_NAME>
The IPsec profile should be applied to the tunnel interface with the following
configuration commands:
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interface <TUNNEL_INTERFACE_NAME>
tunnel protection ipsec profile <IPSEC_PROFILE_NAME>
no shutdown
For more information, refer to “Configuring IPsec Virtual Private Networks,” “Mitigating
Recent VPN Vulnerabilities,” and “Eliminating Obsolete Transport Layer Security (TLS)
Protocol Configurations” [6], [7], [8].
Return to Contents
3. Security maintenance
Outdated hardware and software may contain publicly known vulnerabilities and provide
an easy mechanism for adversaries to exploit the network. These vulnerabilities are
mitigated by regularly upgrading the hardware and software to newer versions that are
supported by the vendor. Additionally, the
integrity of downloaded software should be
verified before and during use. Security
maintenance should be performed on a
regular basis to ensure devices continue to
operate securely.
3.1 Verify software and configuration integrity
An adversary can introduce malicious software into network devices by modifying
operating system files, the executable code running in memory, or the firmware or
bootloader that loads the operating system of a network device. Software that has been
maliciously modified on a network device can be used by an adversary to violate data
integrity, exfiltrate sensitive information, and cause a denial of service (DoS).
NSA recommends verifying the integrity of operating system files installed and running
on devices by comparing the cryptographic hash of the file with the known good hash
published by the vendor. When upgrading operating system files, perform the same
integrity verification on the files prior to and after installation to ensure no modifications
were made. A basic online hash can be computed on an operating system image file
with the following exec command:
verify /sha512 <PATH:filename>
Upgrade hardware and
software to ensure
efficiency and security.
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Older devices may only support a Message Digest 5 (MD5) hash, which can be
computed with the following exec command:
verify /md5 <PATH:filename>
The computed hash can be compared with the information published for the file on the
Support pages of https://www.cisco.com/ [12]. More information on network device
verification is available in the “Network Device Integrity (NDI) Methodology,” “Network
Device Integrity (NDI) on Cisco IOS Devices,” and “Validate Integrity of Hardware and
Software” documents [30], [31], [32].
Rather than performing these more complex software modifications, an adversary may
choose to simply change the configuration. Configuration changes can be a sign that a
device has been compromised.
NSA also recommends implementing a configuration change control process that
securely creates device configuration backups to detect unauthorized modifications.
When a configuration change is needed, document the change and include the
authorization, purpose, and mission justification. Periodically verify that modifications
have not been applied by comparing current device configurations with the most recent
backups. If suspicious changes are observed, verify the change was authorized.
3.2 Maintain proper file system and boot management
Many network devices have at least two different configurations, one or more saved in
persistent storage and an active copy running in memory. Permanent changes to the
configuration should be saved or committed to prevent configuration inconsistencies if
the device is rebooted or loses power. Configuration changes can be saved on a device
with the following exec command:
copy running-config startup-config
If changes are meant to be temporary, NSA recommends inserting comments before
the updated configuration lines to include why it was made and when it can be removed,
and removing the comments and temporary changes at the appropriate time. If the
device does not support comments, insert comments in a backup copy of the
configuration to compare with the version on the device.
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Use an encrypted protocol when copying configurations remotely, such as Secure File
Transfer Protocol (SFTP) or Secure Copy Protocol (SCP). The copy mechanism used to
backup or archive configurations, and the backup repository, must be protected from
unauthorized access.
NSA also recommends checking for unused or unnecessary files on each device and
removing them with the following exec commands:
dir /recursive all-filesystems
delete <PATH:filename>
Older operating system files or outdated backup configuration files stored on the device
are most likely unnecessary, and should be removed. Storing multiple versions of
software provides an adversary the opportunity to reload outdated software and
reintroduce vulnerabilities patched in newer versions of the operating system.
3.3 Maintain up-to-date software and operating systems
Maintaining up-to-date operating systems and stable software protects against critical
vulnerabilities and security issues that have been identified and fixed in newer releases.
Devices running outdated operating systems or vulnerable software are susceptible to a
variety of published vulnerabilities, and exploiting these devices is a common technique
used by adversaries to compromise a network.
NSA recommends upgrading operating systems and software on all devices to the
latest stable version available from the vendor. Upgrading the operating system may
require additional hardware or memory upgrades, and obtaining a new software version
may require a maintenance or support contract with the vendor. Some network
infrastructure devices may not support an auto-update feature, so it may be necessary
to implement a requisition and installation process to obtain the latest software from the
vendor.
Please see the support page for the corresponding vendor in Table I: Vendor support
pages to determine the latest operating system for a particular device.
Table I: Vendor support pages
Vendor
URL
Arista Networks
https://www.arista.com/en/support/
[9]
Aruba Networks
https://www.arubanetworks.com/support-services/
[10]
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Vendor
URL
Broadcom
https://www.broadcom.com/support
[11]
Cisco Systems
https://www.cisco.com/c/en/us/support/index.html
[12]
Dell
https://www.dell.com/support/home/en-us/
[13]
Extreme Networks
https://www.extremenetworks.com/support/ [14]
F5
https://www.f5.com/services/support [15]
Fortinet
https://www.fortinet.com/support
[16]
Hewlett Packard Enterprise (HPE)
https://www.hpe.com/us/en/services.html
[17]
International Business Machines (IBM)
https://www.ibm.com/mysupport/
[18]
Juniper Networks
https://support.juniper.net/support/
[19]
Linksys
https://www.linksys.com/us/support/
[20]
NETGEAR
https://www.netgear.com/support/
[21]
Palo Alto Networks
https://support.paloaltonetworks.com/support/
[22]
Riverbed Technology
https://support.riverbed.com/
[23]
Ruckus Networks
https://support.ruckuswireless.com/
[24]
SonicWall
https://www.sonicwall.com/support/
[25]
TRENDnet
https://www.trendnet.com/support/
[26]
Tripp Lite
https://www.tripplite.com/support/
[27]
Ubiquiti
https://help.ui.com/hc/en-us/
[28]
WatchGuard
https://www.watchguard.com/wgrd-support/overview
[29]
3.4 Stay current with vendor-supported hardware
Vendors eventually stop supporting specific hardware platforms and, if a failure occurs,
these end-of-life devices cannot be serviced. In addition to device instability and
memory requirement concerns, there is an increased risk of an adversary exploiting the
device due to a lack of software updates to fix known vulnerabilities. Newer, vendor-
supported hardware platforms have improved security features, including protection
from known vulnerabilities.
Once a vendor publishes an end-of-life notice or announces that a device will no longer
be supported, NSA recommends constructing a plan to upgrade or replace affected
devices with newer equipment, according to vendor recommendations. Outdated or
unsupported devices should be immediately upgraded or replaced to ensure the
availability of network services and security support.
Please see the support page for the corresponding vendor in Table I: Vendor support
pages to determine if a particular device is supported by the vendor.
Return to Contents
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4. Authentication, authorization, and accounting (AAA)
Centralized AAA servers provide a consolidated mechanism to manage administrative
access to devices, and the accounts created are more challenging for an adversary to
compromise since credentials are not stored directly on devices. Properly configuring
these servers provides an authoritative source for managing and monitoring access,
improves consistency of access control,
reduces configuration maintenance, and
reduces administrative costs. All devices
should first be configured to use modern
AAA services with the following
configuration example command for Cisco
IOS devices:
aaa new-model
Applying the above configuration ensures a device will not use legacy authentication
and authorization methods.
4.1 Implement centralized servers
All devices should be configured to use centralized AAA servers. NSA recommends
implementing at least two AAA servers to ensure availability, and assist with detecting
and preventing adversary activities. If one server becomes unavailable due to
scheduled maintenance or for other reasons, the remaining servers will continue
providing the centralized AAA services. The servers should be:
Configured to authenticate devices with a unique and complex pre-shared key to
ensure only authorized devices can use the AAA services (see 5.5 Create strong
passwords).
Configured to use the same protocol (e.g., TACACS+, RADIUS, or LDAP) for
consistency, and use encrypted transport when supported (e.g., RadSec,
Diameter, LDAPS, or IPsec encapsulated).
Synchronized with each other to ensure consistency of user credentials and
access controls.
A server group with multiple AAA servers can be configured with the following
configuration commands:
Control access and
reduce maintenance
through use of AAA.
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aaa group server {tacacs+ | radius | ldap} <GROUP_NAME>
server-private <IP_ADDRESS_1> key <KEY_1>
server-private <IP_ADDRESS_2> key <KEY_2>
Some older devices may utilize the keywords
tacacs-server
and
radius-server
in the
configuration, which prevents assigning a unique key to each server.
NSA recommends replacing these lines with the above configuration format, and
assigning a unique pre-shared key to each server. If an adversary obtains the pre-
shared key to one server, the key needs to be revoked, but other servers with different
keys can continue to be used by the devices.
4.2 Configure authentication
Authentication verifies the identity of a person or entity. All devices should be configured
to use centralized servers for AAA services first, and local administrator accounts as a
backup method only if all the centralized servers are unavailable. The same applies to
privileged level authentication; devices should only use the local privileged-level
password if all centralized servers are unavailable. This order of precedence will
prevent an adversary, who obtains local administrator account credentials, from logging
into the devices since access will usually be controlled by the AAA servers.
NSA recommends configuring centralized authentication for
login
and
enable
(privileged) access as the primary method, as shown in the following configuration
commands:
aaa authentication login default group <GROUP_NAME> local
aaa authentication enable default group <GROUP_NAME> enable
Using the
default
keyword ensures the configuration is applied globally in all instances
when an explicit authentication list is not specified. If a custom named list is used
instead, it would be necessary to explicitly apply this list to all instances where AAA is
used and potentially leave some management services incorrectly configured and open
to compromise. The
default
list is always applied when a custom named list is not
explicitly applied.
The
<GROUP_NAME>
should be the custom name of the AAA server group (defined
previously) that includes the IP addresses of the centralized AAA servers and their
associated keys.
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The
line
keyword should not be used as these passwords are not securely stored in
the configuration and do not provide accountability.
The
none
keyword should never be used since it disables authentication.
4.3 Configure authorization
Authorization validates that a person or entity has permission to access a specific
resource or perform a specific action. The organization, situation, and the device’s
purpose will dictate authorized administrator commands. At the least, authorization
should be applied to starting an exec session (shell) and execution of other shell
commands, including configuration commands. Authorization should also be explicitly
applied to the console, as it may not be automatically applied by default.
NSA recommends adequately restricting what legitimate administrators are authorized
to execute to prevent an adversary from performing unauthorized actions with a
compromised account. Most administrators use privilege level 1 for user level access
and privilege level 15 for privileged level access.
Authorization should be applied to both of these levels and any other privilege levels
used by administrators with the following configuration commands:
aaa authorization console
aaa authorization exec default group <GROUP_NAME> local
aaa authorization commands 1 group <GROUP_NAME> local
aaa authorization commands 15 group <GROUP_NAME> local
aaa authorization config-commands
The
default
list should be used to ensure the configuration is applied everywhere.
The
<GROUP_NAME>
should be the custom name of the AAA server group (defined
previously) that includes the IP addresses of the centralized AAA servers and their
associated keys.
If desired, the
if-authenticated
keyword can be applied after the
local
keyword. If an
administrator is successfully logged in and all the centralized AAA servers become
unavailable, the administrator will no longer be authorized to execute commands. The
if-authenticated
keyword ensures an authenticated user will continue to be
authorized to execute commands. However, be cautious with this keyword as it could
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potentially give an administrator access beyond what is configured on the centralized
AAA servers.
The
none
keyword should never be used since it disables authorization.
4.4 Configure accounting
Accounting keeps records of all relevant resources accessed or actions performed,
holding administrators accountable. Waiting until an event is stopped to generate an
accounting record is insufficient, as a particular action could take an unreasonable
amount of time to complete before the record is generated. Accounting records can be
collected for several other event types, but it depends on the organization and purpose
of devices.
NSA recommends that system configuration changes be centrally recorded, and a
process implemented to periodically review these records to detect potential malicious
activities. At a minimum, accounting records should be collected when an exec session
(shell) is started and stopped, and when shell commands are started and stopped.
Similar to authorization, accounting of
commands
must be applied to all administrator
privilege levels with the following configuration commands:
aaa accounting exec default start-stop group <GROUP_NAME>
aaa accounting commands 1 default start-stop group <GROUP_NAME>
aaa accounting commands 15 default start-stop group <GROUP_NAME>
The
default
list should be used to ensure the configuration is applied everywhere.
The
<GROUP_NAME>
should be the custom name of the AAA server group (defined
previously) that includes the IP addresses of the centralized AAA servers and their
associated keys.
4.5 Apply principle of least privilege
Least privilege is a security concept that authorizes access to a person or entity at the
lowest privilege level necessary to perform authorized tasks. Many common tasks do
not require privileged level access, such as viewing status of network interfaces or
reviewing routing tables. To implement least privilege, administrators should initially log
in with the lowest privilege level necessary. This provides an additional layer of security
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that an adversary must circumvent to fully compromise a device. It also prevents
administrators from inadvertently making configuration changes to a device.
NSA recommends that all accounts be configured with privilege level 1 or 0, and require
administrators to enter additional credentials to elevate to a higher privilege level to
perform required tasks. Privilege levels should be periodically reviewed, and
unnecessary accesses removed to prevent inadvertent use of privileged level
commands at lower privilege levels.
The privilege level of individual local accounts can be changed with the
privilege
keyword. Assign local accounts to privilege level 1 with the following configuration
command:
username <USER_NAME> privilege 1
Note:
This does not change the account password.
All administrator accounts logging in at privilege level 1 will be required to execute the
enable
command and provide additional credentials to elevate to a higher privilege
level. In addition to reviewing all local administrator accounts and ensuring they are
assigned the least privilege level, it is also necessary to review all accounts configured
on the centralized AAA servers.
Similarly, the same concept should be applied to the console (CON), auxiliary (AUX),
and virtual teletype (VTY) lines. When AAA authorization is properly configured, it
should not be dependent on the configuration of the lines. However, it is a best practice
to ensure the lines are configured to the least privilege level with the following
configuration commands:
line con 0
privilege level 1
line aux 0
privilege level 1
line vty 0 4
privilege level 1
line vty 5 15
privilege level 1
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Depending on the device, it may be necessary to apply a similar configuration to other
lines as well. If VTY lines 5 through 15 do not exist on a particular device, it is not
necessary to execute those commands.
4.6 Limit authentication attempts
Limiting the number of authentication attempts and introducing a login delay prevents
an adversary from performing brute force password cracking against a device in an
attempt to obtain access.
NSA recommends restricting failed remote administration attempts to a maximum of
three or less with the following configuration example command for Cisco IOS devices:
aaa authentication attempts login 3
Similarly, the same concept of three or less failed attempts should be applied to Secure
Shell (SSH) sessions with the following configuration command:
ip ssh authentication-retries 3
NSA also recommends introducing a delay of at least one second between login
attempts to significantly slow down brute force attempts with the following configuration
command:
login delay 1
Return to Contents
5. Local administrator accounts and passwords
Local accounts are vital to the management of network devices. If centralized
authentication fails, the local accounts provide administrators with access to the network
devices to troubleshoot and diagnose network issues. Local accounts should be unique,
authenticated with a unique and complex
password, and provide accountability for
individual administrators. If a password policy
does not exist for the organization, establish
and enforce a new policy. Periodically review
and revise the policy, when necessary.
Create unique local
accounts with complex
passwords.
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This section focuses primarily on local accounts and passwords. Traditional network
devices use legacy methods for managing local accounts, and they may not support the
recommended mechanisms for composing, changing, and verifying passwords. The
simplistic nature of these local accounts requires different recommendations to be
applied. This is in contrast to the centralized AAA servers where multi-factor
authentication, password complexity, previous password comparison, and other
concepts can be properly implemented.
5.1 Use unique usernames and account settings
Most devices have default administrative credentials which are advertised to the public,
and they often grant full administrative access to a device. Maintaining these settings
offers the adversary an easy entry into the network to connect and potentially gain
privileged level access to anonymously monitor or reconfigure the device.
NSA recommends removing all default configurations and reconfiguring each device
with a unique and secure account for each administrator. Do not introduce any new
devices into the network without first changing the default administrative settings and
accounts.
Note:
The default user account on some devices cannot be removed.
Accounts can be used by individual administrators or shared among a group. However,
if multiple administrators use a group account to access the device, accountability
cannot be enforced as configuration changes will not be associated to a specific
individual. For this reason, adversaries target group accounts to gain unauthorized
access to devices.
NSA further recommends disabling all shared or group administrator accounts, and
using a unique account for each administrator to provide access for configuration
changes and to ensure accountability on each device. If group accounts are necessary,
NSA recommends monitoring these accounts to detect any suspicious activity. It may
not be feasible to create a backup local account for each administrator, but a single
group account known to every administrator does not provide individual accountability.
NSA also recommends that local accounts only be used in emergency situations when
the centralized AAA servers are unavailable. Unique local emergency account
passwords should be maintained by a trusted individual who does not have direct
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access to the devices. During an event, administrators can request the local account
and password and, once the emergency situation has ended, the trusted individual can
then change the password. This will prevent password reuse and ensure accountability.
All other authentication requests should occur via the centralized AAA servers.
5.2 Change default passwords
Most devices are assigned a default password, or sometimes no password, to allow an
administrator easy access prior to initial configuration. Many of these passwords are
public knowledge and typically do not need to be changed for the device to work
properly. They are prime targets for malicious automated scanners (botnets) to exploit,
as the default credentials provide privileged level access to the device.
NSA recommends removing all default passwords and assigning a unique, complex,
and secure password to all levels of access, including both user and privileged levels.
Additionally, when introducing new devices into the network, change the default user
and privileged level passwords before attaching the device to the network.
5.3 Remove unnecessary accounts
Some devices are configured with unnecessary accounts by default. Since they may be
rarely used or not used at all, these accounts are often overlooked. If possible, rename
or remove default accounts that are not associated with a particular administrator.
NSA recommends that the number of accounts authorized to log onto devices should be
limited to what is necessary; all others should be removed. When an administrator
leaves an organization or changes roles, the associated accounts should be disabled or
removed. On Cisco IOS devices, remove a local account with the following configuration
command:
no username <NAME>
5.4 Store passwords with secure algorithms
Passwords are generally stored in the configuration of a device or in a local database,
as clear text, encrypted, or a one-way hash. Clear text should never be used, and some
encryption or hash functions are considered weak and could easily be broken using
publicly available tools. An adversary could accumulate passwords or hashes from the
configuration or local database by using a network analyzer or by compromising a
central management system that stores configuration files. The clear text and weak
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algorithm passwords could be easily cracked and used to obtain user or privileged level
access to a device. Cisco IOS supports the following one-way hash and encrypted
types:
Type 0 passwords should not be used because they are stored in clear text
Type 4 password hashes should not be used because they are easy to crack
Type 5 (MD5) password hashes should be avoided except on older operating
systems that do not support Types 6, 8, or 9
Type 6 passwords are AES-encrypted and should only be used for passwords
that need to be encrypted instead of hashed (such as for VPN keys), or on
systems that do not support Type 8 (which typically implies that Type 9 is also
unavailable)
Type 7 passwords should not be used because they are easily reversible, even
though they are encrypted
Type 8 (SHA-256 PBKDF2) password hashes are recommended
Type 9 (Scrypt) password hashes are not approved by the National Institute of
Standards and Technology (NIST)
For more information on the above password types, see “Cisco Password Types: Best
Practices” [33].
NSA recommends that all passwords on a device be stored using the most secure
algorithm available, and never stored as clear text. One-way hash algorithms are
irreversible and generally should be used for storing passwords. However, if one-way
hash algorithms are unavailable, the passwords should be encrypted with a strong
unique key.
When creating a user account or assigning a password, some devices require the
algorithm to be specified. Special attention should be given to privileged level accounts,
but this guide also applies to user accounts, management ports, authenticated routing
protocols, VPN keys, and any place where a password may be specified in the device’s
configuration.
Prevent clear text passwords with the following configuration command:
service password-encryption
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Store a Type 8 password hash for a local account with the following configuration
command:
username <NAME> algorithm-type sha256 secret <PASSWORD>
Note:
The
algorithm-type
keyword is not stored in the saved configuration in
nvram:/startup-config
; instead the
secret
keyword is replaced with
secret 8
prior to
the hashed password.
If reversible encrypted passwords are needed (such as for VPN keys), use Type 6 AES
instead of Type 7 passwords with the following configuration commands:
password encryption aes
key config-key password-encrypt <KEY>
The
<KEY>
should be a unique and complex password that is used to generate the key
to encrypt Type 6 passwords. It should not be a default, weak, or easily guessable
password, and should not be reused elsewhere in the configuration. An adversary that
guesses this key could use it to decrypt all Type 6 passwords stored in the
configuration. Once this key is set, it generally does not need to be retained.
Note:
The
key config-key password-encrypt
configuration command is not stored in
the saved configuration in
nvram:/startup-config
.
Since it does not need to be retained, NSA recommends using a unique key for every
device which will prevent an adversary from using the same key to decrypt the Type 6
passwords on all of the devices.
Note:
If the key is ever changed, the Type 6 encrypted passwords will need to be
manually set again.
5.5 Create strong passwords
A device configured with a weak password increases an adversary’s ability to
compromise that device. An adversary may be able to easily guess a weak password or
crack it using publicly available password cracking tools (e.g., dictionary or brute force
attempts). Once privileged level access is obtained, an adversary can make
configuration changes that potentially compromise other devices on the network.
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NSA recommends assigning a unique and complex password to all levels of access,
including both user and privileged level accesses. Unique and complex passwords
should also be used for routing authentication, time synchronization, VPN tunnels,
Simple Network Management Protocol (SNMP) community strings, and anywhere else
passwords are stored in the configuration. Passwords should meet the following
complexity requirements:
Use all the different character classes (uppercase, lowercase, numbers, and
special characters)
Be at least 15 characters long
Not be based on unmodified words or acronyms
Not be a keyboard walk
Not be the same as a username
Not be related to the network, organization, location, local sports team, or other
function identifiers
Not be identical or similar to the last password or passwords assigned elsewhere
Not be a default, blank, or publicly known password
An organization’s password policy may not require passwords managed through the
centralized AAA servers to adhere to all of these recommendations, especially when
coupled with multi-factor authentication and other principals. The above guidance
should at least be applied to local accounts and other passwords that are stored in the
configuration of a network device, where centralized security controls cannot be applied.
NSA highly discourages using SNMP version 1 or 2c. For more information, refer to 7.1
Disable clear text administration services and 7.8 Remove SNMP read-write community
strings.
An adversary with knowledge of the network, location, programs, etc. could easily guess
(or know) these terms, thus aiding them in cracking the passwords.
NSA also recommends checking for weak passwords on a regular basis to enforce the
organization’s password policy. Password complexity should be checked before setting
a new password. Network administrators should periodically review network device
configurations to identify the use of weak password algorithms.
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5.6 Utilize unique passwords
Assigning the same password to multiple accounts or multiple levels of access could
impact accountability and authorization. If an administrator accesses the device via an
unencrypted protocol, an adversary could use a network analyzer to collect the
password from network traffic. If an adversary gains user level access, they could
potentially reuse the same password to also gain privileged level access.
Assigning the same password to multiple devices allows an adversary to compromise
numerous devices at the same time, without any extra effort. If the same password was
assigned to a majority of the devices, an adversary would only need to compromise a
single password to obtain privileged level access to all of those devices.
NSA recommends assigning a unique, complex, and secure password for each account
and privileged level on each device.
NSA also recommends checking for password reuse across multiple accounts and
levels of access, and across multiple devices. Identical hashes can be an indication of
password reuse.
5.7 Change passwords as needed
Periodically changing passwords has historically led to the use of weaker passwords,
and enforcing this policy may not be necessary if users follow the guidance in 5.5
Create strong passwords. The initial creation of strong passwords is a more effective
method of reducing successful password compromises.
NSA recommends changing a password immediately if the password or password hash
has been compromised, and storing it securely as described in 5.4 Store passwords
with secure algorithms. Given enough time and resources, every password can be
guessed or brute force cracked, which is an attempt at all possible character
combinations. Compromised passwords that have not been changed give an adversary
even more time to use these techniques. Additionally, if an adversary eventually cracks
an old password, they may continue attempting variations and guess the current
password if it was based on a previous password.
Unfortunately, it can be difficult to discern when a password has been compromised,
especially local passwords stored in the configuration. Traditional network devices are
significantly more simplistic in how they store and transmit the configuration, which
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includes passwords and password hashes. Furthermore, emailing network device
configurations or storing them in unprotected file shares could constitute a compromise,
because the passwords and password hashes stored in the configurations are left
unprotected. Additionally, passwords stored with a weak algorithm should be
considered compromised, since they are significantly easier to crack.
If the confidentiality of passwords cannot be maintained or the organization desires to
regularly attempt to evict actors who may have compromised passwords without being
detected, NSA recommends establishing an aging policy that incorporates changing
passwords on a regular basis. Changing local passwords can be significantly more
cumbersome than centralized passwords, so it is necessary to choose a time frame that
is reasonably practical for network administrators to follow, while reducing the amount of
time an adversary can utilize a potentially compromised password. If the device does
not support long passwords, it is recommended that passwords be changed more
frequently to prevent an adversary from cracking a password that is still in use.
Note:
If
a password is stored using a convoluted Type 9 secret where the
secret 9
password hash begins with
$14$
, it indicates that the password was not recently
changed. The password hash was converted from Type 5 during a previous operating
system upgrade. The convoluted Type 9 secret should be removed by changing the
password to a Type 8 secret with the
algorithm-type sha256
keywords, as previously
described in 5.4 Store passwords with secure algorithms.
Return to Contents
6. Remote logging and monitoring
Logging is an important mechanism for recording device activities and tracking network
security events. It provides administrators with the ability to review the logs for
suspicious activities and to investigate incidents. An incomplete logging configuration on
a device can lead to missing or inaccurate information and difficulty correlating events
that occurred on a device or the network. Proper
logging includes sending logs to multiple remote
log servers, synchronizing the clock to multiple
authenticated time sources, and implementing log
management policies and procedures. A security
information and event management (SIEM)
Enable and configure
logging to identify
malicious activity.
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system can be used to aggregate and analyze logs received by the remote log servers.
Logs should be retained as recommended by the Office of Management and Budget
(OMB) Memorandum M-21-31 [34].
6.1 Enable logging
Log messages will only be generated on network devices when logging is enabled.
Devices should be configured to send log messages to a local log buffer and centralized
log servers simultaneously.
NSA recommends enabling syslog logging, setting the local log buffer to 16 megabytes
or greater, and establishing a procedure to verify the logs are received and reviewed on
a regular basis. Most devices should be able to support the larger buffer size, but it can
be decreased for a particular device if there is insufficient memory.
Ensure that syslog logging is enabled with the following configuration command:
logging on
Increase the maximum local log buffer with the following configuration command:
logging buffered 16777216 informational
Note:
This will also change the logging level to informational, as both values must be
set simultaneously.
6.2 Establish centralized remote log servers
Log messages sent to remote log servers are less vulnerable to compromise or erasure,
ensuring that the messages will not be impacted in the event a device is compromised,
rebooted, or the local log buffer becomes full. Multiple log servers are critical when
defending against DoS effects and reducing a single point-of-failure.
NSA recommends establishing at least two remote, centralized log servers to ensure
monitoring, redundancy, and availability of device log messages. If supported, ensure
the log messages are encrypted in transit to prevent unauthorized disclosure of
sensitive information. Outbound syslog messages can only be encrypted on a Cisco
IOS device by creating an IPsec tunnel between the device and the remote syslog
servers, as described in 2.6 Limit and encrypt virtual private networks (VPNs).
Configure at least two remote log servers with the following configuration commands:
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logging host <IP_ADDRESS_1>
logging host <IP_ADDRESS_2>
6.3 Capture necessary log information
Devices configured with sufficient information in the logs provide administrators with the
information they need to analyze events related to an incident, including correlating
multiple events or events occurring on other devices.
NSA recommends setting the trap and buffer logging levels on each device to at least
syslog level “informational” (code 6) to collect all necessary information. Devices can be
configured for “debugging” (code 7), but the increased number of generated messages
may slow down the log review process. Set both the trap and buffer logging levels to
informational with the following configuration commands:
logging trap informational
logging buffered 16777216 informational
Note:
This will also set the maximum size of the local log buffer, as both values must be
set simultaneously.
Logging can also be enabled on the console and VTY lines with the
console
and
monitor
keywords, respectively. These methods will immediately alert administrators
that are logged in, but the messages are not retained. It is not necessary to enable
logging for these methods unless it is desired by the administrators. These logging
mechanisms can be disabled with the following configuration commands:
no logging console
no logging monitor
NSA also recommends using Coordinated Universal Time (UTC) for the time zone,
especially if the network spans multiple time zones. All log messages should contain a
properly configured timestamp with the full date including the year, the time including
seconds and milliseconds, and the time zone. Ensure the time zone is properly set and
enable all the features listed above with the following configuration commands:
clock timezone UTC 0 0
service timestamps log datetime msec localtime show-timezone year
service timestamps debug datetime msec localtime show-timezone year
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Finally, NSA also recommends enabling log messages to indicate when a user was
successful or unsuccessful at logging into the system. Even though these events are
recorded on the centralized AAA servers when accounting is properly configured, this
information is not logged in the local buffer. Ensure these events are logged with the
following configuration commands:
login on-failure log
login on-success log
6.4 Synchronize clocks
The Network Time Protocol (NTP) is used to synchronize device clocks worldwide and
to ensure that the timestamps included with log messages are reasonably accurate. To
provide this reliability, each device should synchronize with at least two trusted time
sources. This accuracy is critical to ensure log message timestamps can be easily
correlated across geographically dispersed time zones, and used to collectively trace a
network incident from one device to another.
NSA recommends that each device and the remote log servers use at least two
trustworthy and reliable time servers to ensure accuracy and availability of information.
Internal time servers should be established as the primary source for all devices, which
should subsequently synchronize with authoritative external sources. This design
decreases the number of external requests and ensures consistency of timestamps in
the event an external time server is unreachable. When deploying time servers on the
network, administrators should confirm that devices can access the time servers, and
that the clocks are synchronized after the configuration has been applied.
NSA also recommends enabling NTP authentication on all devices to prevent clock
tampering, and configuring strong, unique NTP authentication keys between the devices
and their specified time source.
Establish the trusted NTP keys and enable NTP authentication with the following
configuration commands:
ntp authentication-key <#1> md5 <KEY>
ntp trusted-key <#1>
ntp authentication-key <#2> md5 <KEY>
ntp trusted-key <#2>
ntp authenticate
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Any number of trusted keys can be established. Note that NTP authentication keys will
be stored in the configuration as Type 7 passwords. Type 6 AES-encrypted passwords
are not supported for NTP authentication.
Synchronize the device with at least two different NTP servers with the following
configuration commands:
ntp server <IP_ADDRESS_1> key <#1>
ntp server <IP_ADDRESS_2> key <#2>
Note:
The number at the end of each command is the trusted NTP key used to
authenticate that particular server.
After waiting for the clock to synchronize, verify synchronization and status of the NTP
servers with the following exec commands:
show ntp status
show ntp associations
Note:
It is necessary to verify clock synchronization after every NTP configuration
change, and it may take several hours for proper synchronization.
Return to Contents
7. Remote administration and network services
Network devices can be managed remotely by
administrators through various services. Some
common network services include SSH,
Hypertext Transfer Protocol (HTTP), SNMP, and
File Transfer Protocol (FTP). These services are
useful for administrators, but they are also
targeted by adversaries to exploit and gain privileged level access to a device. All of
them must be properly configured to reduce the probability of a compromise.
7.1 Disable clear text administration services
Clear text protocols pass traffic across the network “in the clear” (i.e., unencrypted) and
were designed prior to widespread use of encryption. Therefore, using these protocols
to remotely administer critical devices may lead to an information disclosure that could
Protect your network
management tools
from adversaries.
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adversely affect device and network security. An adversary could compromise a device
or service by collecting usernames, passwords, configuration information, and other
sensitive data through common information retrieval techniques (e.g., network analyzer
or packet capture utility).
NSA recommends using encrypted services to protect network communications and
disabling all clear text administration services (e.g., Telnet, HTTP, FTP, SNMP 1/2c).
This ensures that sensitive information cannot be easily obtained by an adversary
capturing network traffic. For detailed information on how to enable encrypted services,
refer to 7.11 Configure remote network administration services.
If a device does not support encrypted protocols, connect a management system
directly to the console or management port, or establish a dedicated out-of-band
management network to reduce the ability of an adversary capturing the clear text
protocols. Disable the Telnet service with the following configuration commands:
line vty 0 4
transport input none
line vty 5 15
transport input none
Depending on the device, it may be necessary to apply a similar configuration to other
lines as well. If VTY lines 5 through 15 do not exist on a particular device, it is not
necessary to execute those commands.
Note:
These commands may also disable other services that are enabled on the lines
by default, including SSH. For detailed information on how to enable SSH, refer to
7.11.1 Configuring SSH for remote administration.
Disable the HTTP service with the following configuration command:
no ip http server
For detailed information on how to enable the secure HTTP service, refer to 7.11.2
Configuring HTTP for remote administration.
Disable versions 1 and 2c of the SNMP and SNMP trap services by removing any
configured community strings with the following configuration commands:
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no snmp-server community <COMMUNITY_STRING>
no snmp-server host <HOSTNAME_OR_IP_ADDRESS> <COMMUNITY_STRING>
For detailed information on how to enable SNMP version 3, refer to 7.11.3 Configuring
SNMP for remote administration.
Disable the Trivial File Transfer Protocol (TFTP) by removing any lines with the
following configuration command:
no tftp-server <FILENAME>
The FTP service is generally not enabled as a listening service, but the protocol can be
used as a client. Remove FTP credentials with the following configuration commands:
no ip ftp username
no ip ftp password
7.2 Ensure adequate encryption strength
Some encrypted services require that a public and private key pair be generated so
clients can connect and authenticate to the server. Additionally, the client and server of
an encrypted connection may collectively establish a private session key for each
unique connection. Encrypted connections that use weak algorithms or a small number
of bits make it easier for an adversary to crack the private session key and decrypt all of
the data transferred during a unique connection.
For guidance on which algorithms and ciphers are NSA-approved for National Security
Systems (NSS), refer to CNSSP 15 [4]. The CNSSP 15 requirements are explained in
the draft IETF documents on Commercial National Security Algorithm (CNSA) Suite
Cryptography for Internet Protocol Security (IPsec), Commercial National Security
Algorithm (CNSA) Suite Profile for TLS and DTLS 1.2 and 1.3, and Commercial
National Security Algorithm (CNSA) Suite Cryptography for Secure Shell (SSH) [5], [35],
[36].
For related requirements and guidance for non-NSS U.S. Government systems, refer to
NIST SP 800-52 Revision 2 Appendix F [37].
These documents contain the latest recommended encryption parameters.
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NSA recommends that 3072 bits or higher be used for asymmetric (public and private)
key generation, 384 bits for elliptic curve cryptography (ECC) keys, and 256 bits for
symmetric encryption keys. Some systems may not support 3072 bits, so it may be
necessary to use 4096 bits instead. For any device that has a smaller key size,
regenerate a new key pair and configure encrypted protocols to only use approved
algorithms. A larger key size may increase the time to connect to the service (due to
extra computations), but is negligible on most devices. For more information on
configuring encrypted services, refer to 7.11 Configure remote network administration
services.
7.3 Utilize secure protocols
Several common administration services implement protocols that contain flaws in the
implementation and exchange of information, which can be exploited by an adversary.
Some protocols, such as SSH, can be configured to be backward compatible and
accept older insecure protocols, along with the newer ones. Older protocols are subject
to man-in-the-middle techniques that can force clients and servers to negotiate weaker
algorithms, possibly without user awareness.
NSA recommends ensuring administration services are using the latest version of
protocols, with the proper security settings adequately enabled. SSH version 2 is the
preferred method for remotely accessing devices. Encrypted HTTP servers should be
configured to only accept Transport Layer Security (TLS) version 1.2 or higher. For
more information on limiting services to specific versions of the protocol, refer to 7.11
Configure remote network administration services.
7.4 Limit access to services
If a large number of devices are permitted to connect to management services, they are
more vulnerable to exploitation. NSA recommends configuring ACLs to allow only
administrative systems to connect to devices for remote management. Devices that do
not have the capability to support ACLs should be placed on a separate network
management segment (e.g. VLAN).
Once created, an ACL should be applied to that VLAN or at the ingress router to restrict
access to this network segment. It may be necessary to implement a separate firewall in
front of critical network segments to restrict what systems can connect to that VLAN.
Consider using Dynamic Host Configuration Protocol (DHCP) with reserved IP
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addresses, or assigning administrative systems with static IP addresses, to make it
easier to define the ACL and limit administrative access to the management services.
Most management services only accept standard ACLs. More than one device or
network can be listed on additional lines with the
permit
keyword. Even though every
ACL has an implied
deny
statement at the end, it is a best practice to explicitly include it
so denied attempts are logged. Create a standard ACL to only permit IP addresses
used by administrators with the following configuration commands:
access-list <ACL#> permit <NETWORK> <WILDCARD_MASK> log
access-list <ACL#> deny any log
For more information on how to apply ACLs to specific administrative services, refer to
7.11 Configure remote network administration services.
NSA also recommends removing unused ACLs from the configuration to reduce
confusion around whether or not they are properly applied. After verifying that a
standard ACL is not applied, remove it with the following configuration command:
no access-list <ACL#>
7.5 Set an acceptable timeout period
Setting a timeout period for idle connections allows sessions to close after a prescribed
time of inactivity. When timeout periods are not set or set too long, it is possible for idle
connections to continue indefinitely or even cause a DoS if limited simultaneous
connections have been set on the device. The DoS will persist until the idle timeout
period has been reached, which could be indefinite if the idle timeout has been
disabled. A longer timeout period provides an adversary with more time to hijack a
session while it is idle.
NSA recommends setting the session timeout for administrative connections to five
minutes or less on all remote devices (e.g., exec-timeout on VTY lines, SSH, console
and auxiliary ports). Do not set the timeout period to zero, as most devices will disable
the timeout function with this setting. For more information on limiting the session
timeout on specific administrative services, refer to 7.11 Configure remote network
administration services.
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7.6 Enable Transmission Control Protocol (TCP) keep-alive
TCP keep-alive messages sent and received from a device allow it to assess the
connection status when no activity has occurred in a given timeframe. These messages
can be used to detect an inadvertent loss in connection and mitigate against potential
network compromises. On some devices, the lack of a TCP keep-alive service causes
established TCP connections to remain open after an inadvertent connection loss on
one end, leaving the session vulnerable to hijacking. Additionally, authentication may
not even be required, especially for unencrypted connections, and the adversary could
simply resume the session, possibly gaining privileged level access.
NSA recommends enabling TCP keep-alive settings for both inbound and outbound
messages for all TCP connections with the following configuration commands:
service tcp-keepalives-in
service tcp-keepalives-out
Note that some devices do not support the configuration of TCP keep-alive messages.
7.7 Disable outbound connections
After authenticating to a device via a management port, a user generally has the ability
to remotely connect to other systems on the network through supported protocols (e.g.,
Telnet and SSH). If an adversary was able to compromise the device or use an
administrator account to gain user level access, this outbound connection could
potentially be used to advance through the network. Properly reviewing device
configurations and leveraging ACLs can prevent unauthorized systems from accessing
network resources.
NSA recommends disabling outbound connections to limit an adversary from moving
through the network with the following configuration commands:
line con 0
transport output none
line vty 0 4
transport output none
line vty 5 15
transport output none
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Depending on the device, it may be necessary to apply a similar configuration to other
lines as well. If VTY lines 5 through 15 do not exist on a particular device, it is not
necessary to execute those commands. It is critical to note that this feature must be
explicitly disabled on the console line.
If outbound connections are required for copying files to or from the devices for
maintenance or integrity verification, restrict it to only SSH and limit the number of
devices that can be accessed via outbound ACLs; revert to the above configuration
once the task is complete.
7.8 Remove SNMP read-write community strings
An SNMP version 1 or 2c read-write community string is similar to a password, and can
be used to access or modify device configurations and operating system files. These
actions generally cannot be done with a read-only community string. Because SNMP
read-write community strings are sent in clear text, they can be exploited by an
adversary to gain complete control of a network device.
NSA recommends removing all SNMP read-write community strings and upgrading to
SNMP version 3 with encryption and authentication. If a version 1 or 2c SNMP read-
write community string is required for remote administration and cannot be removed, it
is recommended that the read-write community string be significantly different from
other community strings to prevent an adversary from guessing the read-write
community string if a read-only community string is obtained.
All version 1 and 2c SNMP community strings can be listed with the following exec
command:
show running-config | include snmp-server community
Note:
A read-write community string will include the
RW
keyword, while a read-only
community string will include the
RO
keyword.
Disable an SNMP read-write community string with the following configuration
command:
no snmp-server community <COMMUNITY_STRING>
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7.9 Disable unnecessary network services
During the initial installation of devices, several TCP and UDP services are enabled by
default, even though the provided features are unnecessary for normal operations.
These services can degrade the security level of the network, offering an adversary
additional access points to exploit a device and leave it susceptible to unauthorized
monitoring, information gathering, and compromise.
For example, Cisco Smart Install is often unnecessary, but when left enabled, an
unauthenticated remote adversary could use this service to obtain a device’s
configuration file, upload a new configuration or operating system image file, or force a
reboot. This has been documented in Cisco’s security advisory cisco-sa-20170214-smi
as a misuse of the protocol, but the security community has observed and
acknowledged this issue as a severe vulnerability exploited by adversaries to obtain
configuration files across the Internet [38], [39].
NSA recommends disabling every unnecessary service on each device. If the service is
required and can support a password and ACLs, create a password based on NSA’s
strong password guidance (see 5.5 Create strong passwords) and apply an ACL to only
allow required systems to connect to the service. If a device does not support ACLs, it
can be moved to a separate VLAN, and an ACL can be applied to the VLAN.
NSA also recommends immediately disabling the Cisco Smart Install service on all
devices with the following configuration command:
no vstack
Even though this service is designed for switches, routers can also be configured as a
Cisco Smart Install director; therefore, it should be explicitly disabled on all devices,
especially when they are first configured.
Disable other unnecessary TCP and UDP services with the following configuration
commands:
no service tcp-small-servers
no service udp-small-servers
no service finger
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7.10 Disable discovery protocols on specific interfaces
Cisco Discovery Protocol (CDP) and Link Layer Discovery Protocol (LLDP) are
broadcast protocols that periodically advertise network topology and device information
to neighboring devices that support the protocol and are listening for packets. This
functionality is enabled by default and can be useful for administrators to obtain
information about the network, but it is also extremely useful to an adversary who can
passively gather network configuration information. An adversary that is able to deploy a
sniffer could collect device model numbers, operating system versions, VLAN
information, and physical and logical addresses, gaining valuable information to exploit
systems on the network.
NSA recommends disabling CDP and LLDP on all devices capable of using these
services. If a service is required for proper network communications (e.g., some Cisco
Voice-over-IP (VoIP) phones), only enable it on point-to-point links between devices that
require the protocol or on voice enabled ports.
CDP and LLDP can be globally disabled with the following configuration commands:
no cdp run
no lldp run
If CDP is required on specific interfaces, it must be globally enabled but disabled on all
other interfaces, as shown in the following configuration commands for a single
interface:
interface <INTERFACE>
no cdp enable
7.11 Configure remote network administration services
This section describes how to properly enable common remote network administration
services.
7.11.1 Configuring SSH for remote administration
Allow inbound SSH connections with the following configuration commands:
line vty 0 4
transport input ssh
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line vty 5 15
transport input ssh
Note:
This may disable other services enabled on the lines by default, such as Telnet. If
VTY lines 5 through 15 do not exist on a particular device, it is not necessary to execute
those commands. Depending on the device, it may be necessary to apply a similar
configuration to other lines as well.
Allowed input transports can be confirmed with the following exec command:
show line <LINE> <LINE_NUMBER>
Disable SSH version 1 connections and only allow version 2 of the protocol with the
following configuration command:
ip ssh version 2
Generate a new asymmetric Rivest-Shamir-Adleman (RSA) key pair for SSH with the
following configuration command:
crypto key generate rsa modulus 3072
Note:
This command will overwrite an existing RSA key pair.
Generate a new asymmetric ECC key pair for SSH with the following configuration
command:
crypto key generate ec keysize 384
Note:
This command will overwrite an existing ECC key pair.
Set the minimum Diffie-Hellman key size to 4096 bits with the following configuration
command:
ip ssh dh min 4096
Note:
Some devices do not support 3072 bits for the Diffie-Hellman key size, so 4096
bits is recommended.
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The encryption, key exchange (KEX), and message authentication code algorithms
accepted by the SSH protocol can be specified (to include the preferred order) with the
following configuration commands:
ip ssh server algorithm encryption <ALGORITHM> [<ALGORITHM> ...]
ip ssh server algorithm kex <ALGORITHM> [<ALGORITHM> ...]
ip ssh server algorithm mac <ALGORITHM> [<ALGORITHM> ...]
For more information on acceptable algorithms, refer to CNSSP 15 [4]. The CNSSP 15
requirements are explained in the draft IETF document on Commercial National
Security Algorithm (CNSA) Suite Cryptography for Secure Shell (SSH) [36].
The configuration of the SSH service can be confirmed with the following exec
command:
show ip ssh
Apply a standard ACL to only permit IP addresses used by administrators with the
following configuration commands:
line vty 0 4
access-class <ACL#> in
line vty 5 15
access-class <ACL#> in
If VTY lines 5 through 15 do not exist on a particular device, it is not necessary to
execute those commands. Note that this ACL would also apply to Telnet if it was
enabled on the lines. Depending on the device, it may be necessary to apply a similar
configuration to other lines as well. Refer to 7.4 Limit access to services for more
information on creating a standard ACL.
Set the session expiration to 5 minutes or less with the following configuration
commands:
line con 0
exec-timeout 5 0
line vty 0 4
exec-timeout 5 0
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line vty 5 15
exec-timeout 5 0
If VTY lines 5 through 15 do not exist on a particular device, it is not necessary to
execute those commands.
Note:
This would also apply to Telnet if it was enabled on the lines. Depending on the
device, it may be necessary to apply a similar configuration to other lines as well.
7.11.2 Configuring HTTP for remote administration
If HTTP is used for management purposes, enable HTTP over TLS with the following
configuration command:
ip http secure-server
Only accept TLS version 1.2 with the following configuration command:
ip http tls-version TLSv1.2
The cipher suites accepted by the encrypted HTTP service can be specified (to include
the preferred order) with the following configuration command:
ip http secure-ciphersuite <CIPHERSUITE> [<CIPHERSUITE> ...]
For more information on acceptable algorithms, refer to CNSSP 15 [4]. The CNSSP 15
requirements are explained in the draft IETF document on Commercial National
Security Algorithm (CNSA) Suite Profile for TLS and DTLS 1.2 and 1.3 [35].
Apply a standard ACL to only permit IP addresses used by administrators with the
following configuration command:
ip http access-class <ACL#>
Note:
This ACL would also apply to the clear text HTTP service if it was enabled. For
more information on creating a standard ACL, refer to 7.4 Limit access to services.
The default idle timeout of an HTTP server connection is 180 seconds (three minutes),
so it is not necessary to change this value.
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7.11.3 Configuring SNMP for remote administration
If SNMP is used for management, enable SNMP version 3 with both authentication and
privacy (encryption) with the following configuration commands:
snmp-server group <SNMPv3_GROUP> v3 priv access <ACL#>
snmp-server user <USER> <SNMPv3_GROUP> v3 auth sha <AUTH_PASSWORD> priv aes
256 <PRIV_PASSWORD> access <ACL#>
First a group must be defined, where the
priv
keyword is equivalent to authPriv (both
authentication and privacy). One or more users must be defined and assigned to a
group. In addition to the authentication and encryption parameters, two different
passwords must be supplied for each user, one for authentication and one for privacy.
Interoperability issues have been observed with AES-192 and AES-256, so it may be
necessary to use AES-128 for encryption instead of AES-256 with the
aes 128
keywords. As shown above, an ACL can be applied to both the group and each
individual user by specifying it as a separate option at the end of each command with
the
access
keyword.
The above SNMP configuration can be tested from a Linux system with the following
shell command:
snmpget -v 3 -u <USER> -a sha -l authPriv -A '<AUTH_PASSWORD>' -x AES \
-X '<PRIV_PASSWORD>' <IP_ADDRESS> 1.3.6.1.2.1.1.5.0
Return to Contents
8. Routing
Routers forward data packets between computer networks. When a router receives a
packet, it uses its routing table and the packet’s network address information to
determine the next hop to reach its
destination. An improper configuration of the
router itself or the dynamic routing protocols
used to populate the routing table could allow
an adversary to redirect packets to a different
destination, allowing sensitive data to be
collected, manipulated, or discarded, which would violate confidentiality, integrity, or
availability.
Configure your routers
for network use vs.
malicious abuse.
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8.1 Disable IP source routing
IP source routing is a rarely used feature that enables the sender of a packet to specify
a pre-determined list of intermediate nodes where it should be forwarded rather than
using the internal routing table to make a decision. Leveraging this setting, an adversary
could transmit packets through a route of their choosing. Along with IP address
spoofing, an adversary can use the IP source routing feature to successfully bypass
ACLs and other network restrictions, essentially choosing its own network path.
Although this vulnerability is associated with routers and how packets are routed, the
functionality can also be exploited on switches.
NSA recommends disabling IP source routing on all devices, not just routers, since this
feature is not required for normal network operations. Depending on the vendor of a
product, it may be necessary to disable forwarding of each IP source route option
individually. A similar feature is available in IPv6, and needs to be disabled separately.
Disable IP source routing with the following configuration commands:
no ip source-route
no ipv6 source-route
8.2 Enable unicast reverse-path forwarding (uRPF)
uRPF is a method of protection against IP spoofing that instructs a router to examine
both the source and destination addresses in the packet. When a packet is received on
an interface, the source address is compared to entries in the routing table and is
forwarded if the return route matches where the packet was received. Otherwise, it is
discarded due to concerns that the source address in the packet may have been
spoofed. If uRPF is not enabled, an adversary may be able to successfully spoof the
source address of IP packets sent into the network.
NSA recommends enabling uRPF on external interfaces of perimeter routers. On
routers, it is necessary for Cisco Express Forwarding (CEF), which provides optimized
lookups for efficient packet forwarding, to be enabled before uRPF. Note that uRPF
should not be enabled on internal interfaces or routers that have asymmetrical routing
(where two or more return routes may exist for a given source address), as this situation
could cause legitimate packets to be discarded. Enable uRPF on a single interface with
the following configuration commands:
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ip cef
interface <INTERFACE>
ip verify unicast reverse-path
8.3 Enable routing authentication
Dynamic routing protocols are used to distribute information to neighboring devices and
provide routes to reach other networks. Network devices will use this information to
populate their routing tables, which are then used to determine the next hop for
forwarding a packet to the requested destination. To control the flow of traffic, an
adversary may inject, modify, or corrupt the routing information sent and received by
neighboring devices. Routing authentication should be enabled to prevent route
manipulation and ensure the routing information received from neighboring devices has
not been manipulated by an unauthorized source.
NSA recommends enabling routing authentication on any dynamic routing protocols
used that receive routing updates from other devices on the network. Enable Open
Shortest Path First (OSPF) routing authentication by enabling it on the OSPF process
for each area and applying the authentication key to each interface associated with that
process with the following configuration commands:
key chain <KEY_CHAIN_NAME>
key <KEY_NUMBER>
key-string <KEY>
cryptographic-algorithm hmac-sha-512
interface <INTERFACE>
ip ospf authentication key-chain <KEY_CHAIN_NAME>
NSA also recommends using a unique key between all neighbors, instead of using the
same authentication key for all interfaces on all devices. If the keys are different, an
adversary would not be able to use a compromised key from one network to inject a
malicious route on another network.
Enable Border Gateway Protocol (BGP) routing authentication by applying a unique
password key to each individual peer with the following configuration commands:
router bgp <AS_NUMBER>
peer <IP_ADDRESS_1> password <KEY>
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Enable Enhanced Interior Gateway Routing Protocol (EIGRP) authentication by creating
a key-chain and applying it to each interface with the following configuration commands:
key chain <KEY_CHAIN_NAME>
key <KEY_NUMBER>
key-string <KEY>
cryptographic-algorithm hmac-sha-512
interface <INTERFACE>
ip authentication key-chain eigrp <AS_NUMBER> <KEY_CHAIN_NAME>
Note:
An arbitrary name and number can be chosen for each key. The
<KEY>
is the
shared key assigned to neighboring devices. The
cryptographic-algorithm
can also be
set to
hmac-sha-384
and still meet CNSSP 15 guidance [4].
Do not use the Routing Information Protocol (RIP). It is slow to converge, does not
scale, and RIP version 1 cannot be configured to authenticate nearby routers, making it
easy for an adversary to exploit the protocol. Devices that only support RIP should use
static or default routes to other devices that support modern routing protocols with
authentication.
Return to Contents
9. Interface ports
The interface ports of network switches physically connect workstations, servers, and
other devices to the network, while the interconnections between routers and switches
define how systems communicate across the network. An adversary must first obtain
physical access to the network to connect
an unauthorized system, or use an
authorized system already on the network to
exploit an existing connection. Properly
configured interface ports can prevent an
adversary from performing exploitation
attempts against the network.
9.1 Disable dynamic trunking
A trunk is a point-to-point link between two devices that exchange VLAN encapsulated
frames. Depending on the traffic being sent over the link, it is possible for an interface
Prevent an adversary
from connecting to
your network.
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port to dynamically configure itself to be either a trunk or an access port. An adversary
that is connected to a dynamic port could instruct it to become a trunk port and
potentially gain access to network traffic without regard to VLAN separation.
NSA recommends disabling dynamic trunking as it is not necessary for an interface port
to dynamically configure itself. When a device is added to the network, ensure that all
interface ports are explicitly configured as either trunk ports or access ports. Systems
that do not handle VLAN encapsulated frames should be connected to a port that is
configured for access only.
Strictly configure an interface port for static access only with the following configuration
commands:
interface <INTERFACE>
switchport mode access
Strictly configure an interface port to be a trunk with the following configuration
commands:
interface <INTERFACE>
switchport mode trunk
9.2 Enable port security
Physical interface ports of devices are often the primary means of restricting physical
access to a network. Port security limits the number of valid MAC addresses allowed to
connect to a switchport, restricting connectivity to only authorized systems. A switchport
not configured to enforce port security could allow an adversary with physical access to
connect an unauthorized system. The adversary could bypass existing security
restrictions, gather network information, probe deeper into the network, or compromise
internal systems.
NSA recommends enabling port security on all active switchports on a device, and
setting the maximum number of allowed MAC addresses for each port to be exactly
one, or two if VoIP capabilities are in use. Port security is not a replacement for a NAC,
such as 802.1X, but should be used when a NAC cannot be implemented. If possible,
assign a fixed MAC address to each switchport that is connected to a known system,
and configure each switchport to either shutdown or send an SNMP trap message when
a port security violation occurs. Port security can be enabled on trunk interfaces;
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however, it is not recommended as it requires knowing the number of devices that have
traffic traversing that specific trunk. A dynamic switchport cannot have port security
enabled simultaneously, and must first be configured for static access before port
security can be enabled.
Enable port security on a static access port with a maximum of one MAC address with
the following configuration commands:
interface <INTERFACE>
switchport mode access
switchport port-security
switchport port-security maximum 1
switchport port-security violation shutdown
switchport port-security mac-address sticky
The
sticky
keyword will allow the device to insert the MAC address in the configuration
once the first authorized system is connected to that port.
Note:
The configuration will need to be saved to retain the information after a reboot. If
shutting down a port is not an acceptable action because of availability concerns, the
shutdown
keyword can be replaced with
restrict
to prevent any additional MAC
addresses from communicating on that port.
9.3 Disable default VLAN
Most switches utilize VLAN 1 as the default, including management ports, which may
provide direct access to the switch for administration. Additionally, some layer 2
protocols (e.g., discovery or trunking) need to be sent on a specific VLAN on trunk links,
and VLAN 1 is generally selected as the default. If mitigation steps are not taken, the
default VLAN may span the entire network, and place authorized systems at a higher
risk of exploitation by unauthorized systems that gain access.
NSA recommends moving all management and operational traffic to different VLANs
(not the default) that separate management traffic from user data and protocol traffic,
and using multiple switches to separate different security levels of network traffic. The
default VLAN should also be logically disallowed on all trunks and access ports that
don’t require it (including disconnected and shutdown ports) to ensure it does not
transmit unnecessary broadcast, multicast, and unknown destination traffic.
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Frames that are sent and received on trunk ports are usually tagged with the VLAN ID
associated with the frame. Any frames received that are not tagged are automatically
placed in the native trunking VLAN associated with that port. The native trunking VLAN
should be assigned the same on both ends of a trunk link. Similarly, frames that are
sent and received on access ports are assigned to the access VLAN associated with
that port. All switchports are assigned to an access VLAN and a native trunking VLAN,
regardless if they are trunk or access ports.
NSA recommends assigning all trunk ports to a unique native trunking VLAN that is only
assigned to trunk ports, and assigning the access VLAN to an unused and disabled
VLAN. Similarly, NSA recommends assigning all access ports to the appropriate access
VLAN, and assigning the native trunking VLAN to another unused and disabled VLAN,
different from the ones used by trunk ports. This configuration will prevent an adversary
from jumping between active VLANs by intentionally tagging traffic that would otherwise
be untagged.
Create a unique trunking VLAN (500) and an unused and disabled access VLAN (997),
both assigned to a trunk port, with the following example configuration commands:
vlan 500
name NATIVE-TRUNK
vlan 997
name UNUSED-ACCESS
shutdown
interface <INTERFACE>
switchport mode trunk
switchport access vlan 997
switchport trunk native vlan 500
switchport trunk allowed vlan 2-4094
This configuration also allows all VLANs, except for the default VLAN 1, to traverse the
trunk. If all configured VLANs are known, NSA recommends allowing only those specific
VLANs, rather than just excluding VLAN 1.
Create an unused and disabled VLAN (998), assigned to the native trunking VLAN of an
access port, with the following example configuration commands:
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vlan 998
name UNUSED-NATIVE
shutdown
interface <INTERFACE>
switchport mode access
switchport access vlan <ACCESS_VLAN#>
switchport trunk native vlan 998
Switchports can also be assigned a third VLAN, if VoIP capabilities are in use, with the
following configuration commands:
interface <INTERFACE>
switchport voice vlan <VOICE_VLAN#>
All of the VLANs associated with individual switchports can be confirmed with the
following exec command:
show interfaces switchport
9.4 Disable unused ports
Leaving unused ports enabled on a device allows an adversary to attach a rogue device
to the network and perform information gathering or compromise attempts. All unused
ports should be disabled and placed in an unused VLAN, which is not the default.
NSA recommends disabling all unused ports on a device by shutting down the
associated interfaces and, if supported by the device, assigning unused ports to an
unused VLAN. This will continue to prevent access to the network, even if the ports
become enabled. Prior to disabling a port, it is necessary to verify that it is truly unused
and nothing is connected. If a device connected to the port is powered off, it may
appear that the switchport is unused.
Shut down all unused interfaces and assign the access and native trunking VLANs to
unused and disabled VLANs with the following example configuration commands:
vlan 999
name UNUSED-DISABLED
shutdown
interface <INTERFACE>
switchport mode access
switchport access vlan 999
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switchport trunk native vlan 998
no switchport voice vlan
shutdown
Note:
It is not necessary to assign the voice VLAN to an unused VLAN, as it will remain
unassigned if VoIP capabilities are not in use.
9.5 Disable port monitoring
Port monitoring is used on a network switch to send a copy of network packets seen on
one switchport to a network monitoring connection on another switchport. A device that
has one or more port monitoring sessions defined allows a set of source ports to be
monitored by a specified destination port, and all traffic being sent to or from the source
ports will also be sent to the destination port.
Port monitoring is typically used for connecting an NIDS, diagnosing a problem, or using
a network analyzer to monitor the network. Depending on the vendor, port monitoring is
also known as “port mirroring” or “port spanning.” An adversary connected to the
destination port of a port monitoring session will be able to collect network traffic sent
through all the source ports specified by the session.
NSA recommends disabling all inactive port monitoring sessions on a device. Port
monitoring should only be enabled for those ports where it is necessary, and all
sessions should be disabled once they are no longer needed.
Note:
Some vendors do not allow traffic to be sent from the destination port of a port
monitoring session, effectively disabling network access from that port. This type of
behavior is desired when a NIDS is connected to a port monitoring session.
List the monitoring sessions defined in the configuration with the following exec
command:
show monitor session [1|2|all]
Note:
If the
all
keyword is supported, it will list all of the defined sessions; otherwise
each individual session number will need to be specified in separate commands,
generally
1
and
2
.
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Remove a monitoring session defined in the configuration with the following
configuration command:
no monitor session <SESSION#>
9.6 Disable proxy Address Resolution Protocol (ARP)
Proxy ARP is a technique in which a proxy server on a network answers ARP requests
for an IP address that is not on that network. It helps devices on a subnet reach remote
subnets, without configuring routing on a default gateway. It can be advantageous since
it can be added to a router without disbursing routing tables from other networks, but
this feature allows adversaries to spoof a system and intercept packets.
NSA recommends disabling proxy ARP on all interfaces unless the device is being used
as a LAN bridge or to allow inbound network address translations (NAT) for multiple
destination IP addresses. It may be necessary to disable proxy ARP on each individual
interface, rather than disabling it globally.
Find interfaces that have proxy ARP enabled with the following exec command:
show ip interface
Disable Proxy ARP on an individual interface with the following configuration
commands:
interface <INTERFACE>
no ip proxy-arp
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10. Notification and consent banners
The technical recommendations provided in this guide can significantly reduce the
probability of an adversary exploiting a vulnerability on the network. Unfortunately, an
adversary or insider may still find a weakness to
compromise, circumvent, or disrupt the network.
Having a notification banner can make clear what
is permissible to anyone who accesses the
system, and add any necessary notices and
disclaimers.
10.1 Present a notification banner
Depending on the organization’s requirements, a
notification banner can provide notice to users that connecting to the network device is
for authorized use only and that any use of the system is subject to monitoring for any
authorized purpose. A legally sufficient banner ensures the network owner and others,
including the Government, can take necessary steps to monitor and secure the network.
However, the precise requirements for such a banner will vary by organization and
jurisdiction.
For example, DoD elements must use a banner that meets the requirements of DoD
Instruction 8500.01. Other U.S. Government entities should implement the requirements
of NIST SP800-53, AC-8. For private sector entities, the Cybersecurity and
Infrastructure Security Agency has issued very helpful guidance on developing an
appropriate banner [40].
NSA recommends that each device be configured to present the full notification banner
whenever a user logs into an information system or connects to any remote service.
Cisco IOS devices have two types of banners; the login banner is displayed prior to a
user logging in, and then the “message of the day” is displayed after the user
successfully authenticates. At a minimum, the notification banner should be displayed to
both authorized and unauthorized users attempting to login. The same or additional
information could be provided to authenticated users after logging in, if desired.
Add a notification banner, with the organization’s banner appropriately inserted, prior to
users logging in with the following configuration command:
Display notifications
of authorized use
and consent to
monitoring.
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banner login ^
INSERT NOTIFICATION BANNER HERE
^
Note:
The caret symbol (“^”) is used as a delimiter so the banner can span multiple
lines, assuming the caret symbol is not used in the banner itself. After this command is
inserted into the configuration, the delimiter will generally appear as “^C” instead of “^”.
Do not type in “^C” as part of the command, otherwise the banner will begin with a “C”.
Add the same notification banner or additional information for authorized users who
have successfully authenticated with the following configuration command:
banner motd ^
INSERT NOTIFICATION BANNER HERE
ADDITIONAL INFORMATION
^
Return to Contents
11. Conclusion
The guidance in this report was generated from a depth and breadth of experience in
assisting NSA customers with evaluating their networks and providing
recommendations to immediately harden network devices. Along with essential
maintenance functions, administrators play a critical role in defending networks against
adversarial threats. Following this guide will assist these network defenders with
implementing cybersecurity best practices, lowering the risk against compromise and
ensuring a more secure and better-protected network.
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Abbreviations
AAA
Authentication, authorization, and accounting
ACL
Access control list
AES
Advanced Encryption Standard
ARP
Address Resolution Protocol
AUX
Auxiliary
BGP
Border Gateway Protocol
CDP
Cisco Discovery Protocol
CEF
Cisco Express Forwarding
CISA
Cybersecurity and Infrastructure Security Agency
CNSA
Commercial National Security Algorithm Suite
CNSSP
Committee on National Security Systems Policy
CON
Console
DHCP
Dynamic Host Configuration Protocol
DMZ
Demilitarized zone
DoS
Denial of service
ECC
Elliptic curve cryptography
ECP
Elliptic curve group modulo a prime
EIGRP
Enhanced Interior Gateway Routing Protocol
ESP
Encapsulating Security Payload
FTP
File Transfer Protocol
HTTP
Hypertext Transfer Protocol
IETF
Internet Engineering Task Force
IKE
Internet Key Exchange
IOS
Internetwork Operating System
IP
Internet Protocol
IPS
Intrusion prevention system
IPsec
Internet Protocol Security
ISAKMP
Internet Security Association and Key Management Protocol
ISP
Internet service provider
KEX
Key exchange
LAN
Local area network
LLDP
Link Layer Discovery Protocol
MAC
Media access control
MD5
Message Digest 5
MODP
Modular Exponent
NAC
Network access control
NAT
Network address translation
NDI
Network Device Integrity
NIDS
Network intrusion detection system
NIST
National Institute of Standards and Technology
NSA
National Security Agency
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NSS
National Security System(s)
NTP
Network Time Protocol
OMB
Office of Management and Budget
OSPF
Open Shortest Path First
RIP
Routing Information Protocol
RSA
Rivest-Shamir-Adleman
SCP
Secure Copy Protocol
SFTP
Secure File Transfer Protocol
SHA
Secure Hash Algorithm
SIEM
Security information and event management
SNMP
Simple Network Management Protocol
SSH
Secure Shell
TCP
Transmission Control Protocol
TFTP
Trivial File Transfer Protocol
TLS
Transport Layer Security
UDP
User Datagram Protocol
uRPF
Unicast reverse-path forwarding
UTC
Coordinated Universal Time
VLAN
Virtual local area network
VoIP
Voice over IP
VPN
Virtual private network
VTY
Virtual teletype
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References
Works cited
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[2] National Security Agency (2019), Segment Networks and Deploy Application-aware Defenses.
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[3] National Security Agency (2021), Selecting and Hardening Remote Access VPN Solutions.
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[4] Committee on National Security Systems (2016), CNSS Policy 15. Available at:
https://www.cnss.gov/CNSS/issuances/Policies.cfm
[5] Corcoran, Jenkins, NSA (2021), Commercial National Security Algorithm (CNSA) Suite
Cryptography for Internet Protocol Security (IPsec). Available at:
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[6] National Security Agency (2020), Configuring IPsec Virtual Private Networks. Available at:
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[7] National Security Agency (2019), Mitigating Recent VPN Vulnerabilities. Available at:
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[8] National Security Agency (2021), Eliminating Obsolete Transport Layer Security (TLS) Protocol
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[9] Arista Networks, Inc. (2022), Support Overview. Available at: https://www.arista.com/en/support/
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[11] Broadcom Inc. (2022), Support and Services. Available at: https://www.broadcom.com/support/
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[13] Dell (2022), Support. Available at: https://www.dell.com/support/home/en-us/
[14] Extreme Networks (2022), Support. Available at: https://www.extremenetworks.com/support/
[15] F5, Inc. (2022), Support | Services. Available at: https://www.f5.com/services/support/
[16] Fortinet, Inc. (2022), FortiCare Technical Support and Services. Available at:
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https://www.hpe.com/us/en/services.html
[18] International Business Machines Corporation (2022), IBM Support. Available at:
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[19] Juniper Networks, Inc. (2022), Support. Available at: https://support.juniper.net/support/
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https://www.linksys.com/us/support/
[21] NETGEAR (2022), Support. Available at: https://www.netgear.com/support/
[22] Palo Alto Networks (2022), Customer Support. Available at:
https://support.paloaltonetworks.com/support/
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[23] Riverbed Technology (2022), Riverbed Support. Available at: https://support.riverbed.com/
[24] CommScope, Inc. (2022), Ruckus Wireless Support. Available at:
https://support.ruckuswireless.com/
[25] SonicWall (2022), Support Portal & Downloads. Available at: https://www.sonicwall.com/support/
[26] TRENDnet Inc. (2022), Customer Support. Available at: https://www.trendnet.com/support/
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[28] Ubiquiti Inc. (2022), Ubiquiti Support and Help Center. Available at: https://help.ui.com/hc/en-us/
[29] WatchGuard Technologies, Inc. (2022), WatchGuard Support. Available at:
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[30] National Security Agency (2016), Network Device Integrity (NDI) Methodology. Available at:
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1/0/NETWORK%20DEVICE%20INTEGRITY%20NDI%20METHODOLOGY.PDF
[31] National Security Agency (2016), Network Device Integrity (NDI) on Cisco IOS devices.
Available at: https://media.defense.gov/2023/Oct/06/2003315572/-1/-
1/0/NETWORK%20DEVICE%20INTEGRITY%20ON%20CISCO%20IOS%20DEVICES.PDF
[32] National Security Agency (2016), Validate Integrity of Hardware and Software. Available at:
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1/0/VALIDATE%20INTEGRITY%20OF%20HARDWARE%20AND%20SOFTWARE.PDF
[33] National Security Agency (2022), Cisco Password Types: Best Practices. Available at:
https://www.nsa.gov/cybersecurity-guidance
[34] Office of Management and Budget (2021), Improving the Federal Government’s Investigative
and Remediation Capabilities Related to Cybersecurity Incidents. Available at:
https://www.whitehouse.gov/wp-content/uploads/2021/08/M-21-31-Improving-the-Federal-
Governments-Investigative-and-Remediation-Capabilities-Related-to-Cybersecurity-Incidents.pdf
[35] Cooley, D, NSA (2021), Commercial National Security Algorithm (CNSA) Suite Profile for TLS
and DTLS 1.2 and 1.3. Available at: https://datatracker.ietf.org/doc/html/draft-cooley-cnsa-dtls-
tlsprofile
[36] Gajcowski, Jenkins, NSA (2021), Commercial National Security Algorithm (CNSA) Suite
Cryptography for Secure Shell (SSH). Available at: https://datatracker.ietf.org/doc/html/draft-
gajcowski-cnsa-ssh-profile
[37] National Institute for Standards and Technology (2020), Special Publication 800-52 Rev. 2:
Guidelines for the Selection, Configuration, and Use of Transport Layer Security (TLS)
Implementations. Available at: https://csrc.nist.gov/publications/detail/sp/800-52/rev-2/final
[38] Cisco Systems, Inc. (2017), Cisco Smart Install Protocol Misuse. Available at:
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20170214-smi
[39] National Security Agency (2017), Cisco Smart Install Protocol Misuse. Available at:
https://www.nsa.gov/cybersecurity-guidance
[40] Cybersecurity and Infrastructure Security Agency (2022), Guidance on Consent Banners.
Available at: https://www.cisa.gov/publication/guidance-consent-banners
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Related guidance
Biden (2021), Executive Order 14028: Improving the Nation’s Cybersecurity. Available at:
https://www.federalregister.gov/documents/2021/05/17/2021-10460/improving-the-nations-
cybersecurity
National Institute of Standards and Technology (2020), Special Publication 800-207: Zero Trust
Architecture. Available at: https://www.nist.gov/publications/zero-trust-architecture
Defense Information Systems Agency (DISA) and National Security Agency (NSA) Zero Trust
Engineering Team (2021), Department of Defense (DOD) Zero Trust Reference Architecture.
Available at:
https://dodcio.defense.gov/Portals/0/Documents/Library/(U)ZT_RA_v1.1(U)_Mar21.pdf
National Institute for Standards and Technology (2020), Special Publication 800-63B: Digital
Identity Guidelines - Authentication and Lifecycle Management. Available at:
https://pages.nist.gov/800-63-3/sp800-63b.html
National Security Agency (2019), Continuously Hunt for Network Intrusions. Available at:
https://www.nsa.gov/cybersecurity-guidance
National Security Agency (2021), Embracing a Zero Trust Security Model. Available at:
https://www.nsa.gov/cybersecurity-guidance
National Security Agency (2020), Hardening Network Devices. Available at:
https://www.nsa.gov/cybersecurity-guidance
