Ubuntu 22.04 LTS SSH server must be configured to use only FIPS-validated key exchange algorithms.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-260533	UBTU-22-255060	SV-260533r953412_rule		Medium

Description
Without cryptographic integrity protections provided by FIPS-validated cryptographic algorithms, information can be viewed and altered by unauthorized users without detection. The system will attempt to use the first algorithm presented by the client that matches the server list. Listing the values "strongest to weakest" is a method to ensure the use of the strongest algorithm available to secure the SSH connection.

Description

Without cryptographic integrity protections provided by FIPS-validated cryptographic algorithms, information can be viewed and altered by unauthorized users without detection. The system will attempt to use the first algorithm presented by the client that matches the server list. Listing the values "strongest to weakest" is a method to ensure the use of the strongest algorithm available to secure the SSH connection.

STIG	Date
Canonical Ubuntu 22.04 LTS Security Technical Implementation Guide	2024-03-21

Details

Check Text ( C-64262r953410_chk )
Verify that the SSH server is configured to use only FIPS-validated key exchange algorithms by using the following command: $ sudo /usr/sbin/sshd -dd 2>&1 \| awk '/filename/ {print $4}' \| tr -d '\r' \| tr '\n' ' ' \| xargs sudo grep -iH 'kexalgorithms' /etc/ssh/sshd_config:KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256 If "KexAlgorithms" does not contain only the algorithms "ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256" in exact order, is commented out, is missing, or conflicting results are returned, this is a finding.

Check Text ( C-64262r953410_chk )

Verify that the SSH server is configured to use only FIPS-validated key exchange algorithms by using the following command:

$ sudo /usr/sbin/sshd -dd 2>&1 | awk '/filename/ {print $4}' | tr -d '\r' | tr '\n' ' ' | xargs sudo grep -iH 'kexalgorithms'
/etc/ssh/sshd_config:KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256

If "KexAlgorithms" does not contain only the algorithms "ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256" in exact order, is commented out, is missing, or conflicting results are returned, this is a finding.

Fix Text (F-64170r953411_fix)
Configure the SSH server to use only FIPS-validated key exchange algorithms. Add or modify the following line in the "/etc/ssh/sshd_config" file: KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256 Restart the SSH server for changes to take effect: $ sudo systemctl restart sshd.service