The Ubuntu operating system SSH server must be configured to use only FIPS-validated key exchange algorithms.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-255906	UBTU-18-010421	SV-255906r880889_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 18.04 LTS Security Technical Implementation Guide	2023-09-08

Details

Check Text ( C-59583r880887_chk )
Verify that the SSH server is configured to use only FIPS-validated key exchange algorithms: $ sudo grep -i kexalgorithms /etc/ssh/sshd_config KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256 If "KexAlgorithms" is not configured, is commented out, or does not contain only the algorithms "ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256" in exact order, this is a finding.

Check Text ( C-59583r880887_chk )

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

$ sudo grep -i kexalgorithms /etc/ssh/sshd_config
KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256

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

Fix Text (F-59526r880888_fix)
Configure the SSH server to use only FIPS-validated key exchange algorithms by adding or modifying the following line in "/etc/ssh/sshd_config": KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256 Restart the "sshd" service for changes to take effect: $ sudo systemctl restart sshd