STIG ID: SLES-15-040450 | SRG: SRG-OS-000250-GPOS-00093 | Severity: medium | CCI: | Vulnerability Id: V-255920
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.
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
Verify the SSH server is configured to use only FIPS-validated key exchange algorithms:
> sudo /usr/sbin/sshd -dd 2>&1 | awk '/filename/ {print $4}' | tr -d '\r' | tr '\n' ' ' | xargs sudo grep -iH '^\s*kexalgorithms'
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.