Vulnerability Discussion

Without cryptographic integrity protections, information can be altered by unauthorized users without detection.

Remote access (e.g., RDP) is access to DOD nonpublic information systems by an authorized user (or an information system) communicating through an external, nonorganization-controlled network. Remote access methods include, for example, dial-up, broadband, and wireless.

Nonlocal maintenance and diagnostic activities are those activities conducted by individuals communicating through a network, either an external network (e.g., the internet) or an internal network.

Local maintenance and diagnostic activities are those activities carried out by individuals physically present at the information system or information system component and not communicating across a network connection.

Encrypting information for transmission protects information from unauthorized disclosure and modification. Cryptographic mechanisms implemented to protect information integrity include, for example, cryptographic hash functions that have common application in digital signatures, checksums, and message authentication codes.

By specifying a cipher list with the order of ciphers being in a "strongest to weakest" orientation, the system will automatically attempt to use the strongest cipher for securing SSH connections.

Check

Verify the SSH client is configured to use only ciphers employing FIPS 140-3 approved algorithms with the following command:

$ sudo grep -r 'Ciphers' /etc/ssh/ssh_config*
Ciphers aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes128-ctr

If any ciphers other than "Ciphers aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes128-ctr" are listed, the "Ciphers" keyword is missing, or the returned line is commented out, or if multiple conflicting ciphers are returned, this is a finding.

Fix

Configure the Ubuntu 24.04 LTS SSH client to use only ciphers employing FIPS 140-3 approved algorithms by updating the "/etc/ssh/ssh_config" file with the following line:

Ciphers aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes128-ctr

Restart the "ssh" service for changes to take effect:

$ sudo systemctl restart ssh