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The Ubuntu operating system must implement DoD-approved encryption to protect the confidentiality of remote access sessions.


Finding ID Version Rule ID IA Controls Severity
V-219307 UBTU-18-010411 SV-219307r877398_rule Medium
Without confidentiality protection mechanisms, unauthorized individuals may gain access to sensitive information via a remote access session. Remote access is access to DoD nonpublic information systems by an authorized user (or an information system) communicating through an external, non-organization-controlled network. Remote access methods include, for example, dial-up, broadband, and wireless. Encryption provides a means to secure the remote connection to prevent unauthorized access to the data traversing the remote access connection (e.g., RDP), thereby providing a degree of confidentiality. The encryption strength of a mechanism is selected based on the security categorization of the information. 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.
Canonical Ubuntu 18.04 LTS Security Technical Implementation Guide 2022-12-06


Check Text ( C-21032r621653_chk )
Verify the SSH daemon is configured to only implement DoD-approved encryption.

Check the SSH daemon's current configured ciphers by running the following command:

# grep -E '^Ciphers ' /etc/ssh/sshd_config

Ciphers aes256-ctr,aes192-ctr, aes128-ctr

If any ciphers other than "aes256-ctr", "aes192-ctr", or "aes128-ctr" are listed, the order differs from the example above, the "Ciphers" keyword is missing, or the returned line is commented out, this is a finding.
Fix Text (F-21031r621654_fix)
Configure the Ubuntu operating system to allow the SSH daemon to only implement DoD-approved encryption.

Add the following line (or modify the line to have the required value) to the "/etc/ssh/sshd_config" file (this file may be named differently or be in a different location if using a version of SSH that is provided by a third-party vendor):

Ciphers aes256-ctr,aes192-ctr,aes128-ctr

In order for the changes to take effect, the SSH daemon must be restarted.

# sudo systemctl restart sshd.service