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The Ubuntu operating system must configure the SSH daemon to use FIPS 140-2 approved ciphers to prevent the unauthorized disclosure of information and/or detect changes to information during transmission.


Finding ID Version Rule ID IA Controls Severity
V-238217 UBTU-20-010044 SV-238217r860821_rule Medium
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, non-organization-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 which 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. Satisfies: SRG-OS-000424-GPOS-00188, SRG-OS-000033-GPOS-00014, SRG-OS-000394-GPOS-00174
Canonical Ubuntu 20.04 LTS Security Technical Implementation Guide 2022-09-07


Check Text ( C-41427r858528_chk )
Verify the SSH daemon is configured to only implement FIPS-approved algorithms by running the following command:

$ grep -r '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.
If conflicting results are returned, this is a finding.
Fix Text (F-41386r653825_fix)
Configure the Ubuntu operating system to allow the SSH daemon to only implement FIPS-approved algorithms.

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

Restart the SSH daemon for the changes to take effect:

$ sudo systemctl restart sshd.service