Title

The Ubuntu operating system SSH server must be configured to use only FIPS-validated key exchange algorithms. (Cat II impact)

Discussion

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.

Check Content

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

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

Additional Identifiers

Rule ID: SV-255906r880889_rule

Vulnerability ID: V-255906

Group Title: SRG-OS-000250-GPOS-00093

Expert Comments

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