The Red Hat Enterprise Linux operating system must be configured so that the SSH daemon is configured to only use Message Authentication Codes (MACs) employing FIPS 140-2 approved cryptographic hash algorithms.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-204595	RHEL-07-040400	SV-204595r603846_rule		Medium

Description
DoD information systems are required to use FIPS 140-2 approved cryptographic hash functions. The only SSHv2 hash algorithm meeting this requirement is SHA. By specifying a hash algorithm list with the order of hashes being in a “strongest to weakest” orientation, the system will automatically attempt to use the strongest hash for securing SSH connections.

STIG	Date
Red Hat Enterprise Linux 7 Security Technical Implementation Guide	2021-03-01

Details

Check Text ( C-4719r622308_chk )
Verify the SSH daemon is configured to only use MACs employing FIPS 140-2-approved hashes. Note: If RHEL-07-021350 is a finding, this is automatically a finding as the system cannot implement FIPS 140-2-approved cryptographic algorithms and hashes. Check that the SSH daemon is configured to only use MACs employing FIPS 140-2-approved hashes with the following command: # grep -i macs /etc/ssh/sshd_config MACs hmac-sha2-512,hmac-sha2-256 If any hashes other than "hmac-sha2-512" or "hmac-sha2-256" are listed, the order differs from the example above, they are missing, or the returned line is commented out, this is a finding.

Check Text ( C-4719r622308_chk )

Verify the SSH daemon is configured to only use MACs employing FIPS 140-2-approved hashes.

Note: If RHEL-07-021350 is a finding, this is automatically a finding as the system cannot implement FIPS 140-2-approved cryptographic algorithms and hashes.

Check that the SSH daemon is configured to only use MACs employing FIPS 140-2-approved hashes with the following command:

# grep -i macs /etc/ssh/sshd_config
MACs hmac-sha2-512,hmac-sha2-256

If any hashes other than "hmac-sha2-512" or "hmac-sha2-256" are listed, the order differs from the example above, they are missing, or the returned line is commented out, this is a finding.

Fix Text (F-4719r622309_fix)
Edit the "/etc/ssh/sshd_config" file to uncomment or add the line for the "MACs" keyword and set its value to "hmac-sha2-512" and/or "hmac-sha2-256" (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): MACs hmac-sha2-512,hmac-sha2-256 The SSH service must be restarted for changes to take effect.