Vulnerability Discussion

Operating systems using encryption are required to use FIPS-compliant mechanisms for authenticating to macOS.

For OpenSSH to utilize the Apple Corecrypto FIPS-validated algorithms, a specific configuration is required to leverage the shim implemented by macOS to bypass the non-FIPS validated LibreSSL crypto module packaged with OpenSSH. Information regarding this configuration can be found in the manual page "apple_ssh_and_fips".

Satisfies: SRG-OS-000033-GPOS-00014, SRG-OS-000120-GPOS-00061, SRG-OS-000125-GPOS-00065, SRG-OS-000250-GPOS-00093, SRG-OS-000393-GPOS-00173, SRG-OS-000394-GPOS-00176

Check

Verify the macOS system is configured to use approved SSH Key Exchange Algorithms within the SSH server configuration with the following command:

/usr/bin/sudo /usr/sbin/sshd -T | /usr/bin/grep "kexalgorithms"

kexalgorithms ecdh-sha2-nistp256

If any algorithms other than "ecdh-sha2-nistp256" are listed, or the "kexalgorithms" keyword is missing, this is a finding.

Fix

Configure the macOS system to use approved SSH Key Exchange Algorithms by creating a plain text file in the /private/etc/ssh/sshd_config.d/ directory containing the following:

KexAlgorithms ecdh-sha2-nistp256

The SSH service must be restarted for changes to take effect.