documentation_complete: true


title: 'Use Only Strong MACs'

description: |-
    Limit the MACs to strong hash algorithms.
    The following line in <tt>/etc/ssh/sshd_config</tt> demonstrates use
    of those MACs:
    <pre>MACs {{{ xccdf_value("sshd_strong_macs") }}}</pre>

rationale: |-
    MD5 and 96-bit MAC algorithms are considered weak and have been shown to increase
    exploitability in SSH downgrade attacks. Weak algorithms continue to have a great deal of
    attention as a weak spot that can be exploited with expanded computing power. An
    attacker that breaks the algorithm could take advantage of a MiTM position to decrypt the
    SSH tunnel and capture credentials and information

severity: medium

identifiers:
    cce@rhel8: CCE-86504-8
    cce@rhel9: CCE-86769-7
    cce@rhel10: CCE-86792-9
    cce@sle12: CCE-92280-7
    cce@sle15: CCE-91396-2
    cce@slmicro5: CCE-93875-3

references:
    cis@sle12: 5.2.14
    cis@sle15: 5.2.14
    nist: AC-17 (2)
    srg: SRG-OS-000250-GPOS-00093

ocil_clause: 'MACs option is commented out or not using strong hash algorithms'

ocil: |-
    Only strong MACs should be used. To verify that only strong
    MACs are in use, run the following command:
    <pre>$ sudo grep -i macs /etc/ssh/sshd_config</pre>
    The output should contain only those MACs which are strong, namely,
    {{{ xccdf_value("sshd_strong_macs") }}} hash functions.
