5.1.15 Ensure sshd MACs are configured
Audit#
Run the following command to verify none of the "weak" MACs are being used:
# sshd -T | grep -Pi -- 'macs\h+([^#\n\r]+,)?(hmac-md5|hmac-md5-96|hmac-ripemd160|hmac-sha1-96|umac-64@openssh\.com|hmac-md5-etm@openssh\.com|hmac-md5-96-etm@openssh\.com|hmac-ripemd160-etm@openssh\.com|hmac-sha1-96-etm@openssh\.com|umac-64-etm@openssh\.com|umac-128-etm@openssh\.com)\b'
Nothing should be returned
Note: Review CVE-2023-48795 and verify the system has been patched. If the system has not been patched, review the use of the Encrypt Then Mac (etm) MACs. The following are considered "weak" MACs, and should not be used:
hmac-md5
hmac-md5-96
hmac-ripemd160
hmac-sha1-96
umac-64@openssh.com
hmac-md5-etm@openssh.com
hmac-md5-96-etm@openssh.com
hmac-ripemd160-etm@openssh.com
hmac-sha1-96-etm@openssh.com
umac-64-etm@openssh.com
umac-128-etm@openssh.com
Remediation#
Edit the /etc/ssh/sshd_config file and add/modify the MACs line to contain a comma separated list of the site unapproved (weak) MACs preceded with a - above any Include entries: Example:
MACs -hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-sha1-96,umac-64@openssh.com,hmac-md5-etm@openssh.com,hmac-md5-96-etm@openssh.com,hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,umac-64-etm@openssh.com,umac-128-etm@openssh.com
Default Value: MACs umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1