5.2.14 Ensure only strong MAC algorithms are used

Information

This variable limits the types of MAC algorithms that SSH can use during communication.

Notes:

-

Some organizations may have stricter requirements for approved MACs. Ensure that MACs used are in compliance with site policy

-

The only "strong" MACs currently FIPS 140-2 approved are hmac-sha2-256 and hmac-sha2-512

The Supported MACs are:

hmac-md5
hmac-md5-96
hmac-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]

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

Solution

Edit the /etc/ssh/sshd_config file and add/modify the MACs line to contain a comma separated list of the site approved MACs

Example:

MACs [email protected],[email protected],hmac-sha2-512,hmac-sha2-256

See Also

https://workbench.cisecurity.org/files/3682