5.1.14 Ensure only strong MAC algorithms are used

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Information

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

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

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

Default Value:

macs [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-sha1

See Also

https://workbench.cisecurity.org/files/3659