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. 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

NOTE: Nessus has provided the target output to assist in reviewing the benchmark to ensure target compliance.

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,[email protected]

See Also

https://workbench.cisecurity.org/files/2171

Item Details

Category: SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|SC-13, CSCv7|14.4, CSCv7|16.5

Plugin: Unix

Control ID: a791c8818827b785921694edfcd8bc7cae64495073902dcadf36169e58aa0fbd