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.

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 hmac-sha1,hmac-sha1-96,hmac-sha2-256,hmac-sha2-512,hmac-md5,hmac-md5-96,hmac-ripemd160,hmac- [email protected],[email protected],[email protected],[email protected],hmac-sha [email protected],[email protected],[email protected],hmac-md5-etm@ope nssh.com,[email protected],[email protected],[email protected],umac- [email protected]

See Also

https://workbench.cisecurity.org/files/3399

Item Details

Category: ACCESS CONTROL, IDENTIFICATION AND AUTHENTICATION, SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|AC-17(2), 800-53|IA-5, 800-53|IA-5(1), 800-53|SC-8, 800-53|SC-8(1), CSCv6|3.4, CSCv7|14.4, CSCv7|16.5

Plugin: Unix

Control ID: 81fa7ade801297f0d43754fbc4cd838b12942d8b4d59c40983b4775189dcd83b