5.2.13 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

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]

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/2873

Item Details

Category: SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|SC-13, CSCv6|3.4, CSCv7|14.4, CSCv7|16.5

Plugin: Unix

Control ID: 493db04d087401cc98c90d47a81697cb72f72f3676eb23ac0fef1e8912b56b7d