5.3.14 Ensure only strong MAC algorithms are used - weak MACs

Information

This variable Specifies the available MAC (message authentication code) algorithms. The MAC algorithm is used in protocol version 2 for data integrity protection. Multiple algorithms must be comma-separated.

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

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/3208

Item Details

Category: SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|SC-13, CSCv6|3.4

Plugin: Unix

Control ID: 08172da78769d1360fb7d7931e5859dd069b929ce95a6a515a64e06082b5b2ad