5.3.15 Ensure only strong MAC algorithms are used - approved 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/3219

Item Details

Category: IDENTIFICATION AND AUTHENTICATION, SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|IA-5, 800-53|SC-8, CSCv7|14.4, CSCv7|16.5

Plugin: Unix

Control ID: 1bab957b08cfc71289f18fd74586f99e8e89d82d7c36b52334840f6b469496a4