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5.1.7 Ensure sshd MACs are configured

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 approved are:

 - HMAC-SHA1
 - HMAC-SHA2-256
 - HMAC-SHA2-384
 - HMAC-SHA2-512

MD5-based, RIPEMD-160, 96-bit-truncated, and 64-bit (UMAC-64) 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

The MAC algorithms available to the SSH server are governed by the system-wide crypto policy. Disable the weak MACs by configuring the system-wide crypto policy as described in the "Configure system wide crypto policy" section.

- IF - CVE-2023-48795 (Terrapin) has not been reviewed and addressed, the Encrypt-then-MAC (EtM) MACs should also be disabled through the system-wide crypto policy.

Once the crypto policy has been updated, reload the SSH daemon:

# systemctl reload-or-restart sshd

Impact:

A too restrictive list of MAC algorithms can lead to a "no matching MAC found" error during connection establishment, when a client and the server share no common algorithm. Verify that peers required to connect to this host support at least one of the retained MACs before applying.

See Also

https://workbench.cisecurity.org/benchmarks/27890

Item Details

Category: CONFIGURATION MANAGEMENT, SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|CM-6, 800-53|SC-8, 800-53|SC-8(1), 800-53|SC-13, CSCv7|14.4, CSCv7|16.5

Plugin: Unix

Control ID: 59aa03ed30dd6e265d1ea523f6a01a56a1cfc0b1a4d62c0a08be8ea0345108fd