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-SHA1
- HMAC-SHA2-256
- HMAC-SHA2-384
- HMAC-SHA2-512
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, supported "strong" MACs:
Example:
MACs [email protected],[email protected],hmac-sha2-512,hmac-sha2-256
Note: The first occurrence of an option takes precedence.