Level 1 - Server Level 1 - Workstation
This variable limits the types of MAC algorithms that SSH can use during communication.
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
Run the following command and verify that output does not contain any unlisted MAC algorithms:
# grep "MACs" /etc/ssh/sshd_config MACs [email protected],[email protected],[email protected],hmac-sha2-512,hmac-sha2-256,[email protected],[email protected],diffie-hellman-group-exchange-sha256
Edit the /etc/ssh/sshd_config file to set the parameter as follows:
MACs [email protected],[email protected],[email protected],hmac-sha2-512,hmac-sha2-256,[email protected]
More information on SSH downgrade attacks can be found here: http://www.mitls.org/pages/attacks/SLOTH
Some organizations may have stricter requirements for approved ciphers. Ensure that MACs used are in compliance with site policy.