======5.2.11 Ensure only approved MAC algorithms are used (Scored)======
=====Profile Applicability=====  
<code>
Level 1 - Server
Level 1 - Workstation 
</code>

=====Description=====
This variable limits the types of MAC algorithms that SSH can use during communication.

=====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

=====Audit===== 
Run the following command and verify that output does not contain any unlisted MAC algorithms:
<Code:bash>
# grep "MACs" /etc/ssh/sshd_config 
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com,curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
</Code>

=====Remediation===== 
Edit the ''/etc/ssh/sshd_config'' file to set the parameter as follows:
<Code:bash>
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com
</Code>

=====References===== 
More information on SSH downgrade attacks can be found here: http://www.mitls.org/pages/attacks/SLOTH

=====Notes===== 
Some organizations may have stricter requirements for approved ciphers. Ensure that MACs used are in compliance with site policy.