cyberstrikeus 5.2.11 Ensure only approved MAC algorithms are used (Scored)
Profile Applicability
- Level 1 - Server
- Level 1 - Workstation
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 Procedure
Run the following command and verify that output does not contain any unlisted MAC algorithms:
grep "MACs" /etc/ssh/sshd_config
Expected Result
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
Remediation
Edit the /etc/ssh/sshd_config file to set the parameter as follows:
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
Default Value
MACs include all available algorithms by default.
References
- More information on SSH downgrade attacks can be found here: http://www.mitls.org/pages/attacks/SLOTH
- CIS Controls: 3.4 - Use Only Secure Channels For Remote System Administration
Profile
- Level 1 - Server
- Level 1 - Workstation