Vulnerability Discussion

A replay attack may enable an unauthorized user to gain access to the operating system. Authentication sessions between the authenticator and the operating system validating the user credentials must not be vulnerable to a replay attack.

An authentication process resists replay attacks if it is impractical to achieve a successful authentication by recording and replaying a previous authentication message.

A privileged account is any information system account with authorizations of a privileged user.

Techniques used to address this include protocols using nonces (e.g., numbers generated for a specific one-time use) or challenges (e.g., TLS, WS_Security). Additional techniques include time-synchronous or challenge-response one-time authenticators.

Satisfies: SRG-OS-000112-GPOS-00057, SRG-OS-000113-GPOS-00058

Check

Verify that the Ubuntu operating system enforces SSH protocol 2 for network access.

Check the protocol versions that SSH allows with the following command:

# grep Protocol /etc/ssh/sshd_config

Protocol 2

If the returned line allows for use of protocol "1", is commented out, or the line is missing, this is a finding.

Fix

Configure the Ubuntu operating system to enforce SSHv2 for network access to all accounts.

Add or update the following line in the "/etc/ssh/sshd_config" file:

Protocol 2

Restart the ssh service.

# systemctl restart sshd.service