The Cisco ASA must be configured to implement replay-resistant authentication mechanisms for network access to privileged accounts.

A replay attack may enable an unauthorized user to gain access to the application. Authentication sessions between the authenticator and the application 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.

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.

Step 1: Verify that FIPS mode is enabled as shown in the example.

fips enable

Step 2: Verify only SSH is configured to only use FIPS-compliant ciphers and that Diffie-Hellman Group 14 is used for the key exchange as shown in the example below.

ssh version 2 ssh cipher encryption fips ssh key-exchange group dh-group14-sha1

Note: The ASA only supports SSHv2.

If the ASA is not configured to implement replay-resistant authentication mechanisms for network access, this is a finding.

Step 1: Enable FIPS mode via the fips enable command.

Step 2: Configure SSH to only use FIPS-compliant ciphers and Diffie-Hellman Group 14 for the key exchange.

ASA(config)# ssh cipher encryption fips ASA(config)# ssh key-exchange group dh-group14-sha