The application must implement replay-resistant authentication mechanisms for network access to nonprivileged accounts.

A replay attack is a man-in-the-middle style attack which allows an attacker to repeat or alter a valid data transmission that may enable unauthorized access to the application. Authentication sessions between the authenticating client and the application server validating the user credentials must not be vulnerable to a replay attack. The protection methods selected to protect against a replay attack will vary according to the application architecture. An authentication process resists replay attacks if it is impractical to achieve a successful authentication by recording and replaying a previous authentication message. A nonprivileged account is any operating system account with authorizations of a nonprivileged user. Techniques used to address this include protocols using nonces (e.g., numbers generated for a specific one time use), challenges (e.g., TLS, WS_Security), and PKI certificates. Additional techniques include time-synchronous or challenge-response one-time authenticators.