What specific server-side security hardening practice for a client's environment directly mitigates replay attacks against their integrated OTP validation endpoint?

The specific server-side security hardening practice that directly mitigates replay attacks against an integrated One-Time Password (OTP) validation endpoint is the immediate server-side invalidation and tracking of successfully used OTPs, often combined with the use of nonces or strict timestamp verification for Time-based OTPs. A replay attack occurs when an attacker intercepts a legitimate, valid OTP submission and then resends the exact same captured data to the server at a later time, attempting to impersonate the user or complete an unauthorized action. An OTP is a password that is valid for only one login session or transaction, typically f....