Do I need to detect and block replay attacks on APIs?

Usually yes, for sensitive APIs—but the need depends on what the API does and how it authenticates requests.

When replay protection matters

You should strongly consider detecting/blocking replays if the API:

• Moves money or other valuable assets
• Changes state (password reset, transfer, purchase, delete, approve, sign, etc.)
• Uses signed requests, HMACs, or bearer tokens
• Exposes webhooks or callback endpoints
• Runs over untrusted networks or long-lived credentials
• Must meet compliance/security requirements

When it may be less critical

Replay attacks are less of a concern for:

• Pure read-only endpoints
• Requests protected by short-lived, one-time tokens
• APIs already using strong transport + auth + idempotency controls where replays don’t cause harm

Important nuance

TLS alone does not prevent replay attacks in all cases. It protects data in transit, but if an attacker gets a valid request (or token), they may resend it unless the API includes replay defenses.

Common defenses

Use one or more of these:

• Short-lived access tokens
• Nonce / unique request ID with server-side deduplication
• Timestamps + expiration window
• Idempotency keys for state-changing operations
• Challenge-response / one-time signatures
• Binding the request to context (method, path, body, client identity)
• Single-use tokens for critical actions

Practical rule

If a repeated request could cause financial loss, unauthorized action, or duplicate side effects, you should design the API to detect or safely ignore replays.

If you want, I can help you decide for a specific API pattern (REST, webhooks, HMAC-signed requests, JWT, etc.).