To stop someone from sending the exact same old webhook notification again to trick an application, what security measure should the webhook receiver use?

To prevent an application from being tricked by someone re-sending an exact copy of an old, legitimate webhook notification, which is a specific type of attack called a replay attack, the webhook receiver must implement mechanisms that ensure each unique message is processed only once. The core security measures involve combining unique message identifiers with time-based validation and cryptographic integrity checks.The primary measure against replay attacks is to include a unique identifier, commonly referred to as a nonce, within the webhook payload. A nonce is a value, such as a random string or a monotonically increasing counter, that is generated by the sender for each individual webhook notification and is intended to be used only once. When the webhook sender dispatches a notification, it embeds a newly generated, unique nonce into the message. Upon receiving a webhook, the receiver first extracts this nonce. It then consults an internal record, typically a database or a high-speed cache, to determine if this specific nonce has been previously encountered and processed. If the nonce is found in the receiver's records, it signifies a replay attempt, and the receiver should immediately reject the notification without further processing. If the nonce is new, the receiver proceeds to process the webhook's content and then records this nonce in its storage, marking it as used, to prevent any subsequent messages with the same nonce from being accepted. This ensures that even if an attacker intercepts and re-sends a legitimate, signed webhook, the receiver will identify it as a duplicate based on the nonce.A crucial complementary security measure is the inclusion of a timestamp in the webhook payload. The timestamp indicates the exact time the webhook notification was originally generated and sent by the sender. Upon receiving a webhook, the receiver compares the embedded timestamp against its own current server time. The receiver should reject any webhook notification where the timestamp is excessively old (e.g., older than a predefined acceptable window, such as five minutes) or significantly in the future. This time-window validation limits the period during which a legitimate webhook can be considered valid, thereby drastically reducing the opportunity for a replay attack. While a timestamp alone does not guarantee uniqueness, as an attacker could replay a message within the valid time window, its combination with a nonce provides robust protection: the timestamp limits the practical window for replay, and the nonce ensures uniqueness within that window.Furthermore, signature verification using a shared secret is an indispensable security measure for webhooks, although its role in preventing replay attacks needs to be clearly understood. The webhook sender computes a cryptographic hash, typically an HMAC (Hash-based Message Authentication Code), over the entire webhook payload, including the nonce and timestamp, using a secret key known only to the sender and receiver. This generated signature is then included in a webhook header (e.g., `X-Hub-Signature`). The receiver, upon receiving the webhook, independently computes its own HMAC using the received payload, the timestamp, the nonce, and its own copy of the shared secret key. It then compares its computed signature with the signature provided by the sender. If the signatures do not match, it indicates that the webhook content has either been tampered with in transit or was sent by an unauthorized party. While signature verification primarily ensures the authenticity (that the message came from the legitimate sender) and integrity (that the message has not been altered) of the message, it does not, on its own, prevent a replay attack of an *authentic and untamperedmessage. An attacker could perfectly replay a validly signed message. Therefore, signature verification is essential to trust the message's origin and content, but it must be used *in conjunctionwith nonces and timestamps, which are also included in the signed payload to protect their integrity, to provide comprehensive protection against both message tampering and the re-sending of old notifications.