7.2.2. Signature Replay

7.2.2. Signature Replay

Since HTTP message signatures allow sub-portions of the HTTP message to be signed, it is possible for two different HTTP messages to validate against the same signature. The most extreme form of this would be a signature over no message components. If such a signature were intercepted, it could be replayed at will by an attacker, attached to any HTTP message. Even with sufficient component coverage, a given signature could be applied to two similar HTTP messages, allowing a message to be replayed by an attacker with the signature intact.

To counteract these kinds of attacks, it's first important for the signer to cover sufficient portions of the message to differentiate it from other messages. In addition, the signature can use the nonce signature parameter to provide a per-message unique value to allow the verifier to detect replay of the signature itself if a nonce value is repeated. Furthermore, the signer can provide a timestamp for when the signature was created and a time at which the signer considers the signature to be expired, limiting the utility of a captured signature value.

If a verifier wants to trigger a new signature from a signer, it can send the Accept-Signature header field with a new nonce parameter. An attacker that is simply replaying a signature would not be able to generate a new signature with the chosen nonce value.