WebAuthn Signature Counters Are Clone Signals, Not Authentication Proof
Two valid WebAuthn assertions for the same credential arrive close together. One carries a signature counter of 42 and the other carries 41. Both signatures verify. The relying party now has evidence that deserves attention, but it does not have cryptographic proof that the second assertion came from an attacker. That distinction is built into WebAuthn’s signature-counter model. The counter is auxiliary state intended to help detect cloned authenticators. It is not part of the primary decision that establishes possession of the credential private key, and some authenticators legitimately keep it at zero.