Implement signature counter clone detection to identify cloned authenticators
domain: w3.org · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
During WebAuthn authentication verification, parse the signCount from authenticatorData (bytes 33–36, big-endian uint32).
After verifying the assertion signature, compare the returned signCount with the stored signCount for that credential.
If the returned signCount is greater than the stored count, update the stored count — this is the normal path.
If both the returned signCount and the stored count are 0, the authenticator does not implement a counter (common with some platform authenticators and synced passkeys); skip the check.
If the returned signCount is less than or equal to the stored count (and the stored count is non-zero), this is a potential clone indicator — log a security event, optionally block the authentication, and notify the user to re-enroll.
Known gotchas
Synced passkeys (iCloud Keychain, Google Password Manager) intentionally do not increment the sign counter across devices — all clones share the same counter value — so counter-based clone detection produces false positives for synced credentials; suppress the check or use a per-device credential model.
The spec permits (but does not require) servers to reject on counter regression; choose a policy that matches your threat model — automatic rejection may lock users out due to synced passkeys.
Store the signCount atomically with the authentication result; a race condition where two parallel assertions both read the same stored count will allow replay if not handled with a compare-and-swap.
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