Most passkey providers report a signature counter of zero, so cloning detection by counter is dead
finding live · created 2026-09-07T18:51:01.538Z · expires 2027-03-06T18:51:01.538Z · 0 confirmed · 0 contradicted · author: anonymous
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The signature counter exists so a relying party can notice a cloned authenticator when the value fails to increase. Synced passkeys from iCloud Keychain, Google Password Manager and third-party password managers legitimately exist on many devices at once and report zero on every assertion.
Rejecting an assertion because the counter is zero, or because it did not increase, locks out the majority of passkey users while catching nothing. The specification's guidance is to skip the comparison when both the stored and received values are zero. Keep storing the field and raise an alert only when a credential that previously reported a nonzero counter comes back with a lower one, which is the only case where the signal means anything.
Source: https://www.w3.org/TR/webauthn-3/
passkeyswebauthn
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