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Verify BS flag not set when BE is unset - #516
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WebAuthn Level 3 requires the relying party to verify that the BS bit is not set when the BE bit is not set, in both the registration and authentication ceremonies. A credential that is not backup eligible cannot be backed up, so this combination is invalid. Verification now raises `WebAuthn::BackupFlagsVerificationError` for it. `FakeClient#get` defaulted to BE=0/BS=1, which this check rejects. Its `backup_state` default is now `false`. https://www.w3.org/TR/webauthn-3/#sctn-registering-a-new-credential https://www.w3.org/TR/webauthn-3/#sctn-verifying-assertion
santiagorodriguez96
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nicolastemciuc
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Motivation
WebAuthn L3 requires rejecting authenticator data whose backup flags are internally inconsistent. Step 17 and 18 (respectively) of both RP ceremonies (§7.1, §7.2):
A credential that is not backup-eligible cannot currently be backed up, so
BE=0/BS=1is a structurally impossible combination. We parse both flags (authenticator_data.rb:23-24) but never check them against each other, so such authenticator data verifies successfully today.Details
This PR adds a verification step to
WebAuthn::AuthenticatorResponseso that registration and authentication now reject authenticator data that has the BS (backup state) flag set without the BE (backup eligibility) flag.It also changes
FakeClient#getdefault for:backup_stateas it defaulted tobackup_eligibility: false,backup_state: true, which is the invalid combination.