Summary

A pinUvAuthToken retains all requested permissions after a successful
user-presence-gated makeCredential or getAssertion operation.

CTAP 2.1 requires clearing all token permissions except lbw after an
operation tests user presence. As a result, a token containing
mc|ga|acfg can perform a touch-gated registration and then immediately
use authenticatorConfig with acfg without another touch.

Details

FidoState::mark_token_used() at crates/rsk-fido/src/state.rs:359
only refreshes last_used_ms; it does not clear permissions.

It is called during token validation from:

• makecredential.rs:451
• getassertion.rs:362

The actual presence checks happen later in the successful operation paths.
After those checks, the implementation does not apply the equivalent of
clearPinUvAuthTokenPermissionsExceptLbw().

authenticatorConfig accepts the remaining acfg permission without a
presence check for regular subcommands such as toggleAlwaysUv and
enableEnterpriseAttestation.

CTAP 2.1 reference:

https://fidoalliance.org/specs/fido-v2.1-ps-20210615/fido-client-to-authenticator-protocol-v2.1-ps-20210615.html#pinuvauthprotocol-clearpinuvauthtokenpermissionsexceptlbw

PoC

Prerequisites:

• A device with a configured PIN.
• A valid protocol-two pinUvAuthToken obtained with permissions
  mc|ga|acfg (0x23).
• A valid pinUvAuthParam for the requested operation.

1. Obtain the multi-permission token.
2. Send authenticatorMakeCredential with the token and up implicitly
   true.
3. Complete the requested physical presence check.
4. Reuse the same token to send authenticatorConfig with
   toggleAlwaysUv or enableEnterpriseAttestation.
5. The configuration command succeeds without another physical presence
   check.

Expected behavior: after step 3, the token permissions are reduced to
lbw (or zero if lbw was not requested), and step 4 fails with
CTAP2_ERR_PIN_AUTH_INVALID.

Impact

A host that holds a valid multi-permission token can carry an
acfg authorization across a user-presence-gated FIDO operation and
modify security-relevant authenticator configuration without the
additional presence boundary required by CTAP 2.1.

This is not unauthenticated: obtaining the token still requires the
appropriate PIN/UV authorization.

AUT_ENABLE and AUT_DISABLE have their own touch checks, so the
no-second-touch impact specifically applies to regular configuration
subcommands such as toggleAlwaysUv and enableEnterpriseAttestation.

Suggested fix: after the successful UP check, retain only:

  ctx.state.paut.permissions &= PERM_LBW;

For getAssertion, use the raw up request value; for makeCredential,
UP is implicitly true. The mask should be applied after the successful
presence check, not immediately after mark_token_used().