Webauthn has a User Verification Downgrade via Default-Open ClientOverridePolicy

Summary

## Summary

In version 5.3.0 of the Symfony bundle, `Webauthn\Bundle\Policy\ClientOverridePolicy` defaulted to allowing all client overrides, including `userVerification`. A client could send `{"userVerification": "discouraged"}` in the assertion or attestation options request to override a server-configured `userVerification: required`, causing the emitted WebAuthn options to instruct the authenticator to skip user verification. The `CheckUserVerification` ceremony step then read the same downgraded options and skipped its check.

## Affected versions

- **Vulnerable**: 5.3.0 - **Patched**: 5.3.1

5.3.0 was released on 2026-05-01 and 5.3.1 was published roughly 18 hours later, on 2026-05-02. Practical exposure window was minimal.

## Note on earlier 5.x versions

Versions 5.0.0 to 5.2.x did not ship `ClientOverridePolicy` (introduced in 5.3.0), so the exact code path described above does not apply. However, on those versions the `ProfileBasedRequestOptionsBuilder` and `ProfileBasedCreationOptionsBuilder` already passed the client-supplied `userVerification` value directly to the options factory, where the profile value is only applied via `??=`. The functional outcome (a client can downgrade `userVerification`) is the same. The recommended mitigation (see below) applies regardless of the version, and users on 5.0.x – 5.2.x are encouraged to upgrade to 5.3.1 or later.

## Severity

This is a **defense-in-depth** issue rather than a primitive that grants authentication on its own:

- The attacker must already possess the victim's authenticator (a stolen security key, an unlocked device). Without that, the downgrade is inconsequential. - The framework exposes the actual UV outcome on the returned authenticator data (`AuthenticatorData::isUserVerified()`). Applications that gate sensitive operations on this flag — as documented — remain protected even on the vulnerable version.

## Mitigation

Applications gating sensitive operations on user verification MUST re-check the `UV` flag on the returned authenticator data after a successful ceremony, regardless of what was requested in the options:

```php if (! $authenticatorData->isUserVerified()) { throw new AccessDeniedHttpException('User verification is required.'); } ```

This is the authoritative signal that user verification actually occurred. The hardened default in 5.3.1 closes the implicit profile-bypass; the application-level check remains the recommended defense in depth and is now documented explicitly in the [User Verification](https://webauthn-doc.spomky-labs.com/webauthn-in-a-nutshell/user-verification#checking-the-uv-flag) guide.

## Fix

`ClientOverridePolicy::canOverride()` now defaults to `false` instead of `true`. The Symfony bundle DI configuration ships `user_verification` overrides as **disabled by default**, with a default `allowed_values` list that excludes `discouraged` even when an operator opts in.

## Credit

Reported by @offset.