Summary There is a Proof of Concept which is able to enumerate the usernames of administrator users. This was possible by performing a timing attack. Details The faulty code exists in ""src/Core/Framework/Api/OAuth/UserRepository.php"" (https://github.com/shopware/shopware/blob/trunk/src/Core/Framework/Api/OAuth/UserRepository.php): public function getUserEntityByUserCredentials( string $username, #[\SensitiveParameter] string $password, string $grantType, ClientEntityInterface $clientEntity ): ?UserEntityInterface { if ($this->loginConfigService->getConfig()?->useDefault === false) { // never allow login via password if the default login is disabled (e.g. using SSO only) return null; } $builder = $this->connection->createQueryBuilder(); $user = $builder->select('user.id', 'user.password') ->from('user') ->where('username = :username') ->setParameter('username', $username) ->fetchAssociative(); // PATH 1: EARLY RETURN WHEN USERNAME IS NOT FOUND if (!$user) { return null; } // PATH 2: VERIFY PASSWORD IF USER IS FOUND if (!password_verify($password, (string) $user['password'])) { return null; } return new User(Uuid::fromBytesToHex($user['id'])); } Subroutine "getUserEntityByUserCredentials()" is called when an auth request is send to "api/oauth/token". If the given username is not found an early return is done (PATH 1). Only if the user is found we verify the password using "password_verify". PHP method "password_verify" by default uses hashing algorithm Argon2id which by design is intentionally 'slow' by introducing a timing cost to an attempt to bruteforce hashes more costly. Since "password_verify" has a notable executable time, PATH 2 where an user is found and verified will be slower on average then PATH 1 where we do an early return for non-existing users. Proposed fix Before doing the early return, "password_verify" a dummy hash. Impact 1. More targeted dictionary/bruteforce attacks. 2. Spear phishing / eases social engineering. 3. Credential stuffing from other data leaks. Authors Niel Duysters (@NielDuysters) and Thomas Brankaer (@tbrankaer)