Authentication bypass via MongoDB operator injection
Description
This web application is vulnerable to authentication bypass through MongoDB operator injection. The application fails to properly validate user input before passing it to MongoDB queries, allowing attackers to inject MongoDB query operators (such as $ne, $gt, or $regex) into authentication fields. By submitting specially crafted payloads like {"$ne": "randomstring"} in username or password fields, an attacker can manipulate the query logic to always evaluate to true, bypassing authentication controls without knowing valid credentials.
Remediation
Implement strict input validation and type casting to prevent MongoDB operator injection:
1. Cast all user inputs to strings before using them in queries:
// PHP Example - Force string type casting
$username = (string)$_POST['username'];
$password = (string)$_POST['password'];
$user = $collection->findOne([
'username' => $username,
'password' => hash('sha256', $password)
]);2. Validate input types and reject objects/arrays:
// Node.js Example - Validate input types
if (typeof username !== 'string' || typeof password !== 'string') {
throw new Error('Invalid input type');
}
const user = await db.collection('users').findOne({
username: username,
password: hashedPassword
});3. Use parameterized queries and avoid passing user input directly into query operators
4. Implement allowlisting for expected input patterns (e.g., alphanumeric characters only for usernames)
5. If using JavaScript code execution (MongoCode), pass variables through the scope parameter rather than string interpolation
6. Consider using an ORM or ODM (Object Document Mapper) that provides built-in protection against injection attacks
7. Implement additional security layers such as rate limiting, account lockout policies, and multi-factor authentication to reduce the impact of authentication vulnerabilities