CWE-639: Authorization Bypass Through User-Controlled Key

What is Authorization Bypass Through User-Controlled Key?

• Overview: Authorization Bypass Through User-Controlled Key (CWE-639) occurs when a system allows unauthorized access to user data by failing to properly verify if the authenticated user has the right to access specific data. This happens when user-controlled inputs, like keys or identifiers, are used to retrieve sensitive information without proper checks.

• Exploitation Methods:

• Attackers modify user-controllable keys, such as URL parameters, hidden form fields, or cookies, to access unauthorized data.
• Common attack patterns include manipulating session IDs or other identifiers to gain access to other users' data.

• Security Impact:

• Direct consequences include unauthorized access to sensitive data, leading to potential data breaches.
• Potential cascading effects involve further exploitation of accessed data for more attacks, like identity theft or data manipulation.
• Business impact includes loss of customer trust, legal penalties, and damage to brand reputation.

• Prevention Guidelines:

• Specific code-level fixes include validating user inputs and ensuring access control checks are performed server-side for all data retrieval operations.
• Security best practices involve implementing role-based access control (RBAC) and ensuring that all identifiers are not predictable or tamperable.
• Recommended tools and frameworks include using secure libraries or frameworks that provide built-in access control mechanisms and input validation features.

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Technical Details

Likelihood of Exploit: High

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

Python Example

from flask import Flask, request, jsonify

app = Flask(__name__)

# Dummy data representing user records
users_data = {
    '1001': {'name': 'Alice', 'role': 'user'},
    '1002': {'name': 'Bob', 'role': 'admin'}
}

@app.route('/get_user_info', methods=['GET'])
def get_user_info():
    user_id = request.args.get('user_id')  # User ID is directly taken from user input
    user_info = users_data.get(user_id)  # No verification if the requesting user is allowed to access this data
    if user_info:
        return jsonify(user_info)
    else:
        return jsonify({'error': 'User not found'}), 404

if __name__ == '__main__':
    app.run()

Explanation:

• Vulnerability: The code allows users to specify any user_id in the request, potentially retrieving data for any user in the system without proper authorization checks.
• Impact: This could allow a malicious user to access sensitive data of other users by simply guessing or iterating through user IDs.

How to fix Authorization Bypass Through User-Controlled Key?

To fix this vulnerability, you need to ensure that the user making the request is authorized to access the data associated with the user_id. This can be done by implementing proper authentication mechanisms, such as session tokens or API keys, and verifying that the authenticated user is authorized to access the requested data.

Steps to Fix:

1. Authenticate the User: Ensure that the user is authenticated, typically by using session tokens or API keys.
2. Verify Authorization: Check that the authenticated user has permission to access the data associated with the user_id.
3. Use Secure Session Management: Associate the session with user-specific data and verify access based on this association.

Fixed Code Example

from flask import Flask, request, jsonify, session

app = Flask(__name__)
app.secret_key = 'supersecretkey'  # Secure your session with a secret key for Flask session management

# Dummy data representing user records
users_data = {
    '1001': {'name': 'Alice', 'role': 'user'},
    '1002': {'name': 'Bob', 'role': 'admin'}
}

# Dummy function to simulate user authentication
def authenticate_user():
    # For demonstration purposes, we set a session user_id (would typically be set after a login process)
    session['user_id'] = '1001'  # Simulate that user with ID 1001 is logged in

@app.route('/get_user_info', methods=['GET'])
def get_user_info():
    authenticate_user()  # Simulate user authentication
    user_id = session.get('user_id')  # Use the user_id from a secured session, not directly from user input
    user_info = users_data.get(user_id)  # Access the data only for the authenticated user
    if user_info:
        return jsonify(user_info)
    else:
        return jsonify({'error': 'User not found'}), 404

if __name__ == '__main__':
    app.run()

Explanation:

• Authentication: The fixed code uses a simulated authentication process to set a user_id in the session.
• Authorization: The user_id is now fetched from the session, ensuring that the user can only access their own data.
• Session Management: Proper session management is used to securely maintain the user state and rights throughout the session.

This approach ensures that users can only access their own data, mitigating the risk of unauthorized access to other users' data.