CWE-302: Authentication Bypass by Assumed-Immutable Data

What is Authentication Bypass by Assumed-Immutable Data?

• Overview: This vulnerability occurs when an authentication process relies on data that is assumed to be unchangeable but can actually be altered by an attacker, allowing them to bypass authentication controls.

• Exploitation Methods:

• Attackers can modify supposed immutable data used in authentication to gain unauthorized access.
• Common attack patterns include manipulating tokens, cookies, or session identifiers that are not properly secured against tampering.

• Security Impact:

• Direct consequences include unauthorized access to systems and data.
• Potential cascading effects involve privilege escalation and further exploitation of network resources.
• Business impact may include data breaches, financial losses, and damage to reputation.

• Prevention Guidelines:

• Use secure methods to verify the integrity and authenticity of data used in authentication.
• Implement cryptographic techniques to sign and validate data elements to ensure they remain unaltered.
• Employ security best practices such as input validation, using HTTPS, and ensuring proper session management.
• Recommended tools and frameworks include those that support secure token handling and validation, such as OAuth, JWT with signature verification, and secure cookie management libraries.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

import hmac
import hashlib

class Authenticator:
    # Assuming the 'session_token' is immutable, which can be manipulated by the attacker
    def validate_user(self, user_id, session_token, actual_token):
        # Directly comparing session_token with actual_token without integrity verification
        if session_token == actual_token:
            return True
        return False

Explanation:

• Vulnerability: The code assumes that session_token is immutable and directly compares it to actual_token. An attacker could manipulate session_token to match actual_token, bypassing authentication. This is a classic example of CWE-302 where the assumption of immutability leads to security flaws.

How to fix Authentication Bypass by Assumed-Immutable Data?

To fix this vulnerability, it is essential to:

1. Ensure Data Integrity: Use cryptographic techniques like HMAC to verify the integrity and authenticity of the tokens.
2. Secure Token Storage: Store session tokens securely using server-side storage mechanisms, preventing client-side tampering.
3. Use Secure Session Management: Implement secure session management that handles token generation, storage, and verification on the server side.

Fixed Code Example

import hmac
import hashlib
import secrets

class Authenticator:
    SECRET_KEY = b'supersecretkey'
    
    # Use HMAC to validate the session token
    def validate_user(self, user_id, session_token, actual_token):
        # Calculate the HMAC of the actual token using a secret key
        expected_hmac = hmac.new(self.SECRET_KEY, actual_token.encode(), hashlib.sha256).hexdigest()
        
        # Compare the computed HMAC with the session token using secure comparison
        if hmac.compare_digest(session_token, expected_hmac):
            return True
        return False

Explanation:

• Fix Explanation: The validate_user method now uses HMAC to ensure that the session_token is both authentic and unaltered. By computing an HMAC of the actual_token using a secret key, and comparing it with the session_token using hmac.compare_digest, we ensure a secure and tamper-proof validation process.
• Best Practices:
  • HMAC: Use HMAC to ensure the integrity and authenticity of tokens.
  • Secure Comparison: hmac.compare_digest is used for constant-time comparison, mitigating timing attacks.
  • Secret Management: The secret key should be kept secure and only accessible by the server, ensuring that token generation and validation remain secure.