c_hash Claim & IdP MixUp: A Fix

TL;DR

The c_hash claim in your id_token helps verify the identity provider (IdP) used for login. If it’s missing or incorrect, an attacker might be able to trick your application into accepting a fake login from a different IdP – this is called an IdP MixUp attack. This guide shows you how to check and use c_hash properly to prevent these attacks.

What’s the Problem?

When a user logs in, your application receives an id_token from the IdP. This token contains information about the login, including who logged in (the subject) and which IdP was used. The c_hash claim is a cryptographic hash of the IdP’s configuration. If an attacker can control the contents of the id_token, they could potentially swap out the real IdP with a malicious one.

How to Fix It

1. Understand Your Expected c_hash Values
   • Your application needs to know what valid c_hash values look like. This means you need to store the expected hashes for each IdP you trust.
   • These hashes are generated from the IdP’s metadata (usually a JSON file). The exact method depends on your identity provider and authentication library, but it generally involves hashing specific parts of the metadata document.
2. Verify the c_hash Claim
   • When you receive an id_token, always check that the c_hash claim exists and matches one of your expected values.
   • If the hash doesn’t match, reject the token immediately. Do not proceed with login.
3. Example Code (Conceptual – Adapt to Your Library)

   # Assuming you have a list of expected c_hash values
   expected_hashes = [
     "some-hash-value-1",
     "some-hash-value-2"
   ]
   
   token = get_id_token()
   
   c_hash_from_token = token.get("c_hash")
   
   if c_hash_from_token is None:
     # Reject the token - c_hash claim is missing
     print("Error: c_hash claim is missing!")
   else if c_hash_from_token not in expected_hashes:
     # Reject the token - c_hash does not match any known values
     print("Error: Invalid c_hash value!")
   else:
     # Token is valid, proceed with login
     print("Token validated successfully.")

   Important: This is a simplified example. Your actual implementation will depend on the authentication library you’re using (e.g., Auth0 SDKs, Okta libraries, etc.). Consult your library’s documentation for specific methods to verify token signatures and claims.

4. Metadata Updates
   • IdP metadata can change over time. Regularly update the c_hash values you store in your application to reflect these changes.
   • Automate this process if possible, or at least have a scheduled task to check for updates.
5. Consider Token Signature Verification
   • While c_hash is important, it’s not a replacement for proper token signature verification (using JWT libraries). Always verify the token’s signature to ensure it hasn’t been tampered with.

Further Security Considerations

• cyber security best practice: Implement robust logging and monitoring to detect suspicious login attempts, including failed c_hash validations.
• Review your authentication library’s documentation for specific recommendations on IdP MixUp prevention.