CN Certificate & Name Constraints: Does it work?

TL;DR

No, a CN-only certificate generally cannot bypass name constraints enforced by the client or server. Name constraints are typically checked against Subject Alternative Names (SANs) and/or the Common Name (CN), but relying solely on the CN is insecure and increasingly uncommon. Modern clients and servers will likely reject a CN-only certificate if it doesn’t meet defined name constraint rules.

Understanding the Problem

Certificates use several fields to identify which websites they are valid for. The most important are:

• Subject Alternative Name (SAN): A list of hostnames and other identifiers the certificate covers. This is the preferred method for specifying allowed domains.
• Common Name (CN): An older field that specifies a single hostname. It’s often used as a fallback if SANs aren’t present, but it’s less secure and increasingly deprecated.

Name constraints are rules set by the certificate authority (CA) or server administrator to limit which names a certificate is valid for. They can restrict certificates to specific domains or subdomains.

Why CN-Only Certificates Are Problematic

1. Security Risks: The CN field is vulnerable to attacks like character encoding issues and wildcard misinterpretations.
2. Browser Support: Modern browsers are phasing out support for CN-only certificates, especially if SANs are missing.
3. Standard Practice: Best practice dictates using SANs instead of relying on the CN field.

How Name Constraints Work

Name constraints can be defined in several ways, but they generally involve specifying:

• Excluded DNS Names: A list of names that are not allowed.
• Included DNS Names: A list of names that are allowed (less common).

The client or server checks the certificate against these constraints to ensure it’s valid for the requested hostname.

Steps to Verify Name Constraint Enforcement

1. Check Certificate Details: Use a browser’s developer tools (usually F12) or an online SSL checker to view the certificate. Look for the SAN field and any name constraint extensions.

   openssl x509 -text -noout -in your_certificate.pem | grep 'Subject Alternative Name'

2. Test with Different Browsers: Try accessing the website with multiple browsers (Chrome, Firefox, Edge) to see if they all accept the CN-only certificate.
3. Server Configuration: If you control the server, review its SSL configuration. Ensure it’s not configured to allow CN-only certificates without proper SAN validation.

   # Example Apache configuration (check for AllowCNName)

4. Use a Command Line Tool: Use tools like openssl s_client to connect to the server and verify certificate chain validation. This can help identify name constraint issues.

   openssl s_client -connect yourdomain.com:443 -showcerts

Example Scenario

Let’s say a certificate has only the CN example.com and a name constraint that excludes *.subdomain.example.com. If you try to access test.subdomain.example.com, most modern clients will reject the certificate because it doesn’t match the SAN (which is missing) and violates the name constraint.

Conclusion

While technically possible in some limited cases, relying on a CN-only certificate to bypass name constraints is unreliable and insecure. Always use certificates with properly configured SANs and ensure your clients and servers enforce name constraint validation for robust security.