How to remediate – SSL Certificate with no Common Name

1. Introduction

This report covers SSL Certificates with no Common Name. This means a website’s security certificate doesn’t specify which domain it applies to. While technically allowed, this can cause compatibility issues with some older systems and browsers. A missing common name could lead to warnings for users or connection failures. This affects web servers using TLS/SSL certificates, particularly those recently generated or misconfigured. Impact on confidentiality is low, integrity is medium due to potential man-in-the-middle risks if not detected, and availability is low as connections may fail.

2. Technical Explanation

The issue occurs when the certificate generation process omits the Common Name (CN) field in the Subject Distinguished Name (DN). This isn’t a direct security flaw but can cause problems with validation, especially on older clients that rely heavily on CN for verification. An attacker could potentially exploit this by presenting a similar certificate without a CN, causing confusion or connection errors.

• Root cause: The certificate request does not include the Common Name field during generation.
• Exploit mechanism: A user attempts to connect to a server with an improperly configured certificate. Older clients may fail validation checks or display warnings. For example, a client using outdated TLS settings might not correctly verify the hostname.
• Scope: Web servers (Apache, Nginx, IIS) and any service utilising SSL/TLS certificates are affected. The issue is independent of specific versions but more common with newer certificate generation methods where CN is optional.

3. Detection and Assessment

You can check for missing Common Names using OpenSSL or by inspecting the certificate details in a web browser.

• Quick checks: Use OpenSSL to view the certificate information. openssl s_client -connect yourdomain.com:443 Look for the “subject=” line; if it doesn’t contain CN=, the certificate lacks a Common Name.
• Scanning: Nessus plugin ID 16859 can identify SSL certificates without a common name. Qualys SSL Labs also reports this issue during a scan. These are examples only and may require updates.
• Logs and evidence: Web server logs won’t directly show this, but browser error messages related to certificate validation could indicate the problem. Check client-side errors for warnings about hostname mismatch or invalid certificates.

openssl s_client -connect yourdomain.com:443

4. Solution / Remediation Steps

The fix involves reissuing the certificate with a valid Common Name included in the Subject DN.

4.1 Preparation

• Ensure you have access to your Certificate Authority (CA) account or tools for generating a new certificate request. A roll back plan is to restore the original certificate files.
• A change window may be required depending on service criticality, and approval from the security team might be needed.

4.2 Implementation

1. Step 1: Generate a new Certificate Signing Request (CSR) including the correct Common Name for your domain. Use your web server’s key generation tool or CA portal.
2. Step 2: Submit the CSR to your Certificate Authority and obtain a signed certificate.
3. Step 3: Install the newly issued certificate on your web server, ensuring it replaces the old one.
4. Step 4: Restart your web server to load the new certificate.

4.3 Config or Code Example

Before

-----BEGIN CERTIFICATE REQUEST-----
... (certificate data without CN) ...
-----END CERTIFICATE REQUEST-----

After

-----BEGIN CERTIFICATE REQUEST-----
... (certificate data with CN=yourdomain.com) ...
-----END CERTIFICATE REQUEST-----

4.4 Security Practices Relevant to This Vulnerability

Regular certificate validation and monitoring are important for maintaining secure connections.

• Practice 1: Implement a robust certificate lifecycle management process, including automated renewal reminders and checks for validity.
• Practice 2: Use input validation when generating CSRs to ensure all required fields (like Common Name) are present and correct.

4.5 Automation (Optional)

If using Let’s Encrypt with Certbot, you can specify the domain name during certificate creation which automatically includes it in the CN field.

certbot certonly --domain yourdomain.com --agree-tos --email [email protected]

5. Verification / Validation

Confirm the fix by inspecting the new certificate details and verifying successful connections.

• Post-fix check: Run openssl s_client -connect yourdomain.com:443 again. The output should now show “subject=CN=yourdomain.com”.
• Re-test: Re-run the Nessus scan (ID 16859) or Qualys SSL Labs test; it should no longer report a missing Common Name.
• Smoke test: Access your website via HTTPS in multiple browsers to confirm connections are established without certificate warnings.

openssl s_client -connect yourdomain.com:443

6. Preventive Measures and Monitoring

For example, update security baselines to require Common Names in all SSL certificates.

• Baselines: Incorporate a check for the presence of a Common Name into your SSL certificate baseline configuration or policy.
• Asset and patch process: Review certificate generation processes regularly to ensure best practices are followed.

7. Risks, Side Effects, and Roll Back

The main risk is service interruption during the certificate replacement.

• Risk or side effect 1: Brief downtime while restarting web services. Mitigation: Schedule changes during low-traffic periods.
• Roll back: Restore the original SSL certificate files and restart your web server.

8. References and Resources

• Vendor advisory or bulletin: Not applicable as this is a configuration issue, not a vendor flaw.
• NVD or CVE entry: No specific CVE exists for missing Common Name, but RFC 5280 details the field requirements.
• Product or platform documentation relevant to the fix: https://datatracker.ietf.org/doc/html/rfc5280#section-4.1.2.6