How to remediate – SSL Certificate Chain Not Sorted

1. Introduction

The SSL Certificate Chain Not Sorted vulnerability means the order of certificates in a service’s X.509 chain is incorrect. This can cause connection failures, especially with older devices. It affects services using TLS/SSL and has a low impact on confidentiality, integrity, and availability if connections fail.

2. Technical Explanation

The problem occurs because some Certificate Authorities (CAs) issue certificate bundles in descending order instead of the required ascending order as defined by RFC 4346 Section 7.4.2. Some SSL implementations, particularly those on embedded systems, cannot process these unordered chains and will refuse to connect. There is no known CVE associated with this specific ordering issue but it can lead to denial-of-service.

• Root cause: Incorrect certificate chain order (descending instead of ascending).
• Exploit mechanism: An attacker does not actively exploit this, but a misconfigured server presenting an incorrectly ordered chain will fail to establish TLS connections with vulnerable clients. A client attempting to connect to the service will receive a connection error.
• Scope: Affects any service using TLS/SSL where the certificate chain is improperly configured. This includes web servers (Apache, Nginx), email servers, and other applications relying on SSL/TLS for secure communication.

3. Detection and Assessment

You can check if a system is vulnerable by examining its certificate configuration or using an online tool to test the connection. A quick check involves inspecting the server’s TLS configuration.

• Quick checks: Use `openssl s_client -connect yourdomain.com:443` and examine the output for the certificate chain order.
• Scanning: Qualys SSL Labs Server Test (https://www.ssllabs.com/ssltest/) will report if the chain is incorrectly ordered. This is an example only, and results may vary.
• Logs and evidence: Examine web server logs for TLS handshake errors related to certificate validation failures. Specific error messages depend on the server software.

openssl s_client -connect yourdomain.com:443

4. Solution / Remediation Steps

Fixing this issue involves reordering the certificates in the certificate chain so they are presented in ascending order, starting with the end-entity certificate and ending with the root CA certificate.

4.1 Preparation

• Ensure you have access to all certificates in the chain. A roll back plan is to restore the original configuration file.
• A change window may be required depending on your organisation’s policies, and approval from a senior administrator might be needed.

4.2 Implementation

1. Step 1: Locate the certificate chain configuration in your web server settings (e.g., Apache’s `SSLCertificateChainFile` directive or Nginx’s `ssl_certificate` and `ssl_certificate_key` directives).
2. Step 2: Reorder the certificates so that the end-entity certificate is listed first, followed by any intermediate certificates, and finally the root CA certificate.
3. Step 3: Restart the web server to apply the changes.

4.3 Config or Code Example

Before

SSLCertificateChainFile /etc/ssl/certs/root_ca.crt
SSLCertificateFile /etc/ssl/certs/intermediate.crt
SSLCertificateKeyFile /etc/ssl/private/server.key

After

SSLCertificateChainFile /etc/ssl/certs/intermediate.crt
SSLCertificateFile /etc/ssl/certs/root_ca.crt
SSLCertificateKeyFile /etc/ssl/private/server.key

4.4 Security Practices Relevant to This Vulnerability

Regularly reviewing your TLS configuration is important for maintaining secure connections. Least privilege can limit the impact if a certificate chain issue causes connection failures.

• Practice 1: Regularly review server configurations, including SSL/TLS settings, as part of routine security maintenance.
• Practice 2: Implement least privilege principles to restrict access to sensitive files and directories containing certificates and keys.

4.5 Automation (Optional)

If using configuration management tools like Ansible, you can automate the certificate chain ordering process.

---
- name: Reorder SSL Certificates
  hosts: webservers
  tasks:
    - name: Ensure correct certificate order in Apache config
      lineinfile:
        path: /etc/apache2/sites-available/yourdomain.conf
        regexp: '^SSLCertificateChainFile'
        lines: - SSLCertificateChainFile /etc/ssl/certs/intermediate.crt
               - SSLCertificateFile /etc/ssl/certs/root_ca.crt
      notify: Restart Apache

5. Verification / Validation

Confirm the fix by rechecking the certificate chain order using `openssl s_client`. A smoke test should verify basic service functionality.

• Post-fix check: Run `openssl s_client -connect yourdomain.com:443` and confirm that the certificates are listed in ascending order (end-entity first, root CA last).
• Re-test: Re-run the Qualys SSL Labs Server Test to verify the issue is resolved.
• Smoke test: Access your website via HTTPS to ensure it loads correctly. Check key functionality like login and form submission.
• Monitoring: Monitor web server logs for TLS handshake errors. A spike in these errors could indicate a regression.

openssl s_client -connect yourdomain.com:443

6. Preventive Measures and Monitoring

Updating security baselines can prevent this issue by enforcing correct certificate chain ordering. Regularly reviewing TLS configurations during deployments is also helpful.

• Baselines: Update your server security baseline to include a requirement for correctly ordered SSL/TLS certificate chains (for example, using CIS benchmarks).
• Asset and patch process: Review TLS configuration changes as part of your regular asset management and patching cycle.

7. Risks, Side Effects, and Roll Back

Incorrectly configuring the certificate chain can lead to connection failures. A roll back involves restoring the original configuration file.

• Risk or side effect 1: Incorrect certificate ordering may cause browsers or other clients to refuse connections.
• Risk or side effect 2: Changes to server configurations could temporarily disrupt service availability.
• Roll back: Restore the original web server configuration file from your backup and restart the web server.

8. References and Resources

• Vendor advisory or bulletin: Check your CA’s documentation for guidance on certificate chain ordering.
• NVD or CVE entry: No specific CVE exists for this issue, but information about TLS/SSL vulnerabilities can be found on the NVD website (https://nvd.nist.gov/).
• Product or platform documentation relevant to the fix: Refer to your web server’s documentation (e.g., Apache, Nginx) for instructions on configuring SSL/TLS certificates.