How to remediate – SSL Certificate Chain Contains RSA Keys Less Than 2048 bits (P…

1. Introduction

The SSL Certificate Chain Contains RSA Keys Less Than 2048 bits vulnerability means that a service is using an X.509 certificate with an RSA key shorter than 2048 bits. This can cause browsers to reject the connection, preventing users from accessing the service. It affects web servers and any other system presenting an SSL/TLS certificate. A successful exploit could lead to loss of confidentiality as connections cannot be established.

2. Technical Explanation

• Root cause: Use of an RSA key shorter than 2048 bits in the certificate chain.
• Exploit mechanism: A user attempts to connect to a server using a browser that enforces the 2048-bit minimum key length requirement, resulting in a connection failure.
• Scope: Web servers (Apache, Nginx, IIS), load balancers, and any service presenting an SSL/TLS certificate are affected.

3. Detection and Assessment

You can check for this vulnerability using command-line tools or by examining the certificate details in a web browser. Scanning tools can also identify short RSA keys.

• Quick checks: Use `openssl s_client -connect :` and examine the certificate chain output for key sizes less than 2048 bits.
• Scanning: Nessus vulnerability ID 107659 can detect this issue as an example only.
• Logs and evidence: Web server logs may show SSL handshake failures related to certificate validation errors.

openssl s_client -connect yourdomain.com:443

4. Solution / Remediation Steps

Replace the vulnerable certificate with one that has an RSA key of 2048 bits or greater, and reissue any certificates signed by the old certificate.

4.1 Preparation

• Ensure you have access to your Certificate Authority (CA) for reissuing certificates. A roll back plan is to restore the original backup certificates.
• A change window may be needed, depending on service criticality and user impact; approval from a senior IT manager might be required.

4.2 Implementation

1. Step 1: Obtain a new certificate with an RSA key of at least 2048 bits from your CA.
2. Step 2: Install the new certificate on the server, following your web server’s documentation (e.g., Apache configuration files, IIS Manager).
3. Step 3: Restart the web service to load the new certificate.
4. Step 4: Verify that the new certificate is being served correctly.

4.3 Config or Code Example

Before

#Example Apache config - insecure
SSLCertificateFile /etc/ssl/certs/old_certificate.pem
SSLCertificateKeyFile /etc/ssl/private/old_key.pem

After

#Example Apache config - secure
SSLCertificateFile /etc/ssl/certs/new_certificate.pem
SSLCertificateKeyFile /etc/ssl/private/new_key.pem

4.4 Security Practices Relevant to This Vulnerability

Regular certificate checks and a strong patch cadence are important for preventing this issue.

• Practice 2: Follow a consistent patch management schedule for your web server software and operating system, ensuring security updates are applied promptly.

4.5 Automation (Optional)

If using configuration management tools like Ansible, you can automate certificate replacement.

---
- name: Update SSL Certificate
  hosts: webservers
  tasks:
    - copy:
        src: /path/to/new_certificate.pem
        dest: /etc/ssl/certs/new_certificate.pem
        owner: root
        group: root
        mode: 0644
    - service: name=apache2 state=restarted

5. Verification / Validation

Confirm the fix by checking the certificate details in a browser and verifying that the key size is at least 2048 bits.

• Post-fix check: Run `openssl s_client -connect :` again. The output should show a key length of 2048 or greater.
• Re-test: Re-run the Nessus scan (ID 107659) to confirm that the vulnerability is no longer detected.
• Smoke test: Verify users can access the website without connection errors. Check core functionality like login and data submission.
• Monitoring: Monitor web server logs for SSL handshake success events.

openssl s_client -connect yourdomain.com:443 | openssl x509 -noout -modulus

6. Preventive Measures and Monitoring

Update security baselines to require 2048-bit RSA keys, and include certificate checks in CI/CD pipelines.

• Baselines: Update your server hardening baseline or group policy to enforce a minimum SSL key length of 2048 bits.
• Pipelines: Integrate SSL certificate validation into your continuous integration (CI) or deployment pipeline to prevent the use of weak certificates.
• Asset and patch process: Review all SSL/TLS certificates quarterly, ensuring they meet current security standards.

7. Risks, Side Effects, and Roll Back

Replacing a certificate can cause temporary service disruption if not done correctly. Always have a roll back plan.

• Roll back: Restore the original SSL certificates from your backup, and restart the web service.

8. References and Resources

• Vendor advisory or bulletin: https://www.cabforum.org/wp-content/uploads/Baseline_Requirements_V1.pdf