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

1. Introduction

The SSL Certificate Chain Contains RSA Keys Less Than 2048 bits vulnerability means that a service is presenting an SSL certificate chain with at least one certificate using an RSA key shorter than 2048 bits. This can cause browsers to distrust the connection, leading to website errors and loss of user confidence. It affects web servers, load balancers, and any system handling TLS connections. A successful exploit could result in confidentiality loss if users are directed to a malicious site or integrity loss if a man-in-the-middle attack is possible.

2. Technical Explanation

• Root cause: The service uses an X.509 certificate with an RSA key length less than 2048 bits.
• Exploit mechanism: An attacker could perform a man-in-the-middle attack, presenting their own certificate and exploiting the client’s acceptance of weaker keys.

3. Detection and Assessment

You can confirm this vulnerability by inspecting the certificate chain presented by the service. A quick check involves using a browser’s developer tools, while thorough assessment requires dedicated scanning tools.

• Quick checks: Use your browser’s developer tools (usually F12) to view the certificate details and check the key length of each certificate in the chain.
• Scanning: Nessus vulnerability ID 69853 can detect this issue. Other scanners may have similar signatures.
• Logs and evidence: Check web server logs for SSL handshake errors related to certificate validation failures.

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

4. Solution / Remediation Steps

Replace the vulnerable certificate with one using an RSA key of at least 2048 bits and reissue any certificates signed by the old certificate.

4.1 Preparation

• Ensure you have access to the Certificate Authority (CA) used to issue the certificate. A roll back plan is to restore the original certificate from backup.
• A change window may be needed, depending on your service availability requirements. Approval from a security or infrastructure team lead might be required.

4.2 Implementation

1. Step 1: Obtain a new SSL certificate with an RSA key length of at least 2048 bits from your CA.
2. Step 2: Install the new certificate on your web server, ensuring it is correctly configured in your web server settings.
3. Step 3: Reissue any intermediate certificates signed by the old certificate if applicable.
4. Step 4: Restart your web service to load the new certificate configuration.

4.3 Config or Code Example

Before

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

After

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

4.4 Security Practices Relevant to This Vulnerability

List only practices that directly address this vulnerability type. Use neutral wording and examples instead of fixed advice. For example: least privilege, input validation, safe defaults, secure headers, patch cadence. If a practice does not apply, do not include it.

• Practice 1: Implement a regular certificate review process to ensure all certificates meet current security standards.
• Practice 2: Use automated tools for certificate management and renewal to prevent expiry or the use of weak keys.

4.5 Automation (Optional)

# Example Ansible task to update certificate (use with caution!)
- name: Update SSL Certificate
  copy:
    src: /path/to/new_certificate.pem
    dest: /etc/ssl/certs/yourdomain.com.crt
    owner: root
    group: root
    mode: 0644
  notify: Restart web server

5. Verification / Validation

Confirm the fix by inspecting the new certificate chain and verifying that all certificates use RSA keys of at least 2048 bits. A service smoke test should confirm normal operation.

• Post-fix check: Use `openssl s_client -connect yourdomain.com:443 | openssl x509 -noout -text` and verify the key length of each certificate in the chain is 2048 bits or greater.
• Re-test: Re-run the Nessus scan (ID 69853) to confirm the vulnerability is no longer detected.
• Smoke test: Access your website via HTTPS and verify that it loads correctly without any certificate errors.
• Monitoring: Monitor web server logs for SSL handshake success events to ensure certificates are being validated correctly.

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

6. Preventive Measures and Monitoring

Update security baselines to require certificates with RSA keys of at least 2048 bits, and add checks in CI/CD pipelines to prevent the deployment of vulnerable certificates.

• Baselines: Update your security baseline or policy to mandate the use of SSL certificates with RSA key lengths of at least 2048 bits.
• Asset and patch process: Review all SSL certificates on a quarterly basis to ensure they meet current security standards.

7. Risks, Side Effects, and Roll Back

Risks include service downtime during the certificate update if misconfigured. A roll back plan involves restoring the original certificate from backup.

• Roll back: Restore the original SSL certificate and private key from backup, then restart your web service.

8. References and Resources

• Vendor advisory or bulletin: