How to remediate – SSL Certificate Signed with the Compromised FortiGate Key

1. Introduction

The SSL Certificate Signed with the Compromised FortiGate Key vulnerability means a service’s security certificate was issued by a compromised authority. This allows attackers to intercept and read encrypted communications, potentially stealing sensitive data like usernames and passwords. Systems using services protected by certificates from this CA are affected. Impact on confidentiality is likely high, integrity medium, and availability low.

2. Technical Explanation

This vulnerability occurs because an SSL certificate was signed by a Certificate Authority (CA) whose private key has been stolen. An attacker possessing the compromised CA’s private key can create fraudulent certificates for any domain, allowing them to perform man-in-the-middle attacks. Exploitation requires the target system to trust certificates issued by this rogue CA. CVE-2012-4948 details a specific instance of this issue.

• Root cause: The private key of a Certificate Authority used by FortiGate devices was compromised.
• Exploit mechanism: An attacker presents a fraudulent certificate signed by the compromised CA to the target system. If the system trusts certificates from that CA, it accepts the malicious certificate and allows the attacker to intercept traffic. For example, an attacker could set up a fake website with a certificate issued by the compromised CA, tricking users into providing their credentials.
• Scope: Systems relying on SSL certificates signed by CAs found within FortiGate devices are affected.

3. Detection and Assessment

• Quick checks: Use an SSL checker tool (like those available online) to view the certificate chain for affected services and identify the issuing CA.
• Scanning: Nessus plugin ID 63487 can detect certificates signed by compromised CAs, but results should be verified manually.
• Logs and evidence: Examine system trust stores for entries related to the identified compromised CA. Specific log files will vary depending on the operating system and application.

openssl s_client -showcerts <hostname>:443

4. Solution / Remediation Steps

To fix this issue, configure your device to use a device-specific CA certificate instead of relying on the compromised authority.

4.1 Preparation

• Ensure you have access to a valid, trusted CA certificate for device-specific use. A roll back plan involves restoring from the backup created in step 1.
• A standard change window may be required depending on your organisation’s policies. Approval from a security team lead is recommended.

4.2 Implementation

1. Step 1: Obtain a device-specific CA certificate from a trusted source.
2. Step 2: Log in to the FortiGate administration interface.
3. Step 3: Navigate to System > Certificates.
4. Step 4: Import the new device-specific CA certificate.
5. Step 5: Configure affected services (e.g., VPN, web servers) to use the imported certificate.
6. Step 6: Remove or disable any trust for certificates issued by the compromised CA.

4.3 Config or Code Example

Before

config system certificate ca
  edit "Compromised CA"
    set trusted 1
  next
end

After

config system certificate ca
  edit "Compromised CA"
    set trusted 0
  next
end
config system certificate ca
  edit "Device Specific CA"
    set trusted 1
  next
end

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this type of issue.

• Practice 1: Least privilege – limit the scope of trust granted to CAs. Only trust authorities that are absolutely necessary.
• Practice 2: Patch cadence – Regularly update FortiGate devices with the latest firmware and security patches to address known vulnerabilities.

4.5 Automation (Optional)

Automation is not generally recommended for this specific issue due to the need for careful certificate management, but configuration management tools can be used to enforce trusted CA lists.

# Example Ansible task - use with caution!
- name: Ensure compromised CA is untrusted
  fortios_certificate_ca:
    name: "Compromised CA"
    trusted: false
  

5. Verification / Validation

Confirm the fix by verifying that the compromised CA is no longer trusted and that affected services are using the new certificate.

• Post-fix check: Run `openssl s_client -showcerts <hostname>:443` again. The output should not list the compromised CA in the certificate chain.
• Re-test: Use an SSL checker tool to confirm that the new device-specific certificate is being used and that the old, compromised certificate is no longer present.
• Monitoring: Monitor system logs for any errors related to certificate validation or SSL connections. Look for events indicating trust failures.

openssl s_client -showcerts <hostname>:443 | grep "Issuer:"

6. Preventive Measures and Monitoring

Implement preventive measures to reduce the risk of similar vulnerabilities in the future.

• Baselines: Update your security baseline or policy to include a list of trusted CAs and regularly review it for unnecessary entries. Consider using CIS benchmarks as a starting point.
• Pipelines: Integrate certificate validation checks into your CI/CD pipeline to prevent deployment of services with untrusted certificates.
• Asset and patch process: Establish a regular patch management cycle for FortiGate devices, ensuring that security updates are applied promptly.

7. Risks, Side Effects, and Roll Back

Changing trusted CA lists can disrupt service if not done carefully.

• Risk or side effect 2: Service interruption during the changeover if affected services are not restarted promptly. Mitigation: Plan for minimal downtime and have a clear restart procedure.
• Roll back: Restore from the backup created in step 4.1. Re-enable trust for the original CA certificate (if necessary).

8. References and Resources

Links to official advisories and documentation related to this vulnerability.

• Vendor advisory or bulletin: Fortinet Security Alerts
• NVD or CVE entry: CVE-2012-4948
• Product or platform documentation relevant to the fix: FortiGate Certificates Documentation