1. Introduction
The SSL Certificate Signed with the Revoked DigiNotar Certificate vulnerability means a service’s certificate was issued by the compromised DigiNotar Certificate Authority. This is important because browsers are removing trust in certificates from this CA, causing connection errors for users and potentially impacting business services. Systems using certificates signed by DigiNotar are affected, including web servers, email servers, and VPN gateways. Impact on confidentiality is likely if attackers can intercept traffic; integrity is at risk if they can present a fraudulent certificate; availability may be impacted as browsers block connections.
2. Technical Explanation
This vulnerability occurs because the SSL certificate for a service was signed by DigiNotar, which suffered a significant compromise in 2011 and is no longer trusted by major browser vendors. Attackers don’t directly exploit this; instead, users’ browsers will refuse to connect to services using these certificates. The precondition is that a system uses an SSL certificate issued by the revoked DigiNotar CA.
- Root cause: A compromised Certificate Authority (DigiNotar) signed the certificate.
- Exploit mechanism: An attacker does not actively exploit this vulnerability. Browsers block connections to services using certificates from the compromised CA, leading to service disruption for users.
- Scope: Any system presenting an SSL certificate issued by DigiNotar is affected. This includes web servers (Apache, Nginx, IIS), email servers, and VPN gateways.
3. Detection and Assessment
You can confirm this vulnerability by examining the certificate chain for certificates signed by DigiNotar. A quick check involves inspecting the certificate in a browser; a thorough method is to use OpenSSL or similar tools.
- Quick checks: In your web browser, view the certificate details of the affected service. Check the “Issued By” field for “DigiNotar”.
- Scanning: Nessus vulnerability ID 33648 can detect this issue as an example only.
- Logs and evidence: Web server logs may show TLS handshake failures related to untrusted certificates, but are not specific to DigiNotar.
openssl s_client -connect yourdomain.com:443 | openssl x509 -noout -issuer
4. Solution / Remediation Steps
The solution is to replace the SSL certificate signed by DigiNotar with one issued by a trusted Certificate Authority. This involves purchasing or generating a new certificate and configuring your service to use it.
4.1 Preparation
- Ensure you have access to a trusted CA or the ability to generate a self-signed certificate (not recommended for public facing services). A roll back plan is to restore the original certificate and configuration.
- A change window may be needed for production systems; approval from the security team might be required.
4.2 Implementation
- Step 1: Purchase a new SSL certificate from a trusted Certificate Authority (e.g., Let’s Encrypt, Sectigo, DigiCert).
- Step 2: Generate a Certificate Signing Request (CSR) on your server.
- Step 3: Submit the CSR to the CA and validate domain ownership as required by the CA’s process.
- Step 4: Download the issued certificate from the CA.
- Step 5: Install the new certificate on your server, configuring it for use with your service.
- Step 6: Restart the affected service to load the new certificate.
4.3 Config or Code Example
Before
# Apache configuration example (showing old certificate path)
SSLCertificateFile /etc/ssl/certs/diginotar_certificate.pem
After
# Apache configuration example (showing new certificate path)
SSLCertificateFile /etc/ssl/certs/new_trusted_certificate.pem
4.4 Security Practices Relevant to This Vulnerability
Regularly review and update your SSL certificates is a key practice. Using a trusted Certificate Authority helps ensure certificate validity. Patch cadence for the web server software is also important.
- Practice 2: Use only certificates from trusted CAs, avoiding self-signed certificates where possible.
4.5 Automation (Optional)
If using Let’s Encrypt, Certbot can automate certificate issuance and renewal.
# Example Certbot command to obtain a new certificate
certbot certonly --webroot -w /var/www/yourdomain -d yourdomain.com
5. Verification / Validation
Confirm the fix by checking that the new certificate is installed and trusted by browsers. Re-run the earlier detection method to verify DigiNotar certificates are no longer present.
- Post-fix check: In your web browser, view the certificate details of the affected service. The “Issued By” field should show a trusted CA other than “DigiNotar”.
- Re-test: Run `openssl s_client -connect yourdomain.com:443 | openssl x509 -noout -issuer` and confirm it does not output “DigiNotar”.
- Smoke test: Verify that users can access the service via HTTPS without browser warnings or errors.
- Monitoring: Monitor web server logs for TLS handshake successes, indicating successful certificate validation.
openssl s_client -connect yourdomain.com:443 | openssl x509 -noout -issuer
# Expected output should show a CA other than DigiNotar
6. Preventive Measures and Monitoring
Update security baselines to require certificates from trusted CAs only. Implement checks in your CI/CD pipeline to validate certificate issuers during deployment, for example using SAST tools.
- Baselines: Update your server hardening baseline or configuration policy to explicitly allow only certificates issued by approved Certificate Authorities.
- Asset and patch process: Review all SSL certificates quarterly, ensuring they are valid and from trusted sources.
7. Risks, Side Effects, and Roll Back
Incorrectly configuring the new certificate can cause service downtime. A roll back plan involves restoring the original certificate files and restarting the service.
- Risk or side effect 2: Changes to SSL configurations can impact other services relying on the same certificates. Mitigation: Document all dependencies and coordinate changes with relevant teams.
- Roll back: 1) Stop the affected service. 2) Restore the original certificate files from backup. 3) Restart the service.
8. References and Resources
- Vendor advisory or bulletin: http://www.nessus.org/u?f3fc8e9a
- NVD or CVE entry: Updated on December 27, 2025