How to remediate – SSL Certificate Information

1. Introduction

The SSL Certificate Information vulnerability involves displaying details about an SSL certificate installed on a system. This is generally informational but can reveal sensitive data like expiry dates and certificate authorities, potentially aiding reconnaissance by attackers. Systems using SSL/TLS for secure communication are affected, including web servers, email servers, and VPN gateways. A successful information disclosure could compromise confidentiality.

2. Technical Explanation

This vulnerability occurs because a plugin attempts to extract the X.509 certificate from any SSL-related port on a system. This is not an exploitable flaw in itself but provides information that can be used in other attacks. There are no specific CVEs associated with simply displaying certificate information, however, misconfigured certificates or weak cipher suites often accompany this issue. An attacker could use the displayed certificate details to identify the web server software and version, then search for known vulnerabilities affecting that specific configuration.

• Root cause: The plugin connects to SSL ports without proper access controls or filtering.
• Exploit mechanism: An attacker passively observes the information provided by the plugin. They use this data in further reconnaissance efforts.
• Scope: Any system running an SSL/TLS service is potentially affected, including Apache, Nginx, IIS, and OpenSSL-based services.

3. Detection and Assessment

Confirming exposure involves checking if the plugin is accessible and displays certificate information. A thorough method would involve scanning all SSL ports for exposed certificates.

• Quick checks: Access the system via HTTPS and inspect the browser’s certificate details.
• Scanning: Nessus plugin ID 10423 can identify SSL certificate information. This is an example only.
• Logs and evidence: Web server access logs may show requests to HTTPS ports, indicating potential exposure.

openssl s_client -connect yourdomain.com:443

4. Solution / Remediation Steps

The primary solution is to restrict access to the plugin or disable it if not required. If needed, ensure only authorised personnel can access certificate information.

4.1 Preparation

• Ensure you have a rollback plan to restore the original configuration. A change window may be required, depending on your environment.

4.2 Implementation

1. Step 1: Identify the process or application hosting the vulnerable plugin.
2. Step 2: Restrict access to the plugin using firewall rules or authentication mechanisms.
3. Step 3: If the plugin is not essential, disable it completely.

4.3 Config or Code Example

Before

# Allow all access to plugin URL
Allow from 192.168.1.0/24
Allow from any

After

# Restrict access to specific IP addresses only
Allow from 10.0.0.0/16
Deny from all

4.4 Security Practices Relevant to This Vulnerability

Least privilege and secure configuration are relevant practices. Least privilege limits the impact if certificate information is exposed, while a secure configuration prevents unnecessary exposure of sensitive data.

• Practice 1: Implement least privilege access controls to limit who can view SSL certificates.
• Practice 2: Regularly review server configurations and disable unused services or plugins.

4.5 Automation (Optional)

# Example Bash script to block access via firewall (adjust for your system)
#!/bin/bash
ufw deny from any to any port 443 comment "Block SSL plugin access"
ufw enable

5. Verification / Validation

Confirm the fix by verifying that unauthorised users can no longer access the certificate information. Re-run the earlier detection method to confirm it is blocked.

• Post-fix check: Attempt to access the plugin URL from an unapproved IP address and verify a connection error or access denied message.
• Re-test: Run `openssl s_client -connect yourdomain.com:443` from an unauthorised network segment and confirm no certificate information is displayed.
• Smoke test: Verify that authorised users can still access the intended SSL/TLS services.
• Monitoring: Monitor web server logs for blocked requests to the plugin URL as a regression check.

openssl s_client -connect yourdomain.com:443 # Should return connection error if blocked

6. Preventive Measures and Monitoring

Regular security baselines and vulnerability scanning can help prevent this issue. For example, update a security baseline to include restrictions on plugin access or implement regular scans for exposed SSL certificates.

• Baselines: Update your server hardening baseline to restrict access to sensitive plugins.
• Pipelines: Include SAST/SCA tools in CI pipelines to identify vulnerable components and configurations.
• Asset and patch process: Review system configurations regularly, at least quarterly, for unnecessary exposures.

7. Risks, Side Effects, and Roll Back

Restricting access may disrupt legitimate users if not configured correctly. Rolling back involves restoring the original configuration or re-enabling the plugin with its previous settings.

• Risk or side effect 2: Service disruption if the plugin is essential and incorrectly disabled. Mitigation: Have a rollback plan in place.
• Roll back: Restore the original configuration files or re-enable the plugin through its management interface.

8. References and Resources

• Vendor advisory or bulletin: N/A – This is a configuration issue, not a specific vendor flaw.
• NVD or CVE entry: N/A – No specific CVE for displaying certificate information.
• Product or platform documentation relevant to the fix: Refer to your web server’s documentation on access control and firewall configuration (e.g., Apache Access Control).