How to remediate – SSL/TLS Anonymous Cipher Suites Supported

1. Introduction

The SSL/TLS Anonymous Cipher Suites Supported vulnerability means a system allows connections without verifying the identity of the connecting client. This weakens security, as attackers can intercept and read sensitive data without being authenticated. It typically affects web servers, email servers, and any service using TLS encryption. A successful exploit could compromise confidentiality, integrity, and availability of data transmitted over these services.

2. Technical Explanation

This vulnerability occurs when a server is configured to accept SSL/TLS cipher suites that do not require client authentication. An attacker can connect to the vulnerable service using an anonymous cipher suite, effectively hiding their identity. The Common Weakness Enumeration (CWE) ID for this issue is 327. For example, an attacker could intercept traffic between a user and a web server without being identified by the server. Affected systems include those running older versions of OpenSSL or other TLS libraries with insecure default configurations.

• Root cause: The server allows cipher suites that do not mandate client authentication during the TLS handshake.
• Exploit mechanism: An attacker initiates a TLS connection, selecting an anonymous cipher suite. The server accepts this connection without requiring a certificate or other form of identification.
• Scope: Web servers (Apache, Nginx, IIS), email servers (Postfix, Sendmail), and any application using TLS/SSL are potentially affected.

3. Detection and Assessment

You can check for this vulnerability by examining the supported cipher suites of a service. A quick check involves connecting to the server and inspecting the TLS configuration. More thorough assessment uses dedicated scanning tools.

• Quick checks: Use OpenSSL to connect to the server and list enabled ciphers: openssl s_client -connect yourserver.example.com:443. Look for cipher suites starting with “ADH” or “eADH”.
• Scanning: Nessus plugin ID 69851 can detect this vulnerability. Qualys SSL Labs also provides a detailed scan of TLS configurations. These are examples only, results may vary.
• Logs and evidence: Server logs may show connections using anonymous cipher suites. Look for entries indicating the use of ciphers without client authentication.

openssl s_client -connect yourserver.example.com:443 | openssl x509 -noout -cipherlist

4. Solution / Remediation Steps

The solution involves reconfiguring the affected application to disable anonymous cipher suites. This ensures that all connections require client authentication, improving security.

4.1 Preparation

• A change window may be needed, depending on service criticality and impact of downtime. Approval from a senior IT administrator is recommended.

4.2 Implementation

1. Step 1: Edit the server’s TLS configuration file (e.g., Apache httpd.conf, Nginx nginx.conf).
2. Step 2: Remove or disable any cipher suites that start with “ADH” or “eADH”.
3. Step 3: Restart the affected service to apply the changes.

4.3 Config or Code Example

Before

SSLCipherSuite ALL

After

SSLCipherSuite ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue. Least privilege limits the impact of a successful exploit, while secure defaults reduce the chance of misconfiguration. Patch cadence ensures timely updates and fixes for known vulnerabilities.

• Practice 1: Implement least privilege to restrict access to sensitive data and services.
• Practice 2: Use secure defaults in TLS configurations, avoiding insecure cipher suites by default.

4.5 Automation (Optional)

Ansible can be used to automate the configuration of TLS settings across multiple servers. This example assumes you have an Ansible inventory and SSH access configured.

---
- hosts: webservers
  tasks:
    - name: Remove anonymous cipher suites from Apache config
      lineinfile:
        path: /etc/apache2/mods-enabled/ssl.conf
        regexp: '^SSLCipherSuite ALL'
        line: 'SSLCipherSuite ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256'
      notify: Restart Apache
  handlers:
    - name: Restart Apache
      service:
        name: apache2
        state: restarted

5. Verification / Validation

• Post-fix check: Run openssl s_client -connect yourserver.example.com:443 | openssl x509 -noout -cipherlist and verify that no cipher suites starting with “ADH” or “eADH” are listed.
• Re-test: Re-run the earlier detection command to confirm the vulnerability is resolved.
• Smoke test: Verify that users can still connect to the service using standard TLS configurations. Test basic web page access or email sending/receiving.
• Monitoring: Monitor server logs for any errors related to TLS connections, which could indicate a configuration issue.

openssl s_client -connect yourserver.example.com:443 | openssl x509 -noout -cipherlist

6. Preventive Measures and Monitoring

Update security baselines to include secure TLS configurations, preventing future misconfigurations. Implement CI/CD pipeline checks to automatically validate TLS settings during deployment. A regular patch review cycle ensures timely updates for known vulnerabilities.

• Baselines: Update your server security baseline or policy to mandate the use of strong cipher suites and disable anonymous ciphers.
• Asset and patch process: Implement a monthly patch review cycle for all servers, including security updates for TLS libraries.

7. Risks, Side Effects, and Roll Back

Changing TLS settings can sometimes cause compatibility issues with older clients. A roll back plan involves restoring the original configuration file and restarting the service.

• Risk or side effect 1: Older clients may not support the new cipher suites, leading to connection errors.
• Risk or side effect 2: Incorrectly configured TLS settings can cause a complete service outage.
• Roll back: Restore the original server configuration file and restart the affected service.

8. References and Resources

• Vendor advisory or bulletin: Check your server vendor’s website for specific TLS configuration guidance.
• NVD or