How to remediate – SSL Anonymous Cipher Suites Supported

1. Introduction

The SSL Anonymous Cipher Suites Supported vulnerability means a service allows connections without verifying who is connecting. This enables attackers to intercept and read sensitive data, like usernames and passwords. It usually affects web servers, email servers, and any application using SSL/TLS encryption. A successful attack could compromise the confidentiality of communications.

2. Technical Explanation

This vulnerability occurs when a server is configured to accept SSL connections using anonymous cipher suites. These ciphers don’t require client certificates, meaning anyone can connect without identification. This allows an attacker to perform a man-in-the-middle attack by intercepting the traffic between the user and the server. CVE-2007-1858 describes this issue. For example, an attacker on the same network could use tools like Wireshark or SSLstrip to capture unencrypted data.

• Root cause: The server allows connections using cipher suites that do not require client authentication.
• Exploit mechanism: An attacker intercepts traffic between a user and the vulnerable server, potentially capturing sensitive information.
• Scope: Web servers (Apache, Nginx, IIS), email servers, and any application utilising SSL/TLS are affected if configured with anonymous cipher suites.

3. Detection and Assessment

You can check for this vulnerability by examining the server’s SSL configuration. A quick check is to use a command-line tool like `openssl s_client`. A thorough method involves using a dedicated security scanner.

• Quick checks: Use `openssl s_client -connect :` and look for cipher suites that include “Anonymous”.
• Scanning: Nessus ID 3a040ada can detect this vulnerability, but results should be verified.
• Logs and evidence: Server logs may show connections using unsupported or weak ciphers. Check SSL/TLS configuration files for enabled anonymous cipher suites.

openssl s_client -connect example.com:443

4. Solution / Remediation Steps

Reconfigure the affected application to disable weak ciphers and require stronger authentication methods. Only use secure SSL/TLS configurations.

4.1 Preparation

• Ensure you have access to the server’s configuration files and understand how to modify them. A roll back plan is to restore the original configuration file.
• Changes may require a short maintenance window, depending on your environment. Approval from a senior administrator might be needed.

4.2 Implementation

1. Step 1: Edit the server’s SSL/TLS configuration file (e.g., Apache’s httpd.conf or Nginx’s nginx.conf).
2. Step 2: Remove any lines that enable anonymous cipher suites.
3. Step 3: Ensure only strong, modern ciphers are enabled.
4. Step 4: Restart the web 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

Using least privilege and secure defaults are important for preventing this issue. Regularly reviewing SSL/TLS configurations can also help identify weak ciphers.

• Practice 1: Least privilege limits the impact if an attacker compromises a connection.
• Practice 2: Secure defaults ensure that strong encryption is used by default, reducing the risk of misconfiguration.

4.5 Automation (Optional)

If using configuration management tools like Ansible, you can automate the removal of anonymous cipher suites.

# Example Ansible task to remove insecure ciphers from Apache config
- name: Remove insecure SSL ciphers
  lineinfile:
    path: /etc/apache2/mods-enabled/ssl.conf
    regexp: '^SSLCipherSuite ALL'
    state: absent
  notify: Restart Apache

5. Verification / Validation

Confirm the fix by rechecking the server’s SSL configuration using `openssl s_client`. Ensure that no anonymous cipher suites are listed. Perform a simple service smoke test to verify functionality.

• Post-fix check: Run `openssl s_client -connect :` and confirm the output does not include “Anonymous” cipher suites.
• Re-test: Re-run the initial scan (Nessus ID 3a040ada) to verify the vulnerability is no longer detected.
• Smoke test: Access a key website page or service function via HTTPS to confirm it works as expected.
• Monitoring: Monitor server logs for any errors related to SSL/TLS connections, which could indicate configuration issues.

openssl s_client -connect example.com:443 | grep "Cipher Suite"

6. Preventive Measures and Monitoring

Update your security baselines to include strong SSL/TLS configurations. Implement automated checks in your CI/CD pipelines to prevent weak ciphers from being deployed. Review patch cycles for relevant updates.

• Baselines: Update a CIS benchmark or internal policy with approved cipher suites.
• Pipelines: Add SAST tools to scan configuration files for insecure settings.
• Asset and patch process: Implement a regular review cycle (e.g., monthly) of server configurations.

7. Risks, Side Effects, and Roll Back

Removing anonymous cipher suites may cause compatibility issues with older clients that do not support modern encryption protocols. The roll back steps are to restore the original configuration file.

• Risk or side effect 2: Incorrectly configured strong ciphers could lead to performance issues.
• Roll back: Restore the original server configuration file and restart the web service.

8. References and Resources

• Vendor advisory or bulletin: http://www.nessus.org/u?3a040ada
• NVD or CVE entry: CVE-2007-1858
• Product or platform documentation relevant to the fix: Consult your web server’s official documentation for SSL/TLS configuration.