How to remediate – SSL Weak Cipher Suites Supported

1. Introduction

The SSL Weak Cipher Suites Supported vulnerability means a service is allowing older, less secure methods for encrypting data during communication. This makes it easier for attackers to intercept and read sensitive information like usernames, passwords, and financial details. Systems using TLS/SSL are usually affected, including web servers, email servers, and VPN gateways. A successful exploit could compromise the confidentiality of data in transit.

2. Technical Explanation

The issue occurs when a server is configured to accept SSL cipher suites that have known weaknesses. Attackers can use tools to connect to the server and negotiate a connection using these weak ciphers, potentially decrypting the traffic. This is easier if the attacker has network access to the same physical network as the target system. The Common Weakness Enumeration (CWE) identifiers associated with this vulnerability are 326, 327, 720, 753, 803 and 928, 934.

• Root cause: The server’s SSL/TLS configuration includes cipher suites that use weak encryption algorithms or key lengths.
• Exploit mechanism: An attacker uses a tool like OpenSSL to connect to the vulnerable server and force negotiation of a weak cipher suite, allowing them to intercept and potentially decrypt traffic. For example, an attacker could use openssl s_client -connect target.example.com:443 -cipher 'ADH-RC4-MD5' to attempt connection with a weak cipher.
• Scope: Affected platforms include servers running Apache, Nginx, Microsoft IIS and OpenSSL. Any version supporting older SSL/TLS protocols is potentially vulnerable.

3. Detection and Assessment

You can check for this vulnerability by examining the server’s SSL configuration or using a security scanner. A quick check involves inspecting the supported cipher suites.

• Quick checks: Use openssl s_client -connect target.example.com:443 and look at the “Cipher Suite” line in the output. If you see ciphers like RC4, DES, or MD5, it indicates a potential vulnerability.
• Scanning: Nessus plugin ID 6527892d can detect weak cipher suites. Other scanners may have similar checks.
• Logs and evidence: Server logs might show the negotiated cipher suite during SSL/TLS handshakes. Check for ciphers known to be weak.

openssl s_client -connect target.example.com:443

4. Solution / Remediation Steps

The solution is to reconfigure the affected application to disable support for weak cipher suites and prioritize stronger, modern ciphers.

4.1 Preparation

• Ensure you have access to the server’s SSL/TLS configuration files. A roll back plan involves restoring the original configuration file.
• A change window may be needed, depending on the impact of restarting services. Approval from a senior administrator might be required.

4.2 Implementation

1. Step 1: Edit your server’s SSL/TLS configuration file (e.g., Apache’s httpd.conf or Nginx’s nginx.conf).
2. Step 2: Remove any lines that explicitly enable weak cipher suites like RC4, DES, MD5, and older versions of TLS (SSLv3, TLSv1, TLSv1.1).
3. Step 3: Add a line to specify strong cipher suites in the preferred order. For example, in Apache, use SSLCipherSuite ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256.
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

Several security practices can help prevent this issue. Least privilege reduces the impact if exploited, while secure defaults ensure stronger configurations are used by default.

• Practice 1: Implement least privilege principles so that compromised accounts have limited access.
• Practice 2: Use secure defaults for SSL/TLS configuration to avoid weak ciphers and protocols.

4.5 Automation (Optional)

# Example Ansible task to update SSLCipherSuite in Apache config
- name: Update SSLCipherSuite
  lineinfile:
    path: /etc/apache2/mods-enabled/ssl.conf
    regexp: '^SSLCipherSuite'
    line: 'SSLCipherSuite ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256'
  notify: Restart Apache

5. Verification / Validation

Confirm the fix by rechecking the server’s SSL configuration and ensuring that weak ciphers are no longer supported. A service smoke test verifies basic functionality.

• Post-fix check: Run openssl s_client -connect target.example.com:443 again. The output should *not* list any of the previously identified weak cipher suites (RC4, DES, MD5).
• Re-test: Re-run the Nessus scan (plugin ID 6527892d) and confirm that it no longer reports the vulnerability.
• Smoke test: Verify you can still access your website or service using a modern browser.

openssl s_client -connect target.example.com:443

6. Preventive Measures and Monitoring

Regularly update security baselines to include strong cipher suite requirements. Implement checks in your CI/CD pipelines to prevent weak configurations.

• Baselines: Update your server hardening baseline or CIS control settings to enforce the use of strong SSL/TLS ciphers and protocols.
• Pipelines: Add a SAST scan step to your deployment pipeline that flags any insecure SSL/TLS configuration in code or infrastructure as code templates.
• Asset and patch process: Review server configurations regularly, at least quarterly, for compliance with security standards.

7. Risks, Side Effects, and Roll Back

Changing SSL/TLS settings can sometimes cause compatibility issues with older clients. Always have a roll back plan.

• Risk or side effect 1: Older browsers or applications might not support the stronger cipher suites, leading to connection errors.
• Risk or side effect 2: Incorrect configuration could disrupt SSL/TLS connections for all users.
• Roll back: Restore the original server configuration file and restart the web service.

8. References and Resources

Link