How to remediate – SSL/TLS Insecure Cipher Suites Supported

1. Introduction

The SSL/TLS Insecure Cipher Suites Supported vulnerability means a server is using older, weaker methods for encrypting data sent over the internet. This can allow attackers to intercept and read sensitive information like usernames, passwords, and financial details. Systems that handle web traffic – such as web servers, email servers, and VPN gateways – are usually affected. A successful exploit could compromise confidentiality of data in transit.

2. Technical Explanation

This vulnerability occurs when a server is configured to accept SSL/TLS ciphers that use export-grade encryption or have key lengths less than 128 bits. These ciphers are considered insecure because they can be broken with relatively simple attacks. An attacker could exploit this by using tools like Wireshark to intercept the encrypted traffic and decrypt it, potentially gaining access to sensitive data. The vulnerability is identified as CWE-326.

• Root cause: The server’s SSL/TLS configuration allows weak ciphers.
• Exploit mechanism: An attacker uses a man-in-the-middle attack to intercept encrypted traffic and decrypt it using the insecure cipher suites supported by the server. For example, an attacker could use tools like `openssl s_client` with specific cipher suite options to connect to the vulnerable server and negotiate a weak connection.
• Scope: Affected platforms include web servers (Apache, Nginx, IIS), email servers, VPN gateways, and any other service using SSL/TLS.

3. Detection and Assessment

You can check if your system is vulnerable by examining its SSL/TLS configuration. A quick check involves using a command-line tool to see the supported ciphers. For a thorough assessment, use an online SSL testing service.

• Quick checks: Use `openssl s_client -connect :` and review the cipher suite list presented in the output. Look for ciphers with names like ‘EXPORT’ or key sizes below 128 bits.
• Scanning: Qualys SSL Labs (ssltest.qualystest.com) provides a detailed analysis of SSL/TLS configurations, including supported ciphers. Nessus plugin ID 39604 can also identify this issue.
• Logs and evidence: Server logs may show the negotiated cipher suite during an SSL/TLS handshake. Check web server access logs for details on the TLS version and cipher used in connections.

openssl s_client -connect example.com:443

4. Solution / Remediation Steps

To fix this issue, reconfigure your application to disable insecure ciphers and prioritize strong encryption algorithms.

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.
• Changes may require a short maintenance window, depending on the application and its dependencies. Approval from relevant IT teams might be needed.

4.2 Implementation

1. Step 1: Identify the SSL/TLS configuration file for your web server (e.g., Apache’s `httpd.conf` or Nginx’s `nginx.conf`).
2. Step 2: Edit the configuration file to remove insecure ciphers from the allowed cipher suite list.
3. Step 3: Prioritize strong, modern ciphers such as TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 or TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256.
4. Step 4: Restart the web server to apply the changes.

4.3 Config or Code Example

Before

SSLCipherSuite DEFAULT

After

SSLCipherSuite ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:TLS_AES_128_CIPHER_SUITE

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. Secure defaults ensure that systems are configured with strong settings from the start.

• Practice 1: Implement least privilege principles, limiting access to sensitive configuration files and services.
• Practice 2: Use secure defaults for SSL/TLS configurations, disabling weak ciphers and enabling only strong encryption algorithms.

4.5 Automation (Optional)

# Example Ansible snippet to update SSL/TLS configuration on multiple servers
- name: Update SSL cipher suites
  lineinfile:
    path: /etc/nginx/nginx.conf
    regexp: 'SSLCipherSuite DEFAULT'
    line: 'SSLCipherSuite ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:TLS_AES_128_CIPHER_SUITE'
  become: true

5. Verification / Validation

• Post-fix check: Run `openssl s_client -connect :` again. The output should *not* include any ciphers with ‘EXPORT’ or key sizes below 128 bits.
• Re-test: Re-run the Qualys SSL Labs test to confirm that the server now receives a passing grade and does not report any insecure cipher suites.
• Smoke test: Verify that users can still access the website or service without issues, confirming that the changes did not break functionality.
• Monitoring: Monitor web server logs for TLS version and cipher suite usage to ensure strong ciphers are being negotiated.

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

6. Preventive Measures and Monitoring

Regular security baselines and automated checks can help prevent this issue from recurring. A consistent patch process ensures that systems are up-to-date with the latest security fixes.

• Baselines: Update your server’s SSL/TLS baseline configuration to include only strong ciphers, following industry best practices (e.g., CIS benchmarks).
• Asset and patch process: Implement a regular patch review cycle for all servers, ensuring that SSL/TLS libraries are updated promptly.

7. Risks, Side Effects, and Roll Back

Changing the SSL/TLS configuration can sometimes cause compatibility issues with older clients. Always have a roll back plan in place.

• Risk or side effect 1: Older browsers or applications may not support the new cipher suites, leading to connection errors.
• Risk or side effect 2: Incorrectly configured ciphers can cause service outages.
• Roll back: Restore the original SSL/TLS configuration file and restart the server. If necessary, revert any changes made through automation tools.

8. References and Resources

Link only to sources