How to remediate – SSL/TLS EXPORT_DHE <= 512-bit Export Cipher Suites Supported (...

1. Introduction

The SSL/TLS EXPORT_DHE <= 512-bit Export Cipher Suites Supported vulnerability means a server allows connections using older, weaker encryption methods. This is a security risk because these ciphers can be broken relatively easily, potentially allowing attackers to read sensitive information. Systems running public facing web servers or any service using TLS are usually affected. Impact on confidentiality is likely, with integrity and availability less directly affected.

2. Technical Explanation

The vulnerability occurs when a server negotiates a connection using EXPORT_DHE cipher suites with key sizes of 512 bits or lower. These ciphers were designed for export restrictions but are now considered too weak due to advances in cryptanalysis. An attacker can use a man-in-the-middle attack to downgrade the session to use these weaker ciphers, enabling them to intercept and decrypt the communication. The vulnerability is tracked as CVE-2015-4000.

• Root cause: Support for weak EXPORT_DHE cipher suites with key sizes less than or equal to 512 bits.
• Exploit mechanism: An attacker intercepts the TLS handshake and forces the server to use a vulnerable cipher suite. This allows them to decrypt the traffic using known weaknesses in the EXPORT_DHE algorithm.
• Scope: Servers running OpenSSL, GnuTLS, or other TLS libraries with support for these ciphers are affected. Specific versions depend on the library and its configuration.

3. Detection and Assessment

• Quick checks: Use an online SSL checker like SSL Labs Server Test (https://www.ssllabs.com/ssltest/) to see if weak ciphers are supported.
• Scanning: Nessus plugin ID 74733 can identify this vulnerability. OpenVAS also has relevant checks, but results should be verified.
• Logs and evidence: Examine TLS handshake logs for cipher suite negotiation. Look for EXPORT_DHE suites with key sizes of 512 bits or lower. The exact log location varies by system.

openssl s_client -connect yourserver.com:443 | openssl x509 -text

4. Solution / Remediation Steps

The solution is to reconfigure the service to remove support for EXPORT_DHE cipher suites. This strengthens encryption and prevents attackers from downgrading connections.

4.1 Preparation

• Ensure you have access to the server’s configuration files and understand how to modify them. A roll back plan involves restoring the original configuration file.
• A change window may be needed for production systems, requiring approval from relevant IT teams.

4.2 Implementation

1. Step 1: Edit your server’s TLS configuration file (e.g., Apache httpd.conf, Nginx nginx.conf).
2. Step 2: Remove or disable any lines that explicitly enable EXPORT_DHE cipher suites.
3. Step 3: Restart the web service or TLS-enabled application to apply the changes.

4.3 Config or Code Example

Before

SSLProtocol all -EXPORT

After

SSLProtocol all -SSLv2 -SSLv3 -TLSv1 -TLSv1.1 

4.4 Security Practices Relevant to This Vulnerability

• Practice 1: Least privilege – limit the impact if a service is compromised by restricting access and permissions.
• Practice 2: Secure defaults – configure services with strong encryption settings from the start, avoiding weak ciphers.
• Practice 3: Patch cadence – Regularly update TLS libraries to address known vulnerabilities.

4.5 Automation (Optional)

# Example Ansible task to remove weak ciphers from Apache configuration
- name: Remove weak cipher suites from Apache config
  lineinfile:
    path: /etc/apache2/mods-enabled/ssl.conf
    regexp: '^SSLProtocol all -EXPORT'
    line: 'SSLProtocol all -SSLv2 -SSLv3 -TLSv1 -TLSv1.1'
  notify: Restart Apache

5. Verification / Validation

• Post-fix check: Run `openssl s_client -connect yourserver.com:443 | openssl x509 -text` and confirm that EXPORT_DHE suites are not listed in the supported cipher list.
• Re-test: Use an online SSL checker (SSL Labs Server Test) to verify weak ciphers are no longer advertised.
• Smoke test: Ensure basic HTTPS functionality is still working by accessing your website or service via a web browser.
• Monitoring: Monitor TLS handshake logs for any unexpected cipher suite negotiation patterns.

openssl s_client -connect yourserver.com:443 | openssl x509 -text

6. Preventive Measures and Monitoring

Update security baselines to prevent this issue, add checks in CI/CD pipelines, and establish a regular patch review cycle.

• Baselines: Update your server hardening baseline or policy to explicitly disable weak cipher suites (for example, CIS benchmark).
• Asset and patch process: Implement a regular patch review cycle for TLS libraries and operating systems.

7. Risks, Side Effects, and Roll Back

Removing weak ciphers should not cause compatibility issues with modern browsers. However, older clients may experience problems. A roll back involves restoring the original configuration file.

• Roll back: Restore the original server configuration file and restart the web service or TLS-enabled application.

8. References and Resources

• Vendor advisory or bulletin: https://weakdh.org/
• NVD or CVE entry: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-4000
• Product or platform documentation relevant to the fix: Consult your TLS library’s documentation for specific configuration instructions.