How to remediate – SSL Resume With Different Cipher Issue

1. Introduction

The SSL Resume With Different Cipher Issue affects systems allowing renegotiation of SSL sessions with a different cipher suite than originally agreed upon. This can allow an attacker to downgrade encryption strength, potentially exposing sensitive data. Systems using SSL/TLS for secure communication are usually affected, including web servers, email servers and VPN gateways. A successful attack could compromise the confidentiality of communications.

2. Technical Explanation

The vulnerability occurs because the SSL implementation doesn’t properly validate the cipher suite during session resumption. An attacker who intercepts the initial handshake can manipulate the session cache to force a weaker cipher on subsequent connections. There is no known CVE associated with this specific issue, but it relates to weaknesses in TLS protocol handling and improper cipher negotiation. For example, an attacker could sniff the start of an SSL connection to a web server, then replay modified data to cause a resumption using a less secure cipher like RC4.

• Root cause: Insufficient validation of cipher suites during SSL session resumption.
• Exploit mechanism: An attacker intercepts and manipulates session cache entries to force the use of a weaker cipher suite on subsequent connections. This can be achieved by replaying modified handshake data or exploiting vulnerabilities in session management.
• Scope: Affected platforms include those running OpenSSL, GnuTLS, and other SSL/TLS implementations without appropriate safeguards. Specific versions depend on the implementation and patch level.

3. Detection and Assessment

Confirming vulnerability requires checking SSL configuration and testing session resumption behaviour. A quick check involves examining the supported cipher suites. Thorough assessment uses tools to attempt a downgrade attack.

• Quick checks: Use openssl s_client -connect example.com:443 and review the “Cipher suite” line in the output. Look for weak or outdated ciphers enabled.
• Scanning: Nessus plugin ID 69807 may detect this issue as an example. Other scanners with SSL/TLS testing capabilities might also identify it.
• Logs and evidence: Examine web server logs for SSL renegotiation events. Look for unusual cipher suite changes during session resumption.

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

4. Solution / Remediation Steps

Fixing this issue involves disabling SSL resume and prioritising strong cipher suites. Follow these steps carefully.

4.1 Preparation

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

4.2 Implementation

1. Step 1: Disable SSL session resumption in your web server configuration. For Apache, add or modify the line SSLSessionCache disabled to your virtual host file.
2. Step 2: Prioritise strong cipher suites by explicitly defining them in your configuration. Remove any weak or outdated ciphers.
3. Step 3: Restart the web server to apply the changes.

4.3 Config or Code Example

Before

SSLSessionCache shmcb_logs=/var/log/apache2/ssl_session_cache.log timeout=300

After

SSLSessionCache disabled

4.4 Security Practices Relevant to This Vulnerability

Several security practices help prevent this issue. Least privilege reduces the impact of a successful attack. Input validation prevents malicious data from being processed. Safe defaults ensure secure configurations are used by default.

• Practice 1: Implement least privilege principles, limiting access to sensitive configuration files and SSL keys.
• Practice 2: Regularly review and update your SSL/TLS configuration to use strong cipher suites and disable weak protocols.

4.5 Automation (Optional)

# Example Ansible snippet to disable SSL session resumption on Apache servers
- name: Disable SSL Session Resumption
  lineinfile:
    path: /etc/apache2/sites-available/{{ site_name }}.conf
    regexp: '^SSLSessionCache'
    line: 'SSLSessionCache disabled'
  notify: Restart Apache

5. Verification / Validation

Confirm the fix by checking SSL configuration and testing session resumption behaviour again. Verify that weak ciphers are no longer enabled.

• Post-fix check: Run openssl s_client -connect example.com:443 | grep "Cipher suite". The output should not list any weak or outdated cipher suites.
• Re-test: Re-run the earlier detection method (Nessus scan) to confirm that the vulnerability is no longer reported.
• Smoke test: Verify that users can still access secure web pages without errors. Check basic functionality like login and data submission.
• Monitoring: Monitor web server logs for SSL renegotiation events and unusual cipher suite changes.

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

6. Preventive Measures and Monitoring

Update security baselines to include strong SSL/TLS configurations. Implement checks in CI pipelines to prevent weak ciphers from being deployed. Establish a regular patch review cycle.

• Baselines: Update your security baseline or policy (for example, CIS control 8) to require strong cipher suites and disable SSL session resumption.
• Asset and patch process: Implement a monthly patch review cycle for all servers, including security updates related to SSL/TLS.

7. Risks, Side Effects, and Roll Back

Disabling SSL session resumption may slightly increase server load. If issues occur, restore the original configuration file.

• Risk or side effect 1: Disabling SSL session resumption might slightly increase CPU usage on the web server due to more frequent full handshakes.
• Risk or side effect 2: In rare cases, disabling session resumption could cause compatibility issues with older clients.
• Roll back: Restore the original web server configuration file from your backup. Restart the web service.

8. References and Resources

• Vendor advisory or bulletin: [https://httpd.apache.org/security/](https://httpd.apache.org/security/)
• NVD or CVE entry: No specific CVE exists for this exact issue, but research TLS protocol vulnerabilities on the NVD website ([https://nvd.nist.gov/](https://nvd.nist.gov/)).
• Product or platform documentation relevant to the fix: [https://httpd.apache.org/docs/2.4/mod_ssl.html#SSLSessionCache](https://httpd.apache.org/docs/2.4/mod_ssl.html#SSLSessionCache)