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How to remediate – SSL 64-bit Block Size Cipher Suites Supported (SWEET32)

Contents

1. Introduction

The SSL 64-bit Block Size Cipher Suites Supported vulnerability, also known as SWEET32, affects systems using TLS connections that support weak 64-bit block ciphers. This is a High severity issue because it allows an attacker to potentially decrypt HTTPS cookies and hijack authenticated sessions. Servers supporting these ciphers are at risk, particularly those handling sensitive data like login credentials or financial information. A successful attack could compromise confidentiality of user data.

2. Technical Explanation

The vulnerability stems from the use of block ciphers with a 64-bit block size in TLS cipher suites. These ciphers are susceptible to a ‘birthday’ attack, where an attacker exploits statistical probabilities to detect collisions and recover plaintext data. An attacker needs to be able to send a large number of requests over the same TLS connection to successfully carry out this attack. The vulnerability is tracked as CVE-2016-2183 and CVE-2016-6329.

  • Root cause: Support for 64-bit block ciphers in TLS configurations.
  • Exploit mechanism: An attacker sends numerous requests to the server over a single TLS connection, attempting to find collisions that reveal XORed data between fixed secrets and known plaintext (e.g., HTTPS cookies).
  • Scope: Servers using OpenSSL versions prior to 1.0.2g are affected if configured to allow 64-bit block ciphers. Other TLS implementations may also be vulnerable.

3. Detection and Assessment

You can check for this vulnerability by examining your server’s TLS configuration. A thorough assessment involves using a security scanner.

  • Quick checks: Use the `openssl s_client` command to view supported cipher suites. Look for ciphers including ‘CBC’ with 64-bit keys (e.g., AES64-CBC).
  • Scanning: Nessus plugin ID 92630 can detect this vulnerability, but requires report paranoia as it does not check for request limits.
  • Logs and evidence: Examine TLS negotiation logs for the presence of vulnerable cipher suites being offered by the server.
openssl s_client -connect yourserver.com:443 | openssl x509 -cipherlist

4. Solution / Remediation Steps

The best solution is to reconfigure your application to disable the use of all 64-bit block ciphers. If that isn’t possible, limit the number of requests allowed over a single TLS connection.

4.1 Preparation

  • Roll back plan: Restore from backup if issues occur. Revert any config changes made to TLS settings.

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 cipher suites that use 64-bit block ciphers.
  3. Step 3: Restart the web server 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 mitigate this vulnerability type. Least privilege reduces the impact of a successful attack, while secure defaults prevent weak configurations. A regular patch cadence ensures timely updates and fixes for known vulnerabilities.

  • Practice 1: Least privilege – limit user access to sensitive data and systems.
  • Practice 2: Secure defaults – configure TLS with strong cipher suites by default.

4.5 Automation (Optional)

# Example Ansible task to update SSL configuration
- name: Remove weak ciphers from Apache config
  lineinfile:
    path: /etc/apache2/mods-enabled/ssl.conf
    regexp: '^SSLCipherSuite ALL'
    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 your TLS configuration and verifying that vulnerable ciphers are no longer offered. Perform a basic service smoke test to ensure functionality remains intact.

  • Post-fix check: Run `openssl s_client -connect yourserver.com:443 | openssl x509 -cipherlist` again and confirm that 64-bit ciphers are not listed.
  • Re-test: Re-run the Nessus scan (ID 92630) to verify it no longer reports the vulnerability.
  • Smoke test: Verify you can still access HTTPS websites without errors.
  • Monitoring: Monitor TLS negotiation logs for any unexpected cipher suite usage.
openssl s_client -connect yourserver.com:443 | openssl x509 -cipherlist

6. Preventive Measures and Monitoring

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

  • Baselines: Update your server hardening baseline to require only strong cipher suites.
  • Asset and patch process: Review security patches weekly, prioritizing those addressing TLS vulnerabilities.

7. Risks, Side Effects, and Roll Back

  • Risk or side effect 1: Compatibility issues with legacy systems using outdated TLS protocols. Mitigation: Test thoroughly with known client configurations.
  • Roll back: Restore your server configuration from the pre-change backup. Restart the web server.

8. References and Resources

Updated on December 27, 2025

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