How to remediate – SSL Medium Strength Cipher Suites Supported (SWEET32)

1. Introduction

SSL Medium Strength Cipher Suites Supported (SWEET32) means a service allows encryption using SSL ciphers considered less secure than modern standards. This poses a risk to data confidentiality, as these ciphers are easier to break. Systems offering public-facing services like web servers and email servers are usually affected. Impact on confidentiality is likely if an attacker intercepts encrypted traffic.

2. Technical Explanation

The vulnerability occurs when a server negotiates SSL connections using cipher suites with key lengths between 64 and 112 bits, or the 3DES encryption suite. An attacker can exploit this by intercepting traffic and attempting to decrypt it more easily than with stronger ciphers. The attack is easier if the attacker is on the same network as the target server. This issue is tracked as CVE-2016-2183.

• Root cause: Use of SSL cipher suites offering medium strength encryption.
• Exploit mechanism: An attacker intercepts encrypted traffic and attempts to decrypt it using known weaknesses in the weaker ciphers. For example, an attacker could use a man-in-the-middle attack on a public Wi-Fi network.
• Scope: Servers running OpenSSL versions prior to 1.0.2g are affected. Other SSL/TLS implementations may also be vulnerable if they support similar cipher suites.

3. Detection and Assessment

You can confirm vulnerability by checking the server’s supported cipher suites, or using a network scanner.

• Quick checks: Use openssl s_client -connect : to view the negotiated cipher suite.
• Scanning: Nessus plugin ID 10428 can detect this vulnerability. Other scanners may have similar plugins.
• Logs and evidence: Check server logs for SSL handshake details, looking for use of cipher suites with key lengths less than 112 bits or the 3DES suite.

openssl s_client -connect example.com:443

4. Solution / Remediation Steps

Reconfigure affected applications to disable medium strength ciphers and prefer stronger 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 change window and approval from security teams.

4.2 Implementation

1. Step 1: Edit the server’s SSL/TLS configuration file (e.g., Apache’s httpd.conf, Nginx’s nginx.conf).
2. Step 2: Remove or disable any cipher suites with key lengths less than 112 bits and remove support for 3DES.
3. Step 3: Restart the affected 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.

• Practice 1: Least privilege – limit access to SSL/TLS configuration files to only authorized personnel.
• Practice 2: Secure defaults – configure servers with strong cipher suites by default, rather than relying on weak or outdated settings.
• Practice 3: Patch cadence – Regularly update OpenSSL and other TLS libraries to the latest versions.

4.5 Automation (Optional)

Ansible can automate configuration changes.

---
- name: Disable weak cipher suites in Apache
  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 checking the server’s supported cipher suites again, and performing a service smoke test.

• Post-fix check: Run openssl s_client -connect : and verify that only strong cipher suites are listed.
• Re-test: Re-run the Nessus scan (plugin ID 10428) to confirm the vulnerability is no longer detected.
• Smoke test: Verify that users can still access HTTPS websites without errors.
• Monitoring: Monitor server logs for SSL handshake failures, which could indicate a configuration issue.

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

6. Preventive Measures and Monitoring

Regular security baselines and pipeline checks can help prevent this vulnerability.

• Baselines: Update your server security baseline to require strong cipher suites, based on recommendations from CIS or NIST.
• Asset and patch process: Implement a regular patch cycle for OpenSSL and other TLS libraries, reviewing configuration changes as part of the process.

7. Risks, Side Effects, and Roll Back

Disabling weak cipher suites could cause compatibility issues with older clients.

• Risk or side effect 2: Incorrect configuration can prevent all SSL/TLS connections. Mitigation: Back up the original configuration file and test carefully.
• Roll back: Restore the original server configuration file. Restart the affected service.

8. References and Resources

Links to official advisories and documentation.

• Vendor advisory or bulletin: https://www.openssl.org/blog/blog/2016/08/24/sweet32/
• NVD or CVE entry: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-2183
• Product or platform documentation relevant to the fix: https://sweet32.info