SSL Medium Strength Cipher Suites Supported (SWEET32)

Overview

Supporting medium strength cipher suites, specifically those vulnerable to the SWEET32 attack, poses a significant security risk to data transmitted over SSL/TLS. These cipher suites typically use 64-bit block ciphers, which are susceptible to birthday attacks that can decrypt sensitive information, such as session cookies.

• Severity: High

Impact

The continued use of medium strength cipher suites, particularly 3DES and Blowfish, can lead to:

• Data Disclosure: Successful exploitation allows attackers to intercept and decrypt HTTPS traffic, gaining access to sensitive data.
• Loss of Data Integrity: Potential for data tampering and unauthorized actions within a user session.
• Compliance Failures: Non-compliance with security standards and regulations requiring strong encryption practices.

Cause

This vulnerability is primarily caused by:

• Legacy Compatibility: Continued support for older cipher suites to maintain compatibility with legacy systems.
• Misconfiguration: Incorrect server configuration that does not prioritize strong cipher suites.
• Lack of Security Awareness: Insufficient awareness about the risks associated with older, weaker encryption algorithms.

Solution

Implementing strong encryption practices and phasing out vulnerable cipher suites are critical steps to mitigate this vulnerability.

1. Disable Weak Ciphers:
   • Update the server configuration to disable medium strength cipher suites, particularly those using 64-bit block ciphers like 3DES and Blowfish.
2. Configure Preferred Cipher Suites:
   • Explicitly specify stronger cipher suites in your server configuration. For example, prefer AES with key sizes of 128 bits or higher.
   • Use server directives to specify cipher order, ensuring the use of the most robust ciphers first.
3. Regular Security Audits:
   • Conduct regular audits and vulnerability assessments to identify and address configurations that permit weak ciphers.
   • Use tools like SSL Labs' SSL Test to evaluate the server’s SSL configuration and make necessary adjustments.

Examples

Disabling Medium Strength Ciphers in Apache:

Edit your Apache SSL configuration to exclude medium strength ciphers:

SSLProtocol all -SSLv3 -TLSv1 -TLSv1.1
SSLCipherSuite HIGH:!aNULL:!MD5:!3DES
SSLHonorCipherOrder on

Restart Apache to apply the changes:

sudo systemctl restart apache2

Testing Cipher Suite Configuration:

Verify that the server no longer supports medium strength ciphers using the following OpenSSL command:

openssl s_client -connect yourserver.com:443 -cipher 3DES

If properly configured, the connection should fail or not utilize the specified weak cipher.

References

• CVE-2016-2183 SWEET32: Birthday attacks on 64-bit block ciphers in TLS and OpenVPN
• Mozilla Security/Server Side TLS

Additional Resources

• OpenSSL Cipher Suite Configuration Generator