How to remediate – SSL Cipher Block Chaining Cipher Suites Supported

1. Introduction

The SSL Cipher Block Chaining Cipher Suites Supported vulnerability means a service allows older, less secure methods of encrypting data. These ciphers aren’t fundamentally broken but can be misused and leak information under certain conditions. This affects servers using OpenSSL or similar libraries to handle HTTPS connections. A successful exploit could compromise the confidentiality of data in transit.

2. Technical Explanation

The remote host uses SSL ciphers that operate in Cipher Block Chaining (CBC) mode. CBC mode combines blocks of data during encryption, offering some protection against simple attacks like Electronic Codebook (ECB). However, improper implementation can lead to information leakage. There is no specific CVE associated with simply *supporting* these cipher suites; the risk lies in their configuration and use alongside other vulnerabilities.

• Root cause: The service has not been configured to disable older or weaker CBC cipher suites.
• Exploit mechanism: An attacker could potentially exploit weaknesses in the implementation of CBC mode, especially when combined with padding oracle attacks, to decrypt encrypted data. This requires a specific configuration and is often complex.
• Scope: Systems using OpenSSL versions prior to 1.1.1 are more likely to be affected if not properly configured. Other SSL/TLS libraries may also be vulnerable depending on their implementation of CBC ciphers.

3. Detection and Assessment

Confirming vulnerability involves checking the supported cipher suites. A quick check shows what is enabled, while a thorough scan identifies specific weak configurations.

• Quick checks: Use OpenSSL to list enabled ciphers with openssl s_client -connect : and review the output for CBC-based suites.
• Scanning: Nessus plugin ID cc4a822a can identify systems supporting vulnerable cipher suites, but results should be verified manually.
• Logs and evidence: Examine SSL/TLS negotiation logs for the ciphers used during connections. Look for entries indicating CBC-based suites are being selected.

openssl s_client -connect example.com:443

4. Solution / Remediation Steps

Fixing this issue involves disabling weak or outdated cipher suites and prioritising stronger, more modern options.

4.1 Preparation

• Ensure you have tested the new configuration in a staging environment first. A roll back plan involves restoring the original configuration file and restarting the web server.
• Change windows should be scheduled during off-peak hours with approval from relevant stakeholders.

4.2 Implementation

1. Step 1: Edit your OpenSSL configuration file (usually openssl.cnf or a similar file).
2. Step 2: Remove or comment out any lines enabling CBC cipher suites that are considered weak or outdated.
3. Step 3: Add stronger, more modern cipher suites to the configuration file.
4. Step 4: Restart your web server service to apply the changes.

4.3 Config or Code Example

Before

CipherSuite TLS_CBC_MD5_SHA1

After

# CipherSuite TLS_CBC_MD5_SHA1  (Commented out weak cipher)
CipherSuite TLS_AES_256_GCM_SHA384

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue.

• Practice 1: Least privilege – limit the number of supported cipher suites to only those required for compatibility and security.
• Practice 2: Secure defaults – configure SSL/TLS with strong, modern ciphers by default.
• Practice 3: Patch cadence – Regularly update OpenSSL or your TLS library to benefit from security fixes and improvements.

4.5 Automation (Optional)

Ansible can be used to manage SSL/TLS configuration files at scale.

---
- name: Disable weak cipher suites
  lineinfile:
    path: /etc/ssl/openssl.cnf
    regexp: '^CipherSuite TLS_CBC_MD5_SHA1'
    state: absent
  notify: Restart web server

5. Verification / Validation

Confirm the fix by checking supported ciphers and ensuring older suites are disabled.

• Post-fix check: Run openssl s_client -connect : again and verify that weak CBC cipher suites are no longer listed.
• Re-test: Re-run the Nessus scan (cc4a822a) to confirm the vulnerability is resolved.
• Smoke test: Verify basic HTTPS functionality by accessing a website hosted on the server.
• Monitoring: Monitor SSL/TLS negotiation logs for any unexpected cipher suite selections.

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

6. Preventive Measures and Monitoring

Regularly update security baselines and incorporate checks into your CI/CD pipeline.

• Baselines: Update your security baseline to include a list of approved cipher suites, following industry best practices (for example, CIS benchmarks).
• Pipelines: Add Static Application Security Testing (SAST) tools to your CI/CD pipeline to identify insecure SSL/TLS configurations.
• Asset and patch process: Implement a regular patch review cycle for OpenSSL and other TLS libraries.

7. Risks, Side Effects, and Roll Back

Disabling cipher suites could cause compatibility issues with older clients.

• Risk or side effect 1: Older browsers or applications may not be able to connect if they do not support the remaining cipher suites.
• Risk or side effect 2: Incorrect configuration can lead to service outages.
• Roll back: Restore the original SSL/TLS configuration file and restart the web server service.

8. References and Resources

Link only to sources that match this exact vulnerability.

• Vendor advisory or bulletin: https://www.openssl.org/docs/manmaster/man1/ciphers.html
• NVD or CVE entry: No specific CVE for supporting CBC suites, but research related padding oracle attacks.
• Product or platform documentation relevant to the fix: https://www.openssl.org/~bodo/tls-cbc.txt