How to remediate – SSL RC4 Cipher Suites Supported (Bar Mitzvah)

1. Introduction

The SSL RC4 Cipher Suites Supported vulnerability, also known as Bar Mitzvah, means a service allows connections using the outdated and insecure RC4 encryption method. This matters because RC4 has known weaknesses that attackers can exploit to decrypt sensitive data like cookies or login details. Systems offering TLS/SSL are usually affected, including web servers, email servers, and VPN gateways. A successful attack could compromise confidentiality of transmitted data.

2. Technical Explanation

The RC4 cipher is flawed in how it generates random numbers, creating predictable patterns. If an attacker can capture enough encrypted traffic using RC4, they may be able to recover the original plaintext. The vulnerability occurs when a server or client negotiates a cipher suite that includes RC4 during TLS/SSL handshake. CVE-2013-2566 and CVE-2015-2808 describe this issue.

• Root cause: Support for the insecure RC4 cipher in negotiated TLS/SSL connections.
• Exploit mechanism: An attacker intercepts encrypted traffic, repeatedly encrypts data with RC4, and uses statistical analysis to derive the plaintext key stream. For example, an attacker could intercept HTTP cookies transmitted over a vulnerable connection.
• Scope: Web servers (Apache, Nginx, IIS), email servers, VPN gateways, any service using TLS/SSL where RC4 cipher suites are enabled.

3. Detection and Assessment

You can check for this vulnerability by examining the supported cipher suites of a service. A thorough method involves scanning with a security tool.

• Quick checks: Use openssl s_client -connect example.com:443 and look for RC4 in the “Cipher Suite” list.
• Scanning: Nessus plugin ID ac7327a0 can detect this vulnerability. Other scanners may have similar checks.
• Logs and evidence: Check TLS/SSL handshake logs for cipher suites negotiated during connections. Look for entries containing “RC4”.

openssl s_client -connect example.com:443

4. Solution / Remediation Steps

The best solution is to disable RC4 ciphers in your application’s TLS/SSL configuration and upgrade to stronger algorithms like AES-GCM.

4.1 Preparation

• Change windows may be needed during peak hours; approval from a security team lead might be required.

4.2 Implementation

1. Step 1: Identify the configuration file for your web server (e.g., Apache’s httpd.conf, Nginx’s nginx.conf).
2. Step 2: Edit the configuration file to remove RC4 cipher suites from the list of supported ciphers.
3. Step 3: Restart the web server to apply the changes.

4.3 Config or Code Example

Before

SSLCipherSuite DEFAULT:!EXP:!SSLv2

After

SSLCipherSuite ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:TLS_AES_128_GCM_SHA256:TLS_AES_256_GCM_SHA384

4.4 Security Practices Relevant to This Vulnerability

Several security practices help prevent this type of vulnerability.

• Practice 1: Least privilege – limit the services that use TLS/SSL and the data they access, reducing potential impact if exploited.
• Practice 2: Secure defaults – configure new systems with strong cipher suites enabled by default, disabling weak algorithms like RC4.
• Practice 3: Patch cadence – Regularly update software to address known vulnerabilities in TLS/SSL libraries.

4.5 Automation (Optional)

# Example Ansible task to remove RC4 ciphers from Apache configuration
- name: Remove RC4 cipher suites from Apache
  lineinfile:
    path: /etc/apache2/mods-enabled/ssl.conf
    regexp: '^SSLCipherSuite.*DEFAULT'
    line: 'SSLCipherSuite ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:TLS_AES_128_GCM_SHA256:TLS_AES_256_GCM_SHA384'
  notify: Restart Apache

5. Verification / Validation

Confirm the fix by checking that RC4 is no longer supported and verifying basic service functionality.

• Post-fix check: Run openssl s_client -connect example.com:443 again; the output should *not* list any cipher suites containing “RC4”.
• Re-test: Re-run the initial scan (Nessus plugin ac7327a0) to confirm the vulnerability is resolved.
• Smoke test: Verify that you can still access your website or service via HTTPS without errors.
• Monitoring: Check TLS/SSL handshake logs for any unexpected cipher suites being negotiated.

openssl s_client -connect example.com:443

6. Preventive Measures and Monitoring

Update security baselines and add checks to your CI pipelines.

• Baselines: Update your security baseline or policy to explicitly prohibit the use of RC4 cipher suites (for example, a CIS control).
• Asset and patch process: Review and apply security patches for TLS/SSL libraries on a regular schedule (e.g., monthly or quarterly).

7. Risks, Side Effects, and Roll Back

Disabling RC4 could cause compatibility issues with older browsers or clients.

• Roll back: Restore the original TLS/SSL configuration file and restart the service.

8. References and Resources

Link only to sources that match this exact vulnerability.

• Vendor advisory or bulletin: https://www.rc4nomore.com/
• NVD or CVE entry: CVE-2013-2566
• Product or platform documentation relevant to the fix: http://www.isg.rhul.ac.uk/tls/