How to remediate – HTTP Proxy Open gopher:// Request Relaying

1. Introduction

The HTTP Proxy Open gopher:// Request Relaying vulnerability means your proxy server is accepting requests using the older Gopher protocol. This could allow attackers to bypass firewall rules and potentially exploit security flaws in the Gopher protocol itself, gaining access to internal systems. Systems running an HTTP proxy are usually affected, especially those with default configurations or that haven’t been updated recently. A successful attack may compromise confidentiality, integrity, and availability of connected services.

2. Technical Explanation

The vulnerability occurs because the HTTP proxy doesn’t filter out Gopher requests (gopher://). Gopher is an older protocol that predates HTTP and isn’t commonly used today. Attackers can exploit this by sending specially crafted Gopher requests to connect to internal ports, like port 70, or leverage flaws in the Gopher protocol itself. There are no known CVEs specifically for this relaying issue but it’s related to older proxy implementations lacking modern security checks. An attacker could use a tool like `curl` with a gopher URL to attempt connection to an internal service.

• Root cause: The HTTP proxy does not properly validate or reject requests using the gopher:// scheme.
• Exploit mechanism: An attacker sends a Gopher request through the proxy, bypassing firewall rules and potentially connecting to internal services on non-standard ports. For example, `curl gopher://internal.example.com:70`.
• Scope: HTTP proxies running with default configurations or without specific Gopher filtering enabled are affected.

3. Detection and Assessment

You can confirm if your system is vulnerable by checking the proxy configuration and attempting a test connection.

• Quick checks: Check your proxy server’s documentation for settings related to allowed protocols. Look for any options that enable or disable Gopher support.
• Scanning: Nessus plugin ID 16389 can detect this issue, but results should be verified manually.
• Logs and evidence: Examine the proxy logs for requests using the gopher:// scheme. The log file location varies depending on your proxy server (e.g., Squid’s access.log).

curl -v gopher://yourproxyip:70

4. Solution / Remediation Steps

The solution is to reconfigure your proxy server to refuse Gopher requests.

4.1 Preparation

• Ensure you have access to modify the proxy’s configuration file. A rollback plan is to restore the original configuration file and restart the proxy service.
• A change window may be required for production systems; approval from a system administrator is recommended.

4.2 Implementation

1. Step 1: Edit your proxy server’s configuration file (e.g., squid.conf).
2. Step 2: Add a rule to deny Gopher requests. For Squid, add `acl no_gopher proto gopher; http_access deny no_gopher`.
3. Step 3: Save the changes to the configuration file.
4. Step 4: Restart the proxy service to apply the new configuration.

4.3 Config or Code Example

Before

# No specific rules for Gopher protocol

After

acl no_gopher proto gopher;
http_access deny no_gopher

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue.

• Practice 1: Least privilege – Configure the proxy server with only the necessary permissions and access rights to reduce the impact of a potential compromise.
• Practice 2: Input validation – Implement strict input validation to block unsafe or unexpected data, including unsupported protocols like Gopher.

4.5 Automation (Optional)

If you use configuration management tools, you can automate this fix.

# Example Ansible task:
- name: Deny Gopher requests in Squid configuration
  lineinfile:
    path: /etc/squid/squid.conf
    regexp: '^acl no_gopher proto gopher;'
    insertafter: EOF
    line: 'acl no_gopher proto gopher; http_access deny no_gopher'
  notify: Restart Squid

5. Verification / Validation

Confirm the fix by checking the proxy configuration and attempting a test connection again.

• Post-fix check: Check your proxy server’s configuration file to ensure the Gopher denial rule is present.
• Re-test: Run `curl -v gopher://yourproxyip:70` again; you should receive a connection refused or access denied error.
• Monitoring: Monitor proxy logs for any unexpected errors related to Gopher protocol, as an example: `grep gopher /var/log/squid/access.log`.

curl -v gopher://yourproxyip:70 # Expected output: Connection refused or access denied

6. Preventive Measures and Monitoring

Update security baselines and implement checks in your CI/CD pipelines to prevent similar issues.

• Baselines: Update your security baseline to include a rule that explicitly denies Gopher requests for all proxy servers.
• Pipelines: Add static analysis tools (SAST) or configuration scanning tools to your CI/CD pipeline to detect insecure configurations, such as allowing unsupported protocols.
• Asset and patch process: Implement a regular patch review cycle to ensure proxy servers are updated with the latest security fixes.

7. Risks, Side Effects, and Roll Back

The main risk is accidentally blocking legitimate traffic if your configuration is incorrect.

• Risk or side effect 2: Service interruption during proxy restart; schedule maintenance windows when possible.
• Roll back: Restore the original proxy configuration file and restart the service.

8. References and Resources

• Vendor advisory or bulletin: Check your proxy server vendor’s website for specific security recommendations.
• NVD or CVE entry: While no direct CVE exists, research related vulnerabilities in older proxy implementations on the NVD website (https://nvd.nist.gov/).
• Product or platform documentation relevant to the fix: Refer to your proxy server’s official documentation for configuration options and best practices.