How to remediate – HTTP Proxy Open Relay Detection

1. Introduction

The HTTP Proxy Open Relay Detection vulnerability identifies web proxy servers that accept unauthenticated HTTP requests. This allows users to route traffic through the proxy, masking their origin IP address and potentially bypassing security controls. Affected systems are typically public-facing web proxies or internal proxies with misconfigured access controls. A successful exploit could lead to a loss of confidentiality as website logs will show requests originating from the proxy server instead of the user’s host.

2. Technical Explanation

The vulnerability occurs when a web proxy is configured to accept connections from any source without authentication. This allows attackers or users to relay HTTP requests through the proxy, effectively hiding their IP address and location. There is no specific CVE associated with this general detection; however, misconfigured proxies are frequently exploited. An attacker could use Nessus (or similar tools) to route a request through the vulnerable proxy when browsing a website, making it appear as if the request originated from the proxy server’s IP address.

• Root cause: Missing or insufficient authentication checks on incoming HTTP requests.
• Exploit mechanism: An attacker sends an HTTP request to the proxy server without providing credentials, and the proxy forwards the request to the destination website.
• Scope: Web proxies running on various platforms (e.g., Linux, Windows) with default or weak configurations are affected.

3. Detection and Assessment

Confirming a vulnerable system involves checking if it accepts unauthenticated HTTP requests. A quick check can be performed using `curl`, while thorough assessment requires scanning tools like Nessus.

• Quick checks: Use `curl` to send an HTTP request through the proxy server without authentication. For example, if the proxy is at 192.168.1.100 and the target website is google.com: curl -x http://192.168.1.100 http://google.com If the request succeeds, the proxy is likely open.
• Scanning: Nessus vulnerability ID 34758 (HTTP Proxy Open Relay Detection) can identify this issue. Other scanners may have similar checks.
• Logs and evidence: Examine proxy server logs for connections from unexpected sources or unauthenticated requests. Log locations vary depending on the proxy software used.

curl -x http://192.168.1.100 http://google.com

4. Solution / Remediation Steps

Fixing this vulnerability requires limiting access to the proxy server to authorized users and hosts.

4.1 Preparation

• Ensure you have a rollback plan in case of issues (restore from backup). A change window may be required for production systems.

4.2 Implementation

1. Step 1: Configure the proxy server to require authentication for all incoming requests. This usually involves enabling user accounts and passwords or IP address whitelisting.
2. Step 2: Restrict access to authorized users or hosts by configuring an Access Control List (ACL).
3. Step 3: Restart the proxy service to apply the changes.

4.3 Config or Code Example

Before

# Sample Squid configuration (insecure)
acl all src 0.0.0.0/0
http_access allow all

After

# Sample Squid configuration (secure)
acl trusted_hosts src 192.168.1.0/24  # Replace with your network
http_access allow trusted_hosts
http_access deny all

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue. Least privilege limits the impact of exploitation, while input validation and secure defaults reduce the attack surface.

• Practice 1: Implement least privilege by granting only necessary access to proxy servers.
• Practice 2: Use strong authentication methods for all users accessing the proxy server.

4.5 Automation (Optional)

If using a configuration management tool, automate the ACL updates and service restarts.

# Example Ansible task to update Squid ACL
- name: Update Squid ACL
  lineinfile:
    path: /etc/squid/squid.conf
    regexp: '^acl all src 0.0.0.0/0$'
    line: 'acl trusted_hosts src {{ proxy_trusted_network }}'
  notify: Restart Squid
- name: Deny access to all other sources
  lineinfile:
    path: /etc/squid/squid.conf
    regexp: '^http_access allow all$'
    line: 'http_access deny all'
  notify: Restart Squid
handlers:
  - name: Restart Squid
    service:
      name: squid
      state: restarted

5. Verification / Validation

Confirm the fix by verifying that unauthenticated requests are no longer accepted and that authorized users can still access the proxy server.

• Post-fix check: Use `curl` to send an HTTP request through the proxy without authentication. The request should now fail with a 407 Proxy Authentication Required error or similar.
• Re-test: Re-run the Nessus scan (ID 34758) and confirm that it no longer detects the vulnerability.
• Smoke test: Verify that authorized users can still access websites through the proxy server without issues.
• Monitoring: Monitor proxy logs for failed authentication attempts or connections from unexpected sources.

curl -x http://192.168.1.100 http://google.com # Expected output: 407 Proxy Authentication Required

6. Preventive Measures and Monitoring

Regularly update security baselines and implement checks in CI/CD pipelines to prevent similar misconfigurations. A sensible patch or config review cycle should be implemented based on the risk assessment.

• Baselines: Update a security baseline (e.g., CIS benchmark) to include proxy server configuration requirements.
• Pipelines: Add checks in CI/CD pipelines to scan for open relay configurations during deployment.
• Asset and patch process: Review proxy server configurations regularly as part of an asset management or change control process.

7. Risks, Side Effects, and Roll Back

• Roll back: Restore the previous proxy server configuration from backup. Restart the proxy service.

8. References and Resources

• Vendor advisory or bulletin: Consult your proxy server vendor’s documentation for specific configuration guidance.
• NVD or CVE entry: This is a general detection, so there isn’t a single CVE. Search NVD for proxy vulnerabilities.
• Product or platform documentation relevant to the fix: Refer to your proxy software documentation (e.g., Squid documentation).