How to remediate – HTTP Proxy CONNECT Request Relaying

1. Introduction

The HTTP Proxy CONNECT Request Relaying vulnerability allows an HTTP proxy server to be used for establishing interactive sessions with third-party sites. This can allow attackers to bypass firewalls and connect to sensitive ports, potentially exposing internal networks to attacks. Affected systems are typically those running HTTP proxy services, such as those used for web filtering or caching. This issue may impact confidentiality, integrity, and availability by allowing unauthorized access to internal resources.

2. Technical Explanation

The vulnerability occurs when the proxy server does not properly restrict CONNECT requests. A CONNECT request is an HTTP method used to establish a tunnel through the proxy to a destination host. If the proxy allows unrestricted CONNECT requests, attackers can use it as a gateway to bypass firewall rules and connect directly to internal services or external sites on non-standard ports. For example, an attacker could initiate a connection to CONNECT http://cvs.example.org:23, potentially establishing a telnet session with the target host.

• Root cause: The proxy server does not validate or restrict CONNECT requests, allowing connections to arbitrary hosts and ports.
• Exploit mechanism: An attacker sends a malicious CONNECT request through the vulnerable proxy server to reach an internal service or external site they shouldn’t have access to.
• Scope: HTTP proxy servers running on various platforms are affected. Specific versions depend on the proxy software implementation.

3. Detection and Assessment

To confirm vulnerability, check if the proxy server allows CONNECT requests to arbitrary hosts. A thorough method involves attempting a connection through the proxy to a known internal service or external site on a non-standard port.

• Quick checks: Check the proxy configuration file for restrictions on CONNECT requests.
• Scanning: Nessus plugin ID 10856 can be used to detect this vulnerability as an example only.
• Logs and evidence: Examine proxy logs for successful CONNECT requests to unexpected hosts or ports. Look for entries containing the “CONNECT” method.

curl -v --proxy http://your-proxy-server:8080 http://internal-service:23

4. Solution / Remediation Steps

Reconfigure your proxy server to refuse CONNECT requests. This is the most effective way to mitigate this vulnerability.

4.1 Preparation

• Ensure you have access to modify the proxy server’s configuration. A roll back plan is to restore the original configuration file and restart the service.
• A change window may be required depending on your environment, with approval from IT security.

4.2 Implementation

1. Step 1: Open the proxy server’s configuration file in a text editor.
2. Step 2: Locate the section that handles CONNECT requests.
3. Step 3: Add or modify the configuration to explicitly deny all CONNECT requests. The exact syntax will vary depending on the proxy software. For example, in Squid you might add “deny connect all”.
4. Step 4: Save the configuration file.
5. Step 5: Restart the proxy service for the changes to take effect.

4.3 Config or Code Example

Before

# No specific restrictions on CONNECT requests

After

deny connect all

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this type of vulnerability. Least privilege reduces the impact if exploited, and input validation blocks unsafe data. Safe defaults ensure that services are configured securely out-of-the-box.

• Practice 1: Implement least privilege by restricting proxy access to authorized users and networks.
• Practice 2: Use input validation to block malicious requests or connections.

4.5 Automation (Optional)

# Example Ansible task to deny CONNECT requests in Squid configuration file
- name: Deny CONNECT requests in Squid config
  lineinfile:
    path: /etc/squid/squid.conf
    regexp: '^deny connect'
    line: 'deny connect all'
    state: present

5. Verification / Validation

Confirm the fix by attempting a CONNECT request through the proxy server to an internal service or external site on a non-standard port. The connection should be refused.

• Post-fix check: Attempt a CONNECT request and verify that it is blocked. Expected output will vary depending on the client, but should indicate a connection refusal.
• Re-test: Re-run the detection command from Section 3 to confirm that the vulnerability is no longer present.
• Smoke test: Verify that standard web browsing functionality still works through the proxy server.
• Monitoring: Monitor proxy logs for any unexpected CONNECT requests or connection attempts.

curl -v --proxy http://your-proxy-server:8080 http://internal-service:23

6. Preventive Measures and Monitoring

Update security baselines to include restrictions on CONNECT requests. Implement checks in CI/CD pipelines to prevent insecure proxy configurations from being deployed. Establish a regular patch or configuration review cycle to identify and address potential vulnerabilities. For example, regularly review CIS benchmarks for relevant settings.

• Baselines: Update your security baseline to require explicit denial of CONNECT requests on all proxy servers.
• Pipelines: Add checks in CI/CD pipelines to scan proxy configurations for insecure settings.
• Asset and patch process: Implement a monthly review cycle for proxy server configurations.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 1: Some applications may stop working if they require CONNECT requests.
• Risk or side effect 2: Users may experience connectivity issues with certain services.
• Roll back:
  1. Step 1: Restore the original proxy configuration file from backup.
  2. Step 2: Restart the proxy service.

8. References and Resources

• Vendor advisory or bulletin: No specific vendor advisory available at this time.
• NVD or CVE entry: No specific CVE entry available at this time.
• Product or platform documentation relevant to the fix: Squid Configuration Examples.