Lab 3: Webhook Tester

SSRF in webhook testing functionality

Difficulty: Medium

Lab Overview

This lab demonstrates an SSRF vulnerability in a webhook testing service. The application makes HTTP requests to user-supplied webhook URLs without proper validation, allowing access to internal services.

Objective: Use SSRF to access internal services, cloud metadata, or local files through webhook testing.

Vulnerable PHP Code 
// Handle webhook test request
if (isset($_GET['webhook']) && !empty($_GET['webhook'])) {
    $webhook_url = $_GET['webhook'];
    
    // Vulnerable: No validation of webhook URL
    try {
        $context_options = [
            'http' => [
                'timeout' => 10,
                'user_agent' => 'WebhookTester/1.0',
                'follow_location' => true,
                'max_redirects' => 5
            ]
        ];
        
        if ($method === 'POST') {
            $context_options['http']['method'] = 'POST';
            $context_options['http']['header'] = 'Content-Type: application/json';
            $context_options['http']['content'] = $payload;
        }
        
        $context = stream_context_create($context_options);
        $response = file_get_contents($webhook_url, false, $context);
        
        if ($response !== false) {
            // Display response content
        }
    } catch (Exception $e) {
        // Error handling
    }
}

// Example vulnerable usage:
// ?webhook=https://example.com/webhook
// ?webhook=http://localhost:8080
// ?webhook=file:///etc/passwd
Webhook Tester Demo
Quick Test URLs:
Vulnerability Details
  • Type: Server-Side Request Forgery (SSRF)
  • Severity: High
  • Parameter: webhook
  • Method: GET
  • Issue: Webhook URL testing without validation
Test Payloads

Try these payloads in the webhook parameter:

  • http://localhost:8080 - Local service
  • http://127.0.0.1:3306 - Database port
  • file:///etc/passwd - Local file
  • http://169.254.169.254/ - Cloud metadata
  • http://localhost:22 - SSH port

Example URLs:

  • 3.php?webhook=http://localhost:8080
  • 3.php?webhook=file:///etc/passwd
Quick Test URLs

Click these links to test the vulnerability:

Real-World Attack Scenarios
Mitigation Strategies
  • Validate and whitelist allowed webhook URLs
  • Block private IP ranges and localhost
  • Use URL parsing libraries to validate URLs
  • Implement proper error handling
  • Use outbound proxies with restrictions
  • Disable dangerous protocols (file://, gopher://)
  • Implement request timeouts and size limits
  • Validate webhook payloads