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Web Cache Deception in Symfony: Attack & Prevention

#cybersecurity #vulnerabilities #symfony #webdev

Symfony is known for its elegance and flexibility, but like any web framework, it can be vulnerable to specific security threats—Web Cache Deception (WCD) being a prime example. In this post, we’ll explore how this attack works, why Symfony apps are susceptible, and how to defend against it with real code examples.

Web Cache Deception in Symfony: Attack & Prevention

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🔍 What is a Web Cache Deception Attack?

A Web Cache Deception attack tricks the caching mechanism (like Varnish or a CDN) into storing and then serving sensitive content to unauthorized users.

A basic WCD attack follows this logic:

  1. The attacker appends a fake static file extension (e.g., .css, .jpg) to a dynamic URL.
  2. If the server doesn't validate this properly and returns user-specific data, the cache stores the response.
  3. Subsequent users accessing that URL may see another user’s private data.

⚠️ Why Symfony is Vulnerable

Symfony handles routes and request data dynamically. If your routes don’t enforce strict file-type validation or cache control headers, you're at risk.

Let’s say you have a route like this:

// src/Controller/ProfileController.php

#[Route('/profile', name: 'user_profile')]
public function profile(Request $request): Response
{
    $user = $this->getUser();
    return $this->render('profile/show.html.twig', [
        'user' => $user,
    ]);
}
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A malicious actor might request:

/profile.css
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If Symfony doesn’t return a 404, and instead serves the logged-in user’s profile, and this response is cached (e.g., by Cloudflare), anyone visiting /profile.css later could see that user’s profile.

🧪 Exploiting WCD in Symfony – A Live Example

Here's a simple proof-of-concept using Symfony’s development server and curl:

curl -H "Cookie: PHPSESSID=attacker-session-id" http://localhost:8000/profile.css
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If this serves the authenticated profile page and gets cached, any user visiting /profile.css will see the same page—even without authentication.

✅ How to Prevent Web Cache Deception in Symfony

1. Add Proper Cache-Control Headers

Use response headers to prevent sensitive pages from being cached.

use Symfony\Component\HttpFoundation\Response;

$response = new Response();
$response->headers->set('Cache-Control', 'no-store, no-cache, must-revalidate, max-age=0');
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2. Return 404 for Invalid Static Routes

Reject fake static paths using Symfony route requirements:

#[Route('/profile', name: 'user_profile', requirements: ['_format' => 'html'])]
public function profile(Request $request): Response
{
    // ...
}
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Or explicitly check for file extensions:

if (preg_match('/\.(css|js|png|jpg)$/', $request->getRequestUri())) {
    throw $this->createNotFoundException();
}
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3. Serve Static Assets Separately

Use a dedicated subdomain (e.g., static.example.com) for static files to isolate static from dynamic routes.

📸 Screenshots from Our Free Website Vulnerability Scanner Tool

Use our website vulnerability scanner to detect Web Cache Deception vulnerabilities and more.

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Screenshot of the free tools webpage where you can access security assessment tools.Screenshot of the free tools webpage where you can access security assessment tools.

Screenshot 2: Sample vulnerability report by our tool to check Website Vulnerability

An Example of a vulnerability assessment report generated with our free tool, providing insights into possible vulnerabilities.An Example of a vulnerability assessment report generated with our free tool, providing insights into possible vulnerabilities.

These scans can identify exposed endpoints, improper cache control headers, and potential WCD weaknesses.

🧠 Related Symfony Security Code Example: Force HTTPS and Secure Headers 

// In services.yaml or a response listener
$response->headers->set('Strict-Transport-Security', 'max-age=31536000; includeSubDomains');
$response->headers->set('X-Content-Type-Options', 'nosniff');
$response->headers->set('X-Frame-Options', 'DENY');
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And enable force HTTPS redirect in security.yaml:

access_control:
    - { path: ^/.*, roles: IS_AUTHENTICATED_ANONYMOUSLY, requires_channel: https }
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📘 Read More on Our Blog

Explore more secure coding tutorials on our official blog:
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🧪 Final Tip: Test Your Symfony Site Now!

Ready to see if your app is vulnerable to Web Cache Deception or other threats?

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If you found this guide helpful, consider giving it a ❤️ and sharing it with your dev or security teams!

Top comments (1)

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xwero profile image
david duymelinck •

Not checking if the user is allowed to view the content is the bigger problem. And there is an easy fix.