Cache Poisoning in Symfony: Detect & Prevent Easily

#cybersecurity #vulnerabilities #symfony #webdev

Symfony is a powerful and widely-used PHP framework—but with great power comes great responsibility. One such overlooked vulnerability in Symfony-based applications is Cache Poisoning.

In this blog, we’ll explore:

What Cache Poisoning is
How it affects Symfony apps
Code examples showing vulnerable and safe implementations
How to detect it using our website vulnerability scanner online free
Practical mitigation strategies

➡️ Check out other useful posts at Pentest Testing Blog

🔍 What is HTTP Cache Poisoning?

HTTP Cache Poisoning is a web vulnerability where attackers manipulate cacheable responses (like via reverse proxies such as Varnish, CDN, or even Symfony’s HttpCache) to serve malicious or incorrect content to other users.

Example Scenario:

Imagine you cache this response:

GET /profile?id=123 HTTP/1.1
Host: example.com

Response:
200 OK
Content-Type: text/html
Cache-Control: public, max-age=300

Now an attacker crafts:

GET /profile?id=123 HTTP/1.1
Host: example.com
X-Forwarded-Host: attacker.com

If your cache keys include X-Forwarded-Host, and Symfony’s HttpCache is not configured properly, this might poison the cache.

⚙️ Vulnerable Symfony Code Example

Let’s see what a vulnerable Symfony controller might look like:

// src/Controller/ProfileController.php

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;

class ProfileController extends AbstractController
{
    public function profile(Request $request): Response
    {
        $id = $request->query->get('id');
        $response = $this->render('profile/show.html.twig', ['id' => $id]);
        $response->setPublic();
        $response->setMaxAge(300); // 5 minutes

        return $response;
    }
}

👎 This controller does not validate headers like Host, X-Forwarded-Host, or X-Original-URL. That’s where cache poisoning slips in.

✅ Safe Symfony Code with Header Normalization

Here’s how to harden it:

// src/EventSubscriber/TrustedHeaderSubscriber.php

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\RequestEvent;

class TrustedHeaderSubscriber implements EventSubscriberInterface
{
    public static function getSubscribedEvents(): array
    {
        return [
            RequestEvent::class => 'sanitizeHeaders',
        ];
    }

    public function sanitizeHeaders(RequestEvent $event)
    {
        $request = $event->getRequest();

        // Strip suspicious headers
        $request->headers->remove('X-Forwarded-Host');
        $request->headers->remove('X-Original-URL');
        $request->headers->remove('X-Rewrite-URL');
    }
}

This effectively blocks header-based manipulation attempts before they can impact the cache behavior.

🧪 Scan Your Symfony App for Free

Want to find out if your Symfony app is vulnerable to cache poisoning or other threats?

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🔐 Real-World Prevention Tips for Symfony

Here are practical ways to prevent cache poisoning in Symfony:

Normalize headers using an event subscriber
Avoid caching based on user input like query params
Use Symfony's ESI or fragments carefully—validate each fragment separately
Avoid third-party middleware that introduces unvalidated headers
Scan regularly with our tool foe a Website Security test

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🔚 Conclusion

Cache poisoning is a serious yet stealthy threat in Symfony apps. It can lead to content spoofing, account hijacking, and privacy leaks—all without compromising the server directly.

With proper header validation, cautious caching, and regular scanning, you can prevent such exploits from affecting your users.

💬 Have you faced cache poisoning in your project? Let us know in the comments on Dev.to!

Want a free scan? DM me or check https://free.pentesttesting.com