Host Header Injection in Symfony: Risks, Examples & Fixes

Host Header Injection is a critical yet often overlooked vulnerability in modern web applications. In this post, we’ll explore how this issue can affect Symfony-based applications, how attackers can exploit it, and most importantly—how to fix it. We'll also walk through actual code examples and demonstrate how to scan for such vulnerabilities using our Website Vulnerability Scanner online free.

🔐 Stay secure—knowledge is your best defense.

🧠 What is Host Header Injection?

Host Header Injection occurs when user-controlled input is used to generate headers in HTTP requests without proper validation. Symfony and other frameworks often rely on the HTTP Host header for routing and URL generation. If left unchecked, this opens doors for:

• Cache poisoning
• Password reset link manipulation
• Open redirect attacks
• Virtual host routing exploits

⚠️ Real-World Exploitation Example in Symfony

Let's simulate how this works using a basic Symfony controller.

Vulnerable Symfony Code:

Here’s a controller that sends a password reset link:

// src/Controller/ResetPasswordController.php

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\Routing\Generator\UrlGeneratorInterface;

class ResetPasswordController extends AbstractController
{
    public function sendResetLink(Request $request)
    {
        $userEmail = $request->request->get('email');

        // Generate URL using current host header
        $resetUrl = $this->generateUrl('app_reset_password', [
            'token' => 'dummy-token'
        ], UrlGeneratorInterface::ABSOLUTE_URL);

        mail($userEmail, 'Password Reset', "Click here: " . $resetUrl);

        return new Response('Reset link sent.');
    }
}

🧨 Problem: The $resetUrl uses the host from the incoming request. If the attacker changes the Host header, they control the link sent to the user.

Simulated HTTP Request:

POST /reset HTTP/1.1
Host: evil.com
Content-Type: application/x-www-form-urlencoded

email=victim@example.com

The email link will be:

https://evil.com/reset-password?token=dummy-token

This lets attackers intercept token-based flows or impersonate your application.

🛡️ Fixing Host Header Injection in Symfony

You can fix this by explicitly specifying the host when generating URLs and by validating the Host header.

✅ Secure Symfony Code Example:

$resetUrl = $this->generateUrl('app_reset_password', [
    'token' => 'dummy-token'
], UrlGeneratorInterface::ABSOLUTE_URL);

// Force known host
$resetUrl = str_replace(parse_url($resetUrl, PHP_URL_HOST), 'yourdomain.com', $resetUrl);

Alternatively, Symfony allows restricting trusted hosts:

Secure with framework.yaml:

# config/packages/framework.yaml
framework:
    trusted_hosts: ['^www\.yourdomain\.com$', '^yourdomain\.com$']

This way, Symfony will reject requests from any untrusted Host headers.

🧪 Detecting Host Header Injection Automatically

Use our free scanner to detect Host Header Injection vulnerabilities and more.

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📷 Sample Vulnerability Report to check Website Vulnerability

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🧑‍💻 Final Thoughts

Symfony is a robust framework—but like any platform, it’s only as secure as your implementation. Host Header Injection is simple to exploit but easy to mitigate with awareness and good development practices. Always validate headers, use hard-coded trusted domains, and perform routine security scans.

Stay safe, scan smart, and protect your users.

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