CRLF Injection in Laravel: Complete Guide with Examples

CRLF Injection in Laravel is a critical yet often overlooked security risk. Modern developers need to understand, detect, and patch CRLF Injection vulnerabilities to protect user data and infrastructure. In this guide, we’ll break down what CRLF Injection is, demonstrate how it affects Laravel apps, share real-world coding examples, and show you how to fix it.

We’ll also explore automated scanning for Website Security tests, link to advanced resources, and highlight our exclusive AI-powered cybersecurity solutions.

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What is CRLF Injection in Laravel?

CRLF Injection in Laravel is a security vulnerability where malicious input allows attackers to inject unexpected carriage return (CR, \r) and line feed (LF, \n) characters into HTTP headers or responses. This can lead to serious issues like HTTP response splitting, web cache poisoning, or even cross-site scripting (XSS) in some cases.

Why is CRLF Injection in Laravel dangerous?

• Attackers can manipulate HTTP headers.
• May cause users to be redirected to malicious sites.
• Can poison server or intermediary caches.
• Allows for phishing or data leakage.

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How CRLF Injection Works

CRLF Injection in Laravel typically occurs when user input is not sanitized before being included in HTTP headers or other sensitive output. Attackers exploit this by inserting %0d%0a (the URL-encoded form of CRLF) to break header structure.

Example payload:

/download?file=report.txt%0d%0aSet-Cookie:%20crlf=attack

If mishandled, this could force the server to set a malicious cookie or manipulate HTTP headers.

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Common Causes in Laravel Applications

• Using header() with unsanitized input
• Dynamic redirects without validation
• Writing user input directly to HTTP headers
• Insecure download or export endpoints

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Real-World CRLF Injection Examples in Laravel

Let’s see some practical, human-written Laravel code examples:

1. Vulnerable Laravel Controller Example

// routes/web.php
Route::get('/download', 'FileController@download');

// app/Http/Controllers/FileController.php
public function download(Request $request)
{
    $filename = $request->input('file');
    // ❌ Vulnerable to CRLF Injection in Laravel!
    return response()->download(storage_path('app/' . $filename));
}

How an attacker could exploit:
Suppose a user sends file=report.txt%0d%0aSet-Cookie:%20hacked=1. This would inject a new HTTP header.

2. Vulnerable Header Injection Example

public function customHeader(Request $request)
{
    $custom = $request->input('custom-header');
    // ❌ Directly inserting user input
    header("X-Custom-Header: $custom");
    return response("Header set");
}

Attack input:
?custom-header=abc%0d%0aX-Injected-Header:%20attack

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Detecting CRLF Injection in Laravel

To detect CRLF Injection in Laravel, look for:

• User input used in HTTP headers without validation
• Logs showing unusual header structures
• Responses containing unexpected headers

Coding Example: Logging Input Usage

public function safeDownload(Request $request)
{
    $filename = $request->input('file');
    \Log::info('User requested file:', ['filename' => $filename]);
    // ...rest of code
}

Pro tip: Always log user inputs related to headers or file downloads.

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Automated CRLF Vulnerability Assessment (With Free Tool)

Before resorting to manual fixes, consider using an automated vulnerability scanner.
Visit our Free Website Vulnerability Scanner to quickly identify CRLF Injection and other vulnerabilities in your Laravel app.

Screenshot of the Website Vulnerability Scanner tool webpage:

Try scanning your site with this tool—it’s fast, secure, and 100% free.

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Screenshot of a sample vulnerability report to check Website Vulnerability:

You’ll get a comprehensive report showing exactly where CRLF Injection in Laravel might be present.

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How to Fix CRLF Injection in Laravel

A. Sanitize User Input

Always filter user input before using it in headers or redirects.

use Illuminate\Support\Str;

public function downloadSafe(Request $request)
{
    $filename = $request->input('file');
    // ✅ Allow only safe filenames
    if (!preg_match('/^[a-zA-Z0-9_\-.]+$/', $filename)) {
        abort(400, "Invalid file name.");
    }
    return response()->download(storage_path('app/' . $filename));
}

B. Use Laravel Response Helpers

Instead of raw header(), use Laravel’s built-in helpers.

return response($content)
    ->header('X-Safe-Header', Str::of($custom)->replace(["\r", "\n"], ''));

C. Remove CR and LF Characters

function sanitizeHeaderValue($value)
{
    return str_replace(["\r", "\n"], '', $value);
}

// Usage in controller
$headerValue = sanitizeHeaderValue($request->input('header'));
return response('OK')->header('X-Custom', $headerValue);

D. Example: Secure Custom Redirect

public function redirectTo(Request $request)
{
    $url = $request->input('url');
    // Validate the URL to avoid CRLF Injection in Laravel
    if (!filter_var($url, FILTER_VALIDATE_URL)) {
        abort(400, "Invalid URL.");
    }
    return redirect()->away($url);
}

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Other Useful Resources

• SQL injection attack mitigation in WordPress
• Top 7 WebSocket Vulnerabilities in Laravel
• Prevent XSSI Attack in Laravel
• WebSocket Vulnerabilities in React JS
• Preventing SQL Injection (SQLi) in Symfony

Explore these topics for a broader perspective on modern web security.

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Conclusion

CRLF Injection in Laravel is a serious yet preventable threat. By understanding the attack, scanning your application, and applying the best coding practices, you can secure your Laravel apps.
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Stay safe—secure your Laravel apps today!

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