Crlf Injection in Laravel with Basic Auth

Crlf Injection occurs when an attacker can inject carriage return (CR, \r) and line feed (\n) sequences into HTTP headers, causing header splitting and potentially allowing response splitting or header injection. In Laravel, this risk can surface in two ways when Basic Auth is involved: during credential handling and when constructing authenticated responses.

First, if your application reads user input (for example via query parameters or request body) and embeds it directly into an HTTP response header after authentication, a malicious user can supply CRLF sequences to terminate the current header and inject new ones. Example: an attacker provides a username like admin%0d%0aX-Injected: malicious in the Authorization header or a related parameter. When Laravel uses that value to build a Location header or similar redirect, the injected CRLF creates a new header, potentially enabling cache poisoning, open redirects, or HTTP response splitting.

Second, in environments behind proxies or load balancers, Basic Auth credentials are often decoded and forwarded. If Laravel then uses parts of the decoded credentials or related headers in unsafe ways (e.g., logging or redirect construction), CRLF Injection may manifest. The framework does not inherently sanitize these values, so unchecked user-controlled data in authenticated contexts can lead to header manipulation.

A concrete scenario: an API endpoint accepts a user-controlled next parameter and, after Basic Auth validation, redirects using return Redirect::to($next). If the attacker sends next=http://example.com%0d%0aSet-Cookie: foo=bar, the injected CRLF can introduce a new header. Even though Basic Auth itself is not the injection vector, the authenticated flow may expose the application to response splitting if user input is not validated before header use.

Because middleBrick tests unauthenticated attack surfaces and checks Data Exposure and Header Injection among 12 parallel security checks, it can identify places where CRLF Injection is possible in combination with authentication flows. The scanner flags risky patterns such as direct use of user input in headers or redirects without sanitization.

Remediation focuses on not trusting user input used in headers or redirects, and ensuring Basic Auth credentials and related data are handled safely. Below are concrete, safe patterns for Laravel.

1. Avoid embedding user input in headers

Never directly use request input (including data derived from Basic Auth credentials or headers) when setting headers or constructing redirects. Use Laravel’s built-in helpers and whitelisting.

<?php
// Unsafe: directly using user input in a redirect
$next = request('next');
return redirect($next); // Risk: CRLF Injection

// Safe: validate and allowlist the target
$allowed = ['https://app.example.com/dashboard', 'https://app.example.com/profile'];
$next = request('next');
if (!in_array($next, $allowed, true)) {
    $next = '/dashboard';
}
return redirect($next);
?>

2. Sanitize and encode when headers must be built

If you must construct headers, strip or encode CRLF characters. For header values, remove \r and \n and reject inputs containing them.

<?php
$username = request('username'); // e.g., could come from Basic Auth decoding in a custom layer
$safeUsername = preg_replace('/[\r\n]/', '', $username);
// Use $safeUsername in logs or non-header contexts; do not use in actual headers with user input
?>

3. Use Laravel’s response methods correctly

When returning responses, prefer framework methods that do not allow header splitting. Avoid manually concatenating header strings.

<?php
// Instead of manually setting Location with user input, use Laravel’s redirect helper safely
return redirect()->intended('/dashboard'); // Uses session-based intended URL, not raw user input

// If you must set custom headers, use the response object and avoid raw user strings
return response('OK')
    ->header('X-Content-Type-Options', 'nosniff')
    ->header('X-Frame-Options', 'DENY');
?>

4. Validate and scope Basic Auth usage

Ensure Basic Auth credentials are only used for authentication and not repurposed as mutable headers or redirect targets. Keep authentication logic separate from output construction.

<?php
// Example middleware check (simplified)
$authHeader = request()->header('Authorization');
if ($authHeader && str_starts_with($authHeader, 'Basic ')) {
    // Decode and validate credentials against your user store
    // Do NOT use the decoded credential strings directly in headers or redirects
    $credentials = base64_decode(substr($authHeader, 6));
    [$user, $pass] = explode(':', $credentials, 2);
    if (!hash_equals($validUser, $user) || !hash_equals($validPass, $pass)) {
        abort(401);
    }
    // Proceed with authenticated request handling, avoiding unsafe header usage
}
?>

These practices reduce the risk of CRLF Injection in authenticated flows. The key is to treat data from Basic Auth and user inputs as untrusted when they influence headers or redirects, and to use allowlists and framework helpers rather than raw concatenation.