Crlf Injection in Laravel with Hmac Signatures

Crlf Injection occurs when an attacker can inject a carriage return (CR, \r) and line feed (\n) sequence into a header or value that is later reflected in HTTP messages. In Laravel, this risk can intersect with Hmac Signatures when user-controlled data is included in the data that is signed or when signature parameters are reflected in headers or cookies without proper sanitization.

Consider a scenario where a Laravel application constructs a query or header value using user input and then signs that payload using hash_hmac. If the user-controlled string contains \r\n sequences and is placed into a header or a signed cookie value, the injected line break can cause the header to be split. This can lead to response splitting or header injection, even though the signature itself remains valid. The signature does not prevent the injected content from changing message boundaries; it only attests to the integrity of the original byte sequence. The framework does not automatically sanitize inputs before they are used in header construction or cookie setting, so the developer must ensure that user data is sanitized before it becomes part of a header or a signed value that influences message formatting.

Another vector involves the Accept header used in API versioning or content negotiation. If an application uses hash_hmac to sign a concatenation of the Accept header value and a secret, and then later reflects the Accept header into a Set-Cookie or Location header without validation, an attacker-supplied \r\n can inject a new header. Because the signature is computed over the attacker-controlled string, the server may still consider the signature valid, but the resulting message structure is compromised. This is not a flaw in hash_hmac itself, but a failure to treat signed inputs as potentially hostile when they influence protocol-level constructs like headers.

In the context of OWASP API Top 10, this maps closely to the Injection family and can lead to cache poisoning, cross-site scripting via injected headers, or unauthorized redirects. Standard mitigations such as input validation and output encoding are essential, and they should be applied before data is used in headers or cookies, even when a cryptographic signature is present. middleBrick can detect such header-injection patterns during unauthenticated scans by analyzing how user-controlled data flows into response headers and by correlating reflected values with signature-generation logic in OpenAPI specifications.

To remediate Crlf Injection risks when using Hmac Signatures in Laravel, treat all user-controlled data as untrusted before it reaches header construction, cookie setting, or signature computation. Do not rely on the cryptographic integrity of the signature alone to enforce message structure. Use explicit allowlists and strict encoding for any value that may influence HTTP message boundaries.

Below are concrete code examples demonstrating secure handling.

1. Sanitize inputs before using them in headers or cookies

Remove or encode CR and LF characters from any user input that will be placed into headers or cookies. Use a strict allowlist for expected values where possible.

<?php
namespace App\Http\Middleware;

use Closure;
use Illuminate\Http\Request;

class SanitizeHeaderInput
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle(Request $request, Closure $next)
    {
        $raw = $request->input('redirect_url');
        // Remove CR and LF to prevent response splitting
        $safe = preg_replace('/[\r\n]/', '', $raw);
        $request->merge(['redirect_url' => $safe]);

        $response = $next($request);

        // Use the sanitized value safely
        if (! empty($safe)) {
            $response->headers->set('X-Redirect-URL', $safe, false);
        }

        return $response;
    }
}

2. Use hash_hmac safely and avoid concatenating unsanitized user input into header-influencing strings

When signing data, keep user input out of the parts of the message that affect headers. If you must sign a composite value, canonicalize and sanitize before hashing, and do not reflect the signed payload into headers verbatim.

<?php
namespace App\Services;

class SecureHmac
{
    protected string $secret;

    public function __construct()
    {
        $this->secret = config('app.hmac_secret');
    }

    /**
     * Create a signature for a user identifier only.
     * Do not include raw user input that may contain control characters.
     */
    public function signUserId(int $userId): string
    {
        $data = sprintf('user_id:%d', $userId);
        return hash_hmac('sha256', $data, $this->secret);
    }

    /**
     * Verify a signature safely.
     */
    public function verifySignature(int $userId, string $signature): bool
    {
        $expected = $this->signUserId($userId);
        return hash_equals($expected, $signature);
    }
}

3. Validate and encode when setting cookies or Location headers

Use Laravel’s cookie and redirect helpers with strict parameters. Do not pass raw user input to header-producing helpers without validation.

<?php
namespace App\Http\Controllers;

use Illuminate\Http\Request;
use Illuminate\Support\Str;

class ApiController extends Controller
{
    public function redirect(Request $request)
    {
        $url = $request->input('url');
        // Validate format and remove dangerous characters
        if (! filter_var($url, FILTER_VALIDATE_URL)) {
            return response()->json(['error' => 'Invalid URL'], 400);
        }
        $safeUrl = preg_replace('/[\r\n]/', '', $url);

        return redirect()->to($safeUrl);
    }

    public function setTokenCookie(Request $request)
    {
        $token = Str::random(40);
        $request->cookie()->forever('api_token', $token, 0, '/', null, false, true);
        // No user-controlled data reflected into Set-Cookie beyond the token itself
        return response()->json(['status' => 'ok']);
    }
}

These practices reduce the risk of Crlf Injection while preserving the integrity of Hmac Signatures. Combine them with input validation libraries and security-focused middleware, and consider using middleBrick’s scans to verify that user-controlled data does not reach headers or cookies in an unsafe manner.