Crlf Injection in Laravel with Dynamodb

Crlf Injection occurs when untrusted input containing carriage return (CR, \r) and line feed (\n) characters is reflected into HTTP headers or logs. In a Laravel application that uses DynamoDB as a persistence layer, this can manifest when user-controlled data (e.g., a username, redirect target, or request ID) is written to DynamoDB and later read and echoed into HTTP responses or server logs without proper sanitization.

Consider a DynamoDB table storing user profiles with a user_id (string) and display_name (string). If a Laravel route retrieves a profile and sets a custom header with the display name, an attacker-supplied display_name containing \r\nSet-Cookie: malicious=1 can inject additional headers:

<?php
// Laravel controller using AWS SDK for DynamoDB
use Aws\DynamoDb\DynamoDbClient;

$client = new DynamoDbClient([
    'region'  => 'us-east-1',
    'version' => 'latest',
]);

$userId = $request->input('user_id');
$item = $client->getItem([
    'TableName' => 'profiles',
    'Key' => ['user_id' => ['S' => $userId]],
]);

$displayName = $item['Item']['display_name']['S'] ?? '';
// Vulnerable: user-controlled data used in a header
header('X-Display-Name: ' . $displayName);
echo 'Profile loaded';
?>

If the DynamoDB item contains display_name: "Alice\r\nSet-Cookie: session=attacker", the resulting HTTP response will include an extra Set-Cookie header, potentially enabling session fixation or other header-based attacks. Additionally, if logs include the raw DynamoDB response and CRLF sequences are not sanitized, log injection can occur, which may facilitate log forging or log poisoning attacks.

The risk is compounded when the DynamoDB data is later used in URLs or server-side includes. For example, a redirect that pulls a path from DynamoDB becomes vulnerable if the path contains CRLF sequences:

$path = $item['Item']['redirect_path']['S'] ?? '/';
header('Location: https://app.example.com' . $path);
exit;

An attacker who stores path = "//evil.example.com\r\nSet-Cookie: x=1" can cause the header injection to create a malicious redirect plus an extra header. While DynamoDB itself does not interpret CRLF, the storage and subsequent use in HTTP-facing contexts create the injection surface.

Remediation focuses on input validation, output encoding, and strict separation of data and control headers. For DynamoDB-stored data used in HTTP contexts, treat all retrieved values as untrusted.

1. Validate and sanitize before storage or use. Reject or encode CRLF characters in user input that may be stored in DynamoDB and later used in headers or redirects. Use Laravel’s validation rules to prevent carriage returns and line feeds at the application boundary:

$request->validate([
    'display_name' => 'string|regex:/^[^\r\n]*$/',
    'redirect_path' => 'string|regex:/^[^\r\n]*$/|url',
]);

2. Encode output when using DynamoDB data in headers. Instead of directly concatenating user data into header strings, use PHP’s header() with strict control and avoid dynamic header names. For display purposes, encode newlines and carriage returns:

$safeName = str_replace(["\r", "\n"], '', $displayName);
header('X-Display-Name: ' . $safeName);
// Or use a structured header helper
header(sprintf('X-Display-Name: %s', strtr($displayName, ["\r" => '', "\n" => ''])));

3. Use a redirect helper that enforces safe URLs. When using DynamoDB-stored paths for redirects, resolve against a whitelist of allowed hosts and ensure no CRLF sequences are present:

$allowedHosts = ['app.example.com', 'cdn.example.com'];
$path = $item['Item']['redirect_path']['S'] ?? '/';
// Normalize and validate
$path = preg_replace('/[\r\n]/', '', $path);
if (str_starts_with($path, '//')) {
    // Reject scheme-relative URLs or parse strictly
    $path = '/' . ltrim($path, '/');
}
$host = parse_url($request->url(), PHP_URL_HOST);
if (!in_array($host, $allowedHosts, true)) {
    $path = '/';
}
header('Location: https://' . $host . $path, true, 302);
exit;

4. Log safely. When writing DynamoDB data to application logs, strip or replace CRLF characters to prevent log injection:

$logMessage = sprintf(
    'User %s viewed profile %s',
    $userId,
    preg_replace('/[\r\n]/', '?', $displayName)
);
info($logMessage);

5. Leverage middleware for consistent filtering. Create a Laravel middleware that scrubs incoming data destined for DynamoDB or immediate header use, removing CR and LF characters where not semantically valid:

class SanitizeCrlf
{
    public function handle($request, Closure $next)
    {
        $input = $request->all();
        array_walk_recursive($input, function (&$value) {
            if (is_string($value)) {
                $value = str_replace(["\r", "\n"], '', $value);
            }
        });
        $request->replace($input);
        return $next($request);
    }
}

By combining strict validation, safe output encoding, and context-aware handling, Laravel applications using DynamoDB can mitigate Crlf Injection while preserving the integrity of stored and retrieved data.