CVE-2024-24754

## Impacted Resources bref/src/Event/Http/Psr7Bridge.php:130-168 ## Description When Bref is used with the Event-Driven Function runtime and the handler is a `RequestHandlerInterface`, then the Lambda event is converted to a PSR7 object. During the conversion process, if the request is a MultiPart, each part is parsed and its content added in the `$files` or `$parsedBody` arrays. To do that, the following method is called with as first argument the result array (`$files` or `$parsedBody`), as second argument the part name, and as third argument the part content: ```php /** * Parse a string key like "files[id_cards][jpg][]" and do $array['files']['id_cards']['jpg'][] = $value */ private static function parseKeyAndInsertValueInArray(array &$array, string $key, mixed $value): void { if (! str_contains($key, '[')) { $array[$key] = $value; return; } $parts = explode('[', $key); // files[id_cards][jpg][] => [ 'files', 'id_cards]', 'jpg]', ']' ] $pointer = &$array; foreach ($parts as $k => $part) { if ($k === 0) { $pointer = &$pointer[$part]; continue; } // Skip two special cases: // [[ in the key produces empty string // [test : starts with [ but does not end with ] if ($part === '' || ! str_ends_with($part, ']')) { // Malformed key, we use it "as is" $array[$key] = $value; return; } $part = substr($part, 0, -1); // The last char is a ] => remove it to have the real key if ($part === '') { // [] case $pointer = &$pointer[]; } else { $pointer = &$pointer[$part]; } } $pointer = $value; } ``` The conversion process produces a different output compared to the one of plain PHP when keys ending with and open square bracket (`[`) are used. Let's take for example the following part: ``` ------WebKitFormBoundary Content-Disposition: form-data; name="key0[key1][key2][" value ------WebKitFormBoundary-- ``` In plain PHP it would be converted to `Array( [key0] => Array ( [key1] => Array ( [key2] => value) ) )`, while in Bref it would be converted to `Array( [key0] => Array ( [key1] => Array ( [key2] => ) ) [key0[key1][key2][] => value )`. ## Impact Based on the application logic the difference in the body parsing might lead to vulnerabilities and/or undefined behaviors. ## PoC 1. Create a new Bref project. 2. Create an `index.php` file with the following content: ```php <?php namespace App; require __DIR__ . '/vendor/autoload.php'; use Nyholm\Psr7\Response; use Psr\Http\Message\ResponseInterface; use Psr\Http\Message\ServerRequestInterface; use Psr\Http\Server\RequestHandlerInterface; class MyHttpHandler implements RequestHandlerInterface { public function handle(ServerRequestInterface $request): ResponseInterface { return new Response(200, [], var_export($request->getParsedBody(),true)); } } return new MyHttpHandler(); ``` 3. Use the following `serverless.yml` to deploy the Lambda: ```yaml service: app provider: name: aws region: eu-central-1 plugins: - ./vendor/bref/bref # Exclude files from deployment package: patterns: - '!node_modules/**' - '!tests/**' functions: api: handler: index.php runtime: php-83 events: - httpApi: 'ANY /upload' ``` 4. Replay the following request after having replaced the `<HOST>` placeholder with the deployed Lambda domain: ``` POST /upload HTTP/2 Host: <HOST> Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryQqDeSZSSvmn2rfjb Content-Length: 180 ------WebKitFormBoundaryQqDeSZSSvmn2rfjb Content-Disposition: form-data; name="key0[key1][key2][" value ------WebKitFormBoundaryQqDeSZSSvmn2rfjb-- ``` 5. Notice how the body has been parsed. 6. Create a `plain.php` file with the following content: ```php <?php var_dump($_POST); ``` 7. Start a PHP server inside the project directory (e.g. `php -S 127.0.0.1:8090`). 8. Replay the following request after having replaced the `<HOST>` placeholder with the PHP server address: ``` POST /plain.php HTTP/1.1 Host: <HOST> Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryQqDeSZSSvmn2rfjb Content-Length: 180 ------WebKitFormBoundaryQqDeSZSSvmn2rfjb Content-Disposition: form-data; name="key0[key1][key2][" value ------WebKitFormBoundaryQqDeSZSSvmn2rfjb-- ``` 9. Notice the differences in the parsing compared to what observed at step 5. ## Suggested Remediation Use the PHP function [`parse_str`](https://www.php.net/manual/en/function.parse-str.php) to parse the body parameters to mimic the plain PHP behavior.