GHSA-4g3v-8h47-v7g6
GHSA-4g3v-8h47-v7g6 is a security vulnerability in astro. O3 Security confirms whether GHSA-4g3v-8h47-v7g6 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Astro: Reflected XSS via unescaped View Transition animation properties
Blast Radius
Weekly download volume for affected packages — a proxy for how broadly this vulnerability is deployed.
astronpmDescription
Summary
Astro's server-side View Transition CSS generator interpolates animation properties into an inline <style> element without escaping them for the CSS and HTML contexts.
An attacker-controlled value passed to an animation property such as duration can contain a </style> sequence, terminate the generated style element, and inject arbitrary HTML or JavaScript.
This is similar to GHSA-8hv8-536x-4wqp, but exploits a different injection point: unescaped View Transition animation values in a server-generated <style> element rather than an unescaped slot name in a hydration template.
Like GHSA-8hv8-536x-4wqp, exploitation requires an application to pass attacker-controlled data to an Astro API. However, the value is subsequently inserted into the HTML response without context-appropriate escaping by Astro.
Details
packages/astro/src/runtime/server/transition.ts
The generated stylesheet is wrapped in a <style> element and marked as HTML-safe:
const css = sheet.toString();
result._metadata.extraHead.push(markHTMLString(`<style>${css}</style>`));
Animation properties are added to the stylesheet without escaping:
if (anim.duration) {
addAnimationProperty(builder, 'animation-duration', toTimeValue(anim.duration));
}
For string values, toTimeValue() returns the input unchanged:
export function toTimeValue(num: number | string) {
return typeof num === 'number' ? num + 'ms' : num;
}
As a result, a duration value containing </style> can escape from the generated style element.
Other TransitionAnimation properties, including easing, direction, delay, fillMode, and name, are serialized by the same animation builder. The following PoC only relies on the official fade() helper and its duration option.
PoC
Using:
[email protected]@astrojs/[email protected]
astro.config.mjs
import node from '@astrojs/node';
import { defineConfig } from 'astro/config';
export default defineConfig({
output: 'server',
adapter: node({ mode: 'standalone' }),
});
src/pages/index.astro
---
import { fade } from 'astro:transitions';
const duration = Astro.url.searchParams.get('duration') ?? '300ms';
---
<html lang="en">
<head>
<meta charset="utf-8" />
<title>PoC</title>
</head>
<body>
<div transition:animate={fade({ duration })}>
Animated content
</div>
</body>
</html>
Payload:
open:
http://localhost:4321/?duration=%3C%2Fstyle%3E%3Cscript%3Ealert(1)%3C%2Fscript%3E%3C!--
The browser interprets </style> as the end of the generated style element and executes the injected script. An alert dialog is displayed when the page is opened.
Impact
An attacker who can control a View Transition animation value can execute arbitrary JavaScript in the origin of the affected Astro application.
The query-based reflected XSS scenario affects on-demand/server-rendered routes, such as:
- projects configured with
output: "server"; - pages using
export const prerender = false; - other server-side data flows that pass attacker-controlled values into a View Transition animation definition.
Successful exploitation may allow access to sensitive page data and authenticated actions available to the victim.
Suggested Fix
Animation values should be serialized using context-appropriate CSS escaping or validation before being added to the generated stylesheet.
Additionally, content inserted into a raw <style> element must not be able to contain an HTML end-tag sequence such as </style>. The final generated CSS should be made safe for the HTML raw-text context before it is passed to markHTMLString().
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | astro | ≥ 2.9.0&&< 7.1.0 | 7.1.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for astro. 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 astro to 7.1.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-4g3v-8h47-v7g6 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-4g3v-8h47-v7g6 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-4g3v-8h47-v7g6. 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-4g3v-8h47-v7g6 in your dependencies?
O3 detects GHSA-4g3v-8h47-v7g6 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.