Astro: Reflected XSS via unescaped View Transition animation properties
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:
astro@7.0.9@astrojs/node@11.0.2
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 software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | astro | >= 2.9.0, < 7.1.0 | 7.1.0 |
References
- CVSS vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N- Weakness (CWE)
CWE-79- Platform
- npm
- Package
astro- Disclosed
- 20.07.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-4g3v-8h47-v7g6 - CC BY 4.0
Vulnerability history: astro
All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 27
- First → latest
- 10.2024 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- High 6
- Medium 15
- Low 5
Disclosures per year
- 2024: 3
- 2025: 11
- 2026: 13
-
Astro: Reflected XSS via unescaped View Transition animation properties this entry
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