MEDIUM

GHSA-4g3v-8h47-v7g6

Astro: Reflected XSS via unescaped View Transition animation properties

Quick fix

GHSA-4g3v-8h47-v7g6 — astro: upgrade to the fixed version with the command below.

npm install astro@7.1.0

Details

## 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.

## 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()`.

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Affected packages

npm / astro

Introduced in: 2.9.0 Fixed in: 7.1.0

Fix npm install astro@7.1.0

Details

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Affected packages

References