Astro: XSS in define:vars via incomplete tag sanitization

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS

• CVE-2026-41067: 0.19% estimated exploitation activity in the next 30 days; percentile 8.61 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The defineScriptVars function in Astro's server-side rendering pipeline uses a case-sensitive regex /<\/script/g to sanitize values injected into inline <script tags via the define:vars directive. HTML parsers close <script elements case-insensitively and also accept whitespace or / before the closing , allowing an attacker to bypass the sanitization with payloads like </Script, </script , or </script/ and inject arbitrary HTML/JavaScript. Details The vulnerable function is defineScriptVars at packages/astro/src/runtime/server/render/util.ts:42-53: This function is called from renderElement at util.ts:172-174 when a <script element has define:vars: The regex /<\/script/g fails to match three classes of closing script tags that HTML parsers accept per the HTML specification §13.2.6.4: 1. Case variations: </Script, </SCRIPT, </sCrIpT — HTML tag names are case-insensitive but the regex has no i flag. 2. Whitespace before : </script , </script\t, </script\n — after the tag name,…

Published by the source

21 Apr 2026, 20:39 UTC

Last modified by the source

5 May 2026, 16:07 UTC

Source last checked

26 Oct 2026, 12:51 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-j687-52p2-xcff

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-j687-52p2-xcff

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# Patch Next advisory brief: CVE-2026-41067

Advisory: Astro: XSS in define:vars via incomplete tag sanitization
IDs: CVE-2026-41067, GHSA-j687-52p2-xcff
Record: https://patchnext.com/advisories/cve-2026-41067 (revision 2, source checked 26 Oct 2026, 12:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: astro 6.1.6. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm astro: affected < 6.1.6; fixed in 6.1.6

## Suggested inspection (read-only)
- `npm ls astro --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on astro.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-j687-52p2-xcff (OSV record GHSA-j687-52p2-xcff lists affected ranges: npm astro >=0 <6.1.6 (fixed))
- Verified: https://github.com/advisories/GHSA-j687-52p2-xcff (GitHub-reviewed advisory GHSA-j687-52p2-xcff lists affected ranges: npm astro >=0 <6.1.6)

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