Marko: XSS via case-insensitive script/style closing tag bypass in runtime HTML escaping

Exploitation

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

CVSS

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

EPSS

• CVE-2026-41591: 0.23% estimated exploitation activity in the next 30 days; percentile 13.47 (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 When dynamic text is interpolated into a <script or <style tag the Marko runtime failed to prevent tag breakout when the closing tag used non-lowercase casing. An attacker able to place input inside a <script or <style block could break out of the tag with </SCRIPT, </Style, etc. and inject arbitrary HTML/JavaScript, resulting in cross-site scripting. Details The affected helpers used case-sensitive regular expressions to detect attempts at closing the surrounding tag: HTML tag names are case-insensitive in the browser parser, so inputs such as </SCRIPT, </Script, or </sTyLe were not matched by these regexes and passed through the helpers unchanged. A browser rendering the output treats the mixed-case end tag as a valid closing tag, terminating the script or style context, and then parses anything that follows as HTML. The Marko compiler routes interpolated values inside <script and <style tags through these helpers automatically (see native-tag.ts:1080-1085), so application…

Published by the source

22 Apr 2026, 19:55 UTC

Last modified by the source

13 May 2026, 13:54 UTC

Source last checked

26 Oct 2026, 12:52 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-x9fj-57fh-c8wq

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-x9fj-57fh-c8wq

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# Patch Next advisory brief: CVE-2026-41591

Advisory: Marko: XSS via case-insensitive script/style closing tag bypass in runtime HTML escaping
IDs: CVE-2026-41591, GHSA-x9fj-57fh-c8wq
Record: https://patchnext.com/advisories/cve-2026-41591 (revision 2, source checked 26 Oct 2026, 12:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @marko/runtime-tags 6.0.164; marko 5.38.36. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @marko/runtime-tags: affected < 6.0.164; fixed in 6.0.164
- npm marko: affected < 5.38.36; fixed in 5.38.36

## Suggested inspection (read-only)
- `npm ls @marko/runtime-tags --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- `npm ls marko --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 @marko/runtime-tags, marko.
- 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-x9fj-57fh-c8wq (OSV record GHSA-x9fj-57fh-c8wq lists affected ranges: npm marko >=0 =0 <6.0.164 (fixed))
- Verified: https://github.com/advisories/GHSA-x9fj-57fh-c8wq (GitHub-reviewed advisory GHSA-x9fj-57fh-c8wq lists affected ranges: npm marko >=0 =0 <6.0.164)

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