CVE-2026-41591

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

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.

Affected

marko-js · marko

CVSS v3.1 GITHUB

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

EPSS

0.27% chance of exploitation in the next 30 days, 17th percentile

Published

2026-05-08

Proof-of-concept exploits (1)

• advisories/GHSA-x9fj-57fh-c8wq

Security advisories

• marko-js/marko/security/advisories/GHSA-x9fj-57fh-c8wq

References

• CVE Record
• NVD entry