fkie_cve-2026-41591

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.

GHSA-x9fj-57fh-c8wq — Marko: XSS via case-insensitive script/style closing tag bypass in runtime HTML escaping

Affected: npm: marko, npm: @marko/runtime-tags

### 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:

```js // packages/runtime-tags/src/html/content.ts const unsafeScriptReg = /<\/script/g; const unsafeStyleReg = /<\/style/g;

// packages/runtime-class/src/runtime/html/helpers/escape-script-placeholder.js const unsafeCharsReg = /<\/script/g;

// packages/runtime-class/src/runtime/html/helpers/escape-style-placeholder.js const unsafeCharsReg = /<\/style/g; ```

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 code following the framework's conventions had no way to detect or compensate for the ga…

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fkie_cve-2026-41591

GHSA-x9fj-57fh-c8wq — Marko: XSS via case-insensitive script/style closing tag bypass in runtime HTML escaping