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

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 gap.

PoC

marko $ const userCode = "</SCRIPT><script>alert(1)//"; <script> const data = ${JSON.stringify(userCode)}; </script>

Would yield the following:

html <script>const data = "</SCRIPT><script>alert(1)//";</script>

Which is then parsed in any WHATWG-compliant browser as: html <script>const data = "</script> <script>alert(1)//";</script>

Impact

Cross-site scripting. Any Marko template that explicitly interpolates untrusted data inside a <script> or <style> block is affected.

Stored XSS is trivial if the value originates from any persisted user input (username, profile bio, comment body, etc.) that is later embedded in a script tag during rendering. Exploitation yields arbitrary JavaScript execution in the victim's browser, enabling session token theft, account takeover, and arbitrary actions as the victim.

Since the internal escapescript and escapestyle helpers are the framework's designated defense against script/style tag breakout, applications following standard Marko patterns had no obvious reason to add a second layer of sanitization.

This does not affect scripts or hydration state serialized by Marko itself — only templates that explicitly interpolate untrusted values inside a <script> or <style> tag.

Patch

Commit 19d4b37d0 — fix: html script, style, and comment escaping.

diff - const unsafeScriptReg = /<\/script/g; + const unsafeScriptReg = /<\/script/gi;

- const unsafeStyleReg = /<\/style/g; + const unsafeStyleReg = /<\/style/gi;

The same commit also introduced an escapecomment helper and corresponding escape-comment-placeholder.js, hardening HTML comment escaping as a related preventative fix. Test fixtures were added under escape-script-case, escape-style-case, and escape-comment.

Workarounds

Upgrade to the patched release. As a short-term mitigation on affected versions, pre-sanitize any untrusted data before it reaches a template position rendered inside a <script> or <style> tag — e.g. normalize </script, </style, and their mixed-case variants before interpolation, or avoid direct interpolation of untrusted values inside these tags entirely.