CVE-2026-41591

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:

// 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

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

Would yield the following:

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

Which is then parsed in any WHATWG-compliant browser as:

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

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.

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 _escape_script and _escape_style 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

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-41591 has a CVSS score of 6.4 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.38.36, 6.0.164); upgrading removes the vulnerable code path.

Kodem intelligence

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

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

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

The same commit also introduced an _escape_comment 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.

1. What is CVE-2026-41591? CVE-2026-41591 is a medium-severity cross-site scripting (XSS) vulnerability in marko (npm), affecting versions < 5.38.36. It is fixed in 5.38.36, 6.0.164. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-41591? CVE-2026-41591 has a CVSS score of 6.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2026-41591?
   • marko (npm) (versions < 5.38.36)
   • @marko/runtime-tags (npm) (versions < 6.0.164)
4. Is there a fix for CVE-2026-41591? Yes. CVE-2026-41591 is fixed in 5.38.36, 6.0.164. Upgrade to this version or later.
5. Is CVE-2026-41591 exploitable, and should I be worried? Whether CVE-2026-41591 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-41591 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-41591?
   • Upgrade marko to 5.38.36 or later
   • Upgrade @marko/runtime-tags to 6.0.164 or later