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

> Attackers can inject arbitrary JavaScript via case-insensitive closing tags in script or style blocks, leading to cross-site scripting.

- **Severity:** Medium 6.4 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 8 May 2026
- **Updated:** 8 May 2026
- **Also known as:** GHSA-x9fj-57fh-c8wq

## What is CVE-2026-41591?

CVE-2026-41591 is a medium-severity cross-site scripting vulnerability in marko-js marko, published on 8 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official 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.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-41591?

4 open source packages ship an affected version, including @marko/runtime-tags and marko. It affects 2 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `@marko/runtime-tags` | npm | All versions | 6.0.164 |
| `@marko/runtime-tags` | npm | All versions | 6.0.164 |
| `marko` | npm | All versions | 5.38.36 |
| `marko` | npm | All versions | 5.38.36 |

Products: Openjsf Marko, Openjsf Marko\/Runtime-Tags.

## How do I fix CVE-2026-41591?

Upgrade @marko/runtime-tags to 6.0.164 or later.

## Is CVE-2026-41591 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-41591 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-41591 and fix it if it is affected.

What it is: CVE-2026-41591 is a medium-severity cross-site scripting vulnerability in marko-js marko, published on 8 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-41591 exists.

Affected packages and fixed versions:
- @marko/runtime-tags (npm): affected all versions; fixed in 6.0.164
- @marko/runtime-tags (npm): affected all versions; fixed in 6.0.164
- marko (npm): affected all versions; fixed in 5.38.36
- marko (npm): affected all versions; fixed in 5.38.36

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-41591/ (Markdown: https://insights.sbomify.com/CVE-2026-41591/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-41591
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-41591
- OSV: https://osv.dev/vulnerability/CVE-2026-41591

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-41591/
Summaries are generated and may contain mistakes; the official description above is the record.