# CVE-2026-5751: justhtml mutation cross-site scripting in custom sanitization policies

Source: https://junglewise.ai/threats/cve-2026-5751-justhtml-mutation-cross-site-scripting-in-custom-sanitization  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-23. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-5751 · Severity: medium · CVSS 6.1 · Published 2026-08-23

justhtml is a Python library for sanitizing HTML content to prevent security attacks. Versions 1.13.0 and earlier contain a flaw in custom sanitization policies that can allow attackers to inject malicious markup when foreign namespaces (SVG, MathML) or raw-text containers are preserved. The injected code appears safe initially but becomes executable when re-parsed, potentially compromising users who view the content. The default secure configuration is unaffected.

Technologies: [justhtml (PyPI)](https://junglewise.ai/threats/technologies/justhtml). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-5751 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-5751 being exploited in the wild.

**How severe is CVE-2026-5751?** Medium severity, with a CVSS score of 6.1, published 23 August 2026.

**Which products and versions are affected?** justhtml (PyPI) ECOSYSTEM:introduced=1.13.0,fixed=1.14.0.

**Is there a fix for CVE-2026-5751?** Yes. justhtml (PyPI): upgrade to 1.14.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-5751 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-5751?** Check whether you run justhtml (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- justhtml (PyPI): upgrade to 1.14.0 or later

  ```sh
  pip install "justhtml>=1.14.0"
  ```


## Technical details

A mutation XSS (mXSS) vulnerability exists in justhtml's custom sanitization policies when drop_foreign_namespaces=False or when foreign elements/raw-text containers are allowlisted. The vulnerability is a parser-differential attack: specially crafted HTML input sanitizes to markup that appears benign under one parser but becomes malicious when re-parsed by a browser or alternative HTML parser. The attack requires custom policy configuration; the default sanitize=True mode is not vulnerable. Attack vector is network-based with user interaction (viewing content). Fixed in version 1.14.0.

## Affected products

- justhtml justhtml <=1.13.0

## Identifiers

- CVE-2026-5751
- GHSA-r758-8hxw-4845

## Timeline

- 2026-04-05: disclosed: GitHub Security Advisory GHSA-r758-8hxw-4845 published
- 2026-08-23: patched: Fixed in justhtml 1.14.0

## References

- [https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-r758-8hxw-4845](https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-r758-8hxw-4845)
- [https://www.vulncheck.com/advisories/justhtml-before-mutation-xss-via-custom-sanitization-policies](https://www.vulncheck.com/advisories/justhtml-before-mutation-xss-via-custom-sanitization-policies)

## Related threats

- [justhtml uncontrolled recursion DoS via deeply nested HTML](https://junglewise.ai/threats/justhtml-uncontrolled-recursion-dos-via-deeply-nested-html-26591bb6)
- [JustHTML uncontrolled recursion DoS via deeply nested HTML](https://junglewise.ai/threats/cve-2026-9769-justhtml-uncontrolled-recursion-dos-via-deeply-nested-html)
- [justhtml mutation XSS in raw-text element serialization](https://junglewise.ai/threats/cve-2026-8630-justhtml-mutation-xss-in-raw-text-element-serialization)
- [justhtml HTML escaping bypass in Markdown conversion](https://junglewise.ai/threats/cve-2026-8445-justhtml-html-escaping-bypass-in-markdown-conversion)
- [justhtml HTML sanitization bypass leading to XSS](https://junglewise.ai/threats/cve-2026-7808-justhtml-html-sanitization-bypass-leading-to-xss)