CVE-2026-5751
justhtml before 1.14.0 Mutation XSS via custom sanitization policies
- State
- Published
- Severity
- 5.1 Medium
- Published
- Aug 23, 2026
- Updated
- Aug 24, 2026
- Reserved
- Apr 7, 2026
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.
Scores
| CVSS | Score | Vector | Source |
|---|---|---|---|
| 4.0 | 5.1 Medium | 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 | VulnCheck |
| 3.1 | 6.1 Medium | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N | VulnCheck |
Weaknesses
- CWE-79 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Affected Products
| Product | Versions |
|---|---|
justhtmlEmilStenstrom |
|
References
- GitHub Security Advisory (GHSA-r758-8hxw-4845)vendor-advisory
- VulnCheck Advisory: justhtml before 1.14.0 Mutation XSS via custom sanitization policiesthird-party-advisory
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