CVE-2026-5751
justhtml before 1.14.0 Mutation XSS via custom sanitization policies
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.
INFO
Published Date :
Aug. 23, 2026, 2:16 p.m.
Last Modified :
Oct. 12, 2026, 4:10 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
Affected Products
The following products are affected by CVE-2026-5751
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
No affected product recoded yet
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | MEDIUM | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 3.1 | MEDIUM | [email protected] | ||||
| CVSS 3.1 | MEDIUM | MITRE-CVE | ||||
| CVSS 4.0 | MEDIUM | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 4.0 | MEDIUM | [email protected] |
Solution
- Update justhtml to version 1.14.0 or newer.
- Ensure custom SanitizationPolicy does not preserve foreign namespaces.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-5751.
| URL | Resource |
|---|---|
| https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-r758-8hxw-4845 | |
| https://www.vulncheck.com/advisories/justhtml-before-mutation-xss-via-custom-sanitization-policies |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-5751 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-5751
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-5751 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-5751 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Translated by [email protected]
Oct. 12, 2026
Action Type Old Value New Value Added Translation Title: justhtml de EmilStenstrom, Description: Las versiones 1.13.0 y anteriores de justhtml contienen una vulnerabilidad de cross-site scripting (mXSS) diferencial de analizador / por mutación al usar una SanitizationPolicy personalizada que preserva espacios de nombres externos (por ejemplo, drop_foreign_namespaces=False con elementos SVG/MathML en lista blanca o contenedores de texto sin formato como <style>). Una entrada especialmente diseñada puede sanearse en un marcado que parece seguro pero que se vuelve inseguro cuando es reanalizado por un navegador o por otro analizador HTML, permitiendo la inyección de marcado. La configuración segura predeterminada (sanitize=True) no se ve afectada. Corregido en 1.14.0. -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Aug. 24, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-5751', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-08-24T19:03:25.322496Z'} -
New CVE Received by [email protected]
Aug. 23, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'EmilStenstrom', 'product': 'justhtml', 'versions': [{'status': 'affected', 'version': '0', 'lessThan': '1.14.0', 'versionType': 'semver'}, {'status': 'unaffected', 'version': '1.14.0', 'versionType': 'semver'}], 'packageURL': 'pkg:pypi/justhtml', 'defaultStatus': 'unaffected'}] Added 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. Added CVSS V4.0 AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X Added CVSS V3.1 AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N Added CWE CWE-79 Added Reference https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-r758-8hxw-4845 Added Reference https://www.vulncheck.com/advisories/justhtml-before-mutation-xss-via-custom-sanitization-policies