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

Quick assessment

Affected
EmilStenstrom justhtml
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

EmilStenstrom JustHTML是EmilStenstrom个人开发者开源的一个纯 Python HTML5 解析器。 EmilStenstrom JustHTML 1.13.0及之前版本存在跨站脚本漏洞，该漏洞源于使用自定义SanitizationPolicy保留外来命名空间时存在解析器差异/突变跨站脚本漏洞，特制输入可清洗为看似安全但被浏览器或其他HTML解析器重新解析时不安全的标记，导致标记注入。

CVSS 6.1 · Medium EPSS 0.26% · P17

Affected Version Matrix 2

VendorProduct Version RangeStatus
EmilStenstrom justhtml < 1.14.0 affected
1.14.0 unaffected
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I. Basic Information for CVE-2026-5751

Vulnerability Information
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Vulnerability Title
justhtml before 1.14.0 Mutation XSS via custom sanitization policies
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
在Web页面生成时对输入的转义处理不恰当(跨站脚本)
Source: CVE Program / CVE List V5
Vulnerability Title
EmilStenstrom JustHTML 跨站脚本漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
EmilStenstrom JustHTML是EmilStenstrom个人开发者开源的一个纯 Python HTML5 解析器。 EmilStenstrom JustHTML 1.13.0及之前版本存在跨站脚本漏洞，该漏洞源于使用自定义SanitizationPolicy保留外来命名空间时存在解析器差异/突变跨站脚本漏洞，特制输入可清洗为看似安全但被浏览器或其他HTML解析器重新解析时不安全的标记，导致标记注入。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
EmilStenstrom justhtml 0 ~ 1.14.0 -

II. Public POCs for CVE-2026-5751

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III. Intelligence Information for CVE-2026-5751

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Vendor Advisories for CVE-2026-5751 (2)

Same Patch Batch · EmilStenstrom · 2026-08-23 · 11 CVEs total

CVE-2026-7808 9.8 CRITICAL justhtml before 1.16.0 Multiple Security Issues via Sanitization
CVE-2026-8445 9.8 CRITICAL justhtml before 1.12.0 Sanitizer Bypass via Markdown
CVE-2026-5388 9.8 CRITICAL justhtml before 1.15.0 Multiple Security Issues
CVE-2026-9769 7.5 HIGH justhtml before 1.10.0 Denial of Service via deeply nested HTML
CVE-2026-4671 7.5 HIGH justhtml before 1.18.0 Denial of Service via CSS Selector
CVE-2026-77088 6.1 MEDIUM justhtml 0.9.0 through 1.21.0 Cross-Site Scripting via code-span
CVE-2026-74793 6.1 MEDIUM justhtml before 3.11.0 XSS via selectedcontent projection
CVE-2026-6827 6.1 MEDIUM justhtml before 1.17.0 Multiple Cross-Site Scripting Vulnerabilities
CVE-2026-8630 6.1 MEDIUM justhtml before 1.12.0 Mutation XSS via Raw Text Elements
CVE-2026-5389 6.1 MEDIUM justhtml before 1.13.0 XSS via code fence breakout

IV. Related Vulnerabilities

Same product: justhtml
Same vendor: EmilStenstrom
Same weakness: CWE-79