EUVD-2026-64654
GHSA-3227-3f97-9f2q
Severity by source
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/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
AC:H reflects the non-default custom policy prerequisite beyond attacker control; scope changes to browser context (S:C) with bounded C:L/I:L XSS impact; UI:R because victim must load the page.
Primary rating from Vendor (VulnCheck).
CVSS VectorVendor: VulnCheck
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/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
Lifecycle Timeline
3Blast Radius
ecosystem impact- 1 pypi packages depend on justhtml (1 direct, 0 indirect)
Ecosystem-wide dependent count for version 1.13.0.
DescriptionCVE.org
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.
AnalysisAI
Mutation XSS (mXSS) in justhtml 1.13.0 and earlier enables markup injection bypass against applications using non-default custom SanitizationPolicy configurations that preserve foreign namespaces. Specially crafted input sanitizes to apparently clean markup that is subsequently reinterpreted as executable script when re-parsed by a browser - a classic parser-differential attack. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires the target application to use justhtml with a custom SanitizationPolicy that explicitly sets drop_foreign_namespaces=False AND either allowlists SVG or MathML elements, or allowlists raw-text containers such as <style>. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 4.0 score of 5.1 (AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N) reflects a network-reachable, unauthenticated attack requiring active user interaction, with impact scoped to the subsequent browser context (SC:L/SI:L) rather than the server itself. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker targeting an application that renders user-supplied HTML through justhtml with a custom foreign-namespace policy (e.g., a rich-text editor supporting SVG) submits a carefully crafted payload - for example, nested SVG or MathML elements with script-bearing attributes - that justhtml's parser evaluates as benign and passes. When another authenticated user's browser loads the sanitized output and its HTML5 parser re-serializes the foreign-namespace subtree, the markup mutates into an active script node, executing attacker-controlled JavaScript in the victim's session context. … |
| Remediation | Upgrade justhtml to version 1.14.0, available via pip (pip install --upgrade justhtml). … Detailed patch versions, workarounds, and compensating controls in full report. |
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Same weakness CWE-79 – Cross-site Scripting (XSS)
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External POC / Exploit Code
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