EUVD-2026-64655
GHSA-f8m6-5gph-fw7f
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 mandatory non-default configuration (custom SVG/MathML policy); scope changed because XSS impacts victim browser separately from server.
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
3DescriptionCVE.org
justhtml before 1.17.0 contains multiple security issues in sanitization, serialization, and programmatic DOM handling. When custom policies preserve foreign namespaces (SVG/MathML), dangerous content such as HTML integration points (SVG <foreignObject>, MathML <annotation-xml encoding="text/html">) and mutation-XSS parser-differential payloads could survive sanitization and become active HTML after reparse; SVG filter="url(...)" and preserved <style> could leave resource-loading CSS (@import, background-image:url()). Programmatic script/style/Comment nodes could serialize into active markup. Additional hardening fixes address sanitize-pipeline cache mutation and DOM parent/child cycles that could cause infinite loops. Most issues affect advanced or custom configurations rather than the default JustHTML(..., sanitize=True) safe path.
AnalysisAI
Sanitization bypass in justhtml (Python HTML sanitization library) before version 1.17.0 enables cross-site scripting in applications using custom policies that preserve SVG or MathML namespaces, custom policies that preserve <style> blocks, or programmatic DOM construction prior to serialization. Crafted HTML input containing mutation-XSS parser-differential payloads, HTML integration points such as SVG <foreignObject> or MathML <annotation-xml encoding="text/html">, or resource-loading CSS constructs can survive sanitization and activate as executable markup after browser reparse. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation of the primary XSS vectors requires the target application to use justhtml with a custom sanitization policy that explicitly preserves SVG or MathML foreign namespace elements (overriding the default deny behavior), or to preserve <style> elements for untrusted HTML input. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N, score 5.1) reflects a network-reachable XSS affecting only the subsequent system (end-user browser), with user interaction required (UI:A) and no direct impact on the server-side vulnerable component (VC/VI/VA all N). … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker submits a crafted HTML fragment containing a mutation-XSS payload that uses an SVG <foreignObject> or MathML <annotation-xml encoding="text/html"> integration point to an application that calls justhtml with a custom policy permitting foreign namespaces (for example, a content platform allowing rich media embeds). The payload appears inert in justhtml's internal DOM and passes sanitization, but when the application renders the sanitized output in a user's browser, the HTML5 parser re-enters HTML content model at the integration point boundary, activating embedded JavaScript. … |
| Remediation | Upgrade justhtml to version 1.17.0, released April 19, 2026, which fully addresses all reported sanitization bypass, serialization, and DOM handling issues. … Detailed patch versions, workarounds, and compensating controls in full report. |
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Cross-site scripting in justhtml versions before 1.13.0 allows attackers who control `<pre>` element content to break ou
Same weakness CWE-79 – Cross-site Scripting (XSS)
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External POC / Exploit Code
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