Mutation XSS via foreign-namespace parser differential
Published Aug 23, 2026 · Updated Aug 23, 2026
Mutation XSS in JustHTML 1.13.0 and earlier allows remote attackers to inject active markup through custom sanitization policies. When a SanitizationPolicy preserves SVG, MathML, or raw-text elements, crafted input sanitizes into apparently safe markup whose browser reparse changes the DOM into active HTML. The default sanitize=True path is unaffected; exploitation requires an application to preserve those constructs and a user to render the re-parsed output.
Summary
What happened
Mutation XSS in JustHTML 1.13.0 and earlier allows remote attackers to inject active markup through custom sanitization policies. When a SanitizationPolicy preserves SVG, MathML, or raw-text elements, crafted input sanitizes into apparently safe markup whose browser reparse changes the DOM into active HTML. The default sanitize=True path is unaffected; exploitation requires an application to preserve those constructs and a user to render the re-parsed output.
The record
- CVE
- CVE-2026-5751
- Published
- Aug 23, 2026
- Updated
- Aug 23, 2026
- Vendor
- Emil Stenström
- Product
- JustHTML
- Classifications
- CWE-79, T1203
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 23, 2026CVE publishedPublication date reported by the CVE source.
- Aug 23, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
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- Affected versionversion=0 <1.14.0
What conditions does exploitation require?
What is affected?
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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