CVE-2026-25543 — HtmlSanitizer
Fix: mganss/HtmlSanitizer@0ac53dcCVE-2026-25543 is a CWE-116 vulnerability in HtmlSanitizer. A fix is available for HtmlSanitizer — see the affected versions and patch details below.
HtmlSanitizer has a bypass via template tag
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-25543.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
Real-World Exposure
HtmlSanitizer.NETHtmlSanitizerReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.
Description
Impact
If the template tag is allowed, its contents are not sanitized. The template tag is a special tag that does not usually render its contents, unless the shadowrootmode attribute is set to open or closed.
The lack of sanitization of the template tag brings up two bypasses:
- it is still possible to forcibly render the contents of a
<template>tag through mutation XSS. The DOM parsers in browsers such as Chromium have a node depth limit of 512 and tags which are beyond that depth are flattened. This in turn allows elements within<template>(which are not sanitized) to be effectively 'popped out'. An example would look like this:<div>[...]<template><script>alert('xss')</script>where[...]denotes at least another 509 opening<div>tags. - If in addition to the template tag, the
shadowrootmodeattribute is allowed throughsanitizer.AllowedAttributes.Add("shadowrootmode");, the simple payload of<div><template shadowrootmode="open"><script>alert('xss')</script>would bypass the sanitizer. This is because such usage of<template>attaches a shadow root to its parent:<div>, and its contents will be rendered.
Note that the default configuration is not affected because the template tag is disallowed by default.
Patches
The problem has been patched in versions 9.0.892 and 9.1.893-beta.
Workarounds
Disallow the template tag. It is disallowed by default.
Resources
https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/template
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | HtmlSanitizer | all versions | 9.0.892dotnet add package HtmlSanitizer --version 9.0.892 |
| .NETNuGet | HtmlSanitizer | ≥ 9.1.878-beta&&< 9.1.893-beta | 9.1.893-betadotnet add package HtmlSanitizer --version 9.1.893-beta |
Affected Products
htmlsanitizerhtmlsanitizer_projectDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for HtmlSanitizer, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update HtmlSanitizer to 9.0.892 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-25543 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
Frequently Asked Questions
Is CVE-2026-25543 in your dependencies?
Find it across NuGet, including transitive dependencies.