HtmlSanitizer has a bypass via template tag

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.3 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N · score source

EPSS

• CVE-2026-25543: 0.27% estimated exploitation activity in the next 30 days; percentile 19.84 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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: 1. 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<scriptalert('xss')</script where [...] denotes at least another 509 opening <div tags. 2. If in addition to the template tag, the shadowrootmode attribute is allowed through sanitizer.AllowedAttributes.Add("shadowrootmode");, the simple payload of <div<template shadowrootmode="open"<scriptalert('xss')</script would bypass the sanitizer.…

Published by the source

3 Feb 2026, 19:22 UTC

Last modified by the source

27 Feb 2026, 20:27 UTC

Source last checked

26 Oct 2026, 17:04 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-j92c-7v7g-gj3f

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# Patch Next advisory brief: CVE-2026-25543

Advisory: HtmlSanitizer has a bypass via template tag
IDs: CVE-2026-25543, GHSA-j92c-7v7g-gj3f
Record: https://patchnext.com/advisories/cve-2026-25543 (revision 1, source checked 26 Oct 2026, 17:04 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: htmlsanitizer 9.1.893-beta, 9.0.892. A vendor fix does not mean a deployment is fixed.

## Affected components
- NuGet htmlsanitizer: affected < 9.0.892; >= 9.1.878-beta, < 9.1.893-beta; fixed in 9.0.892, 9.1.893-beta

## Suggested inspection (read-only)
- NuGet htmlsanitizer: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on htmlsanitizer.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-j92c-7v7g-gj3f (GitHub-reviewed advisory GHSA-j92c-7v7g-gj3f lists affected ranges: NuGet htmlsanitizer >=0 =9.1.878-beta <9.1.893-beta)

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