AngleSharp HTML5 Spec Compliance: mXSS via annotation-xml HTML Integration Point Bypass

Exploitation

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

CVSS

6.9 · CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:H/A:N

EPSS

• CVE-2026-54570: 0.33% estimated exploitation activity in the next 30 days; percentile 26.27 (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.

Summary The HTML specification requires that a MathML <annotation-xml element with encoding="text/html" or encoding="application/xhtml+xml" is treated as an HTML integration point. Content inside it must be parsed as HTML, not MathML. AngleSharp does not implement this correctly. As a result, the parser produces a DOM tree that differs from what a browser will build (different namespaces if encoding="text/html" is not treated) when given the same serialized output. Two bugs combine to make this exploitable: - Missing HtmlTip flag: MathAnnotationXmlElement is never assigned NodeFlags.HtmlTip based on its encoding attribute, so the Consume() dispatch always routes tokens to Foreign() instead of Home() (HTML mode). - Unescaped < in attribute values: HtmlMarkupFormatter.WriteAttributeValue() does not escape < or characters, only & and ". This allows injected markup to break out of attribute values on re-parse. See Escape "<" and "" in attributes when serializing HTML #6235 Details In…

Published by the source

17 Jul 2026, 21:17 UTC

Last modified by the source

17 Jul 2026, 21:17 UTC

Source last checked

26 Oct 2026, 17:05 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-pgww-w46g-26qg

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# Patch Next advisory brief: CVE-2026-54570

Advisory: AngleSharp HTML5 Spec Compliance: mXSS via annotation-xml HTML Integration Point Bypass
IDs: CVE-2026-54570, GHSA-pgww-w46g-26qg
Record: https://patchnext.com/advisories/cve-2026-54570 (revision 1, source checked 26 Oct 2026, 17:05 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: anglesharp 1.5.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- NuGet anglesharp: affected < 1.5.0; fixed in 1.5.0

## Suggested inspection (read-only)
- NuGet anglesharp: 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 anglesharp.
- 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-pgww-w46g-26qg (GitHub-reviewed advisory GHSA-pgww-w46g-26qg lists affected ranges: NuGet anglesharp >=0 <1.5.0)

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