# CVE-2026-54570: AngleSharp: HTML5 Spec Compliance: mXSS via annotation-xml HTML Integration Point Bypass

> Attackers can exploit a parser differential in AngleSharp to bypass sanitizers and execute malicious scripts via mutation XSS.

- **Severity:** Medium 6.9 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 18 Aug 2026
- **Updated:** 18 Aug 2026
- **Also known as:** ECHO-dcab-2378-985b, GHSA-pgww-w46g-26qg

## What is CVE-2026-54570?

CVE-2026-54570 is a medium-severity cross-site scripting vulnerability in AngleSharp, published on 18 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

AngleSharp is a .NET library for parsing angle bracket based hyper-texts. Prior to 1.5.0, MathAnnotationXmlElement in AngleSharp/Mathml/Dom/Internal/MathAnnotationXmlElement.cs is not treated as an HTML integration point when its encoding attribute is text/html or application/xhtml+xml, causing Consume in AngleSharp/Html/Parser/HtmlDomBuilder.cs to route tokens through foreign-content parsing instead of HTML parsing. A sanitizer can therefore observe a different DOM from the browser that reparses the serialized output. An attacker can combine this namespace differential with markup-breaking characters in an attribute value so that an element hidden from the sanitizer becomes active script-capable HTML after browser reparse, resulting in mutation cross-site scripting. This issue is fixed in version 1.5.0.

Weakness: CWE-80 (Improper Neutralization of Script-Related HTML Tags in a Web Page)

## Which versions are affected by CVE-2026-54570?

2 open source packages ship an affected version, including AngleSharp.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `AngleSharp` | NuGet | All versions | 1.5.0 |
| `AngleSharp` | NuGet | All versions | 1.5.0 |

## How do I fix CVE-2026-54570?

Upgrade AngleSharp to 1.5.0 or later.

## Is CVE-2026-54570 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-54570 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-54570 and fix it if it is affected.

What it is: CVE-2026-54570 is a medium-severity cross-site scripting vulnerability in AngleSharp, published on 18 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-54570 exists.

Affected packages and fixed versions:
- AngleSharp (NuGet): affected all versions; fixed in 1.5.0
- AngleSharp (NuGet): affected all versions; fixed in 1.5.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-54570/ (Markdown: https://insights.sbomify.com/CVE-2026-54570/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-54570
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-54570
- OSV: https://osv.dev/vulnerability/CVE-2026-54570

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-54570/
Summaries are generated and may contain mistakes; the official description above is the record.