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

• Severity: medium (CVSS 6.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-18 · updated 2026-09-18
• Weakness: CWE-80

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.

Affected products

• AngleSharp: before 1.5.0

References

• https://github.com/AngleSharp/AngleSharp/commit/8033a5c690af9c2c443f58e34875343fa4cc1d07
• https://github.com/AngleSharp/AngleSharp/releases/tag/1.5.0
• https://github.com/AngleSharp/AngleSharp/security/advisories/GHSA-pgww-w46g-26qg
• https://github.com/AngleSharp/AngleSharp/security/advisories/GHSA-pgww-w46g-26qg

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