Why does HtmlSanitizer decode '&centerId=2' incorrectly in my .NET app?

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I'm using HtmlSanitizer in my .NET web app to prevent XSS attacks, for example:

CSHARP
var sanitiser = new HtmlSanitizer();
var result = sanitiser.Sanitize(rawText);

However, when the input contains certain sequences starting with an ampersand, the sanitizer decodes them in unexpected ways. For instance:

sanitiser.Sanitize("&pounds=10") returns £s=10
sanitiser.Sanitize("&centerId=2") returns ¢erId=2

I expected the sanitizer to preserve the literal text (i.e. &centerId=2). It appears that portions like &cent are being recognized as valid HTML entities and converted accordingly.

How can I prevent this behavior and ensure the original text remains unchanged?

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

The behavior you’re seeing isn’t a bug in HtmlSanitizer—it’s the expected HTML entity decoding behavior. When you pass a string like "&centerId=2", the sanitizer (and the underlying parser) sees "&cent" as the valid HTML entity for the cent sign (¢) and then leaves the remainder ("erId=2") as literal text.

Here’s what’s happening step‑by‑step:

The string "&pounds=10" is interpreted as "&pounds", which is resolved to the British pound sign (£), followed by "s=10".
The string "&centerId=2" contains "&cent", a valid entity which becomes ¢, then follows "erId=2".

If your intention is to keep the literal text (i.e., not decode the entity), you have a couple of options:

Pre‑encode ampersands:
Instead of passing raw ampersands, encode them as &. For example:

CSHARP
var safeText = "&amp;centerId=2";
var result  = sanitiser.Sanitize(safeText);
// result will now contain "&centerId=2" in the output.

Handle encoding before sanitizing non‑HTML data:
If your input isn’t meant to be HTML, consider encoding it (for example using HttpUtility.HtmlEncode) before running it through the sanitizer, since the sanitizer is intended for HTML content rather than arbitrary text.

By ensuring that ampersands are correctly encoded, you let the sanitizer know that you want to preserve them literally rather than interpret them as the start of an HTML entity.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by LunarPathfinder270 • 1 month ago

It's because it's being parsed as HTML and it gets validly consumed as a legacy HTML entity. I believe that this is intended default behavior from HtmlSanitizer's parser, AngleSharp. Anything you pass in to sanitiser.Sanitize will get parsed as HTML and have its HTML entities consumed.

Before HtmlSanitizer does any sanitization, it parses it as an HTML document using AngleSharp:

CSHARP
/// <summary>
/// Sanitizes the specified HTML body fragment. If a document is given, only the body part will be returned.
/// </summary>
/// <param name="html">The HTML body fragment to sanitize.</param>
/// <param name="baseUrl">The base URL relative URLs are resolved against. No resolution if empty.</param>
/// <returns>The sanitized HTML document.</returns>
public IHtmlDocument SanitizeDom(string html, string baseUrl = "")
{
    var parser = HtmlParserFactory();
    var dom = parser.ParseDocument("<!doctype html><html><body>" + html);

    if (dom.Body != null)
        DoSanitize(dom, dom.Body, baseUrl);

    return dom;
}

When var dom = parser.ParseDocument(...) is called, this is the point when your string gets transformed from &center to ¢er. If you step through the code in a debugger and execute dom.Body.ChildNodes.ToHtml(), you can see that the string is already transformed before the call to DoSanitize happens.

We can see this is also true if we make an HTML snippet with only &center or &pounds as the HTML content - this is just how HTML gets parsed:

&pounds
&center

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According to this answer on Why do HTML entity names with dec < 255 not require semicolon?, it's valid markup to specify these HTML entities (cents, pounds) without a semicolon because their hexadecimal value is less than 256.

Apparently, AngleSharp offers an option IsNotConsumingCharacterReferences that could help us here (discussed here), but that option isn't exposed to us through HtmlSanitizer's API. If it were, you would instead get the output &pounds=10 or &centerId=2, which is still not your expected output. I don't think HtmlSanitizer will work for you in the way you expect it to here.

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