HtmlSanitizer is a .NET library for cleaning HTML fragments and documents from constructs that can lead to XSS attacks. Prior to versions 9.0.892 and 9.1.893-beta, if the template tag is allowed, its contents are not sanitized. The template tag is a special tag that does not usually render its contents, unless the shadowrootmode attribute is set to open or closed. This issue has been patched in versions 9.0.892 and 9.1.893-beta.

Affected: NuGet: HtmlSanitizer

### Impact

If the `template` tag is allowed, its contents are not sanitized. The `template` tag is a special tag that does not usually render its contents, unless the `shadowrootmode` attribute is set to `open` or `closed`.

The lack of sanitization of the template tag brings up two bypasses:

1. it is still possible to forcibly render the contents of a `<template>` tag through mutation XSS. The DOM parsers in browsers such as Chromium have a node depth limit of 512 and tags which are beyond that depth are flattened. This in turn allows elements within `<template>` (which are not sanitized) to be effectively 'popped out'. An example would look like this: `<div>[...]<template><script>alert('xss')</script>` where `[...]` denotes at least another 509 opening `<div>` tags. 2. If in addition to the template tag, the `shadowrootmode` attribute is allowed through `sanitizer.AllowedAttributes.Add("shadowrootmode");`, the simple payload of `<div><template shadowrootmode="open"><script>alert('xss')</script>` would bypass the sanitizer. This is because such usage of `<template>` attaches a shadow root to its parent: `<div>`, and its contents will be rendered.

Note that the default configuration is not affected because the `template` tag is disallowed by default.

### Patches

The problem has been patched in versions [9.0.892](https://www.nuget.org/packages/HtmlSanitizer/9.0.892) and [9.1.893-beta](https://www.nuget.org/packages/HtmlSanitizer/9.1.893-beta).

### Workarounds

Disallo…