Advisory Weakness Curation

HtmlSanitizer has a bypass via template tag ### 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 Disallow the `template` tag. It is disallowed by default. ### Resources https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/template

HtmlSanitizer has a bypass via template tag 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.

HtmlSanitizer has a bypass via template tag 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.