Project: syntax-tree/hast-util-sanitize

Package: hast-util-sanitize@4.1.0

  1. Dependents: 0
  2. hast utility to sanitize nodes
  1. util 145
  2. utility 141
  3. unist 132
  4. html 124
  5. hast 73
  6. hast-util 46
  7. clean 6
  8. sanitize 3
  9. xss 2


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hast utility to make trees safe.


What is this?

This package is a utility that can make a tree that potentially contains dangerous user content safe for use. It defaults to what GitHub does to clean unsafe markup, but you can change that.

When should I use this?

This package is needed whenever you deal with potentially dangerous user content.

The plugin rehype-sanitize wraps this utility to also sanitize HTML at a higher-level (easier) abstraction.


This package is ESM only. In Node.js (version 14.14+ and 16.0+), install with npm:

npm install hast-util-sanitize

In Deno with esm.sh:

import {sanitize} from 'https://esm.sh/hast-util-sanitize@4'

In browsers with esm.sh:

<script type="module">
  import {sanitize} from 'https://esm.sh/hast-util-sanitize@4?bundle'


import {u} from 'unist-builder'
import {h} from 'hastscript'
import {sanitize} from 'hast-util-sanitize'
import {toHtml} from 'hast-util-to-html'

const tree = h('div', {onmouseover: 'alert("alpha")'}, [
    {href: 'jAva script:alert("bravo")', onclick: 'alert("charlie")'},
  u('text', '\n'),
  h('script', 'alert("charlie")'),
  u('text', '\n'),
  h('img', {src: 'x', onerror: 'alert("delta")'}),
  u('text', '\n'),
  h('iframe', {src: 'javascript:alert("echo")'}),
  u('text', '\n'),
  h('math', h('mi', {'xlink:href': 'data:x,<script>alert("foxtrot")</script>'}))

const unsanitized = toHtml(tree)
const sanitized = toHtml(sanitize(tree))



<div onmouseover="alert(&#x22;alpha&#x22;)"><a href="jAva script:alert(&#x22;bravo&#x22;)" onclick="alert(&#x22;charlie&#x22;)">delta</a>
<img src="x" onerror="alert(&#x22;delta&#x22;)">
<iframe src="javascript:alert(&#x22;echo&#x22;)"></iframe>
<math><mi xlink:href="data:x,<script>alert(&#x22;foxtrot&#x22;)</script>"></mi></math></div>



<img src="x">



This package exports the identifiers defaultSchema and sanitize. There is no default export.

sanitize(tree[, schema])

Sanitize a tree.


New, sanitized, tree (Node).


Default schema (Schema).

Follows GitHub style sanitation.


Schema that defines what nodes and properties are allowed (TypeScript type).

The default schema is defaultSchema, which follows how GitHub cleans. If any top-level key is missing in the given schema, the corresponding value of the default schema is used.

To extend the standard schema with a few changes, clone defaultSchema like so:

import {h} from 'hastscript'
import deepmerge from 'deepmerge' // You can use `structuredClone` in modern JS.
import {sanitize, defaultSchema} from 'hast-util-sanitize'

const schema = deepmerge(defaultSchema, {attributes: {'*': ['className']}})

const tree = sanitize(h('div', {className: ['foo']}), schema)

// `tree` still has `className`.
// {
//   type: 'element',
//   tagName: 'div',
//   properties: {className: ['foo']},
//   children: []
// }

Map of tag names to allowed property names (Record<string, Array<string | [string, ...Array<string | number | boolean | RegExp>]>, optional).

The special key '*' as a tag name defines property names allowed on all elements.

The special value 'data*' as a property name can be used to allow all data properties.

For example:

attributes: {
  a: ['href'],
  img: ['src', 'longDesc'],
  // …
  '*': [
    // …

Instead of a single string, which allows any property value of that property name, it’s also possible to provide an array to allow several values. For example, input: ['type'] allows the type attribute set to any value on inputs. But input: [['type', 'checkbox', 'radio']] allows type only when set to one of the allowed values ('checkbox' or 'radio').

You can also use regexes, so for example span: [['className', /^hljs-/]] allows any class that starts with hljs- on span elements.

This is how the default GitHub schema allows only disabled checkbox inputs:

attributes: {
  // …
  input: [
    ['type', 'checkbox'],
    ['disabled', true]
  // …

Attributes also plays well with properties that accept space- or comma-separated values, such as class. Say you wanted to allow certain classes on span elements for syntax highlighting, that can be done like this:

// …
span: [
  ['className', 'token', 'number', 'operator']
// …

Map of tag names to required property names and their default property value (Record<string, Record<string, unknown>>, optional).

If the defined keys do not exist in an element’s properties, they are added and set to the specified value.

Note that properties are first checked based on the schema at attributes, so properties could be removed by that step and then added again through required.

For example:

required: {
  input: {type: 'checkbox', disabled: true}

List of allowed tag names (Array<string>, optional).

For example:

tagNames: [
  // …

Map of property names to allowed protocols (Record<string, Array<string>>, optional).

The listed property names can be set to URLs that are local (relative to the current website, such as this, #this, /this, or ?this) or remote (such as https://example.com), in which case they must have a protocol that is allowed here.

For example:

protocols: {
  href: ['http', 'https', 'mailto'],
  // …
  longDesc: ['http', 'https']

Map of tag names to a list of tag names which are required ancestors (Record<string, Array<string>>, optional).

Elements with these tag names will be ignored if they occur outside of one of their allowed parents.

For example:

ancestors: {
  li: ['ol', 'ul'],
  // …
  tr: ['table']

List of property names that clobber (Array<string>, optional).

For example:

clobber: ['name', 'id']

Prefix to use before clobbering properties (string, optional).

For example:

clobberPrefix: 'user-content-'

List of tag names to strip from the tree (Array<string>, optional).

By default, unsafe elements are replaced by their children. Some elements should however be entirely stripped from the tree.

For example:

strip: ['script']

Whether to allow comment nodes (boolean, default: false).

For example:

allowComments: true

Whether to allow doctype nodes (boolean, default: false).

allowDoctypes: true


This package is fully typed with TypeScript. It exports the additional type Schema.


Projects maintained by the unified collective are compatible with all maintained versions of Node.js. As of now, that is Node.js 14.14+ and 16.0+. Our projects sometimes work with older versions, but this is not guaranteed.


By default, hast-util-sanitize will make everything safe to use. But when used incorrectly, deviating from the defaults can open you up to a cross-site scripting (XSS) attack.

Use hast-util-sanitize after the last unsafe thing: everything after it could be unsafe (but is fine if you do trust it).


See contributing.md in syntax-tree/.github for ways to get started. See support.md for ways to get help.

This project has a code of conduct. By interacting with this repository, organization, or community you agree to abide by its terms.


MIT © Titus Wormer