sanitize-html

What is sanitize-html?

The sanitize-html npm package is designed to clean up user-submitted HTML, preventing XSS attacks by sanitizing any HTML code input by users. It allows developers to specify a whitelist of HTML tags and attributes that are allowed, and it will strip out all other tags and attributes that are not explicitly allowed.

What are sanitize-html's main functionalities?

Sanitizing HTML

This feature allows you to remove any unwanted HTML tags and content that could lead to XSS attacks, leaving only the content that is deemed safe according to the specified rules.

const sanitizeHtml = require('sanitize-html');
const dirtyHtml = '<script>alert("XSS");</script><p>Valid content</p>';
const cleanHtml = sanitizeHtml(dirtyHtml);
console.log(cleanHtml); // Output: '<p>Valid content</p>'

Allowing a set of HTML tags

This feature lets you specify which HTML tags are allowed in the sanitized output, effectively filtering out all other tags that are not part of the whitelist.

const sanitizeHtml = require('sanitize-html');
const dirtyHtml = '<div><p>Some text</p><script>Bad script</script></div>';
const cleanHtml = sanitizeHtml(dirtyHtml, {
  allowedTags: ['div', 'p']
});
console.log(cleanHtml); // Output: '<div><p>Some text</p></div>'

Configuring allowed attributes for tags

This feature allows you to configure which attributes are allowed for specific tags, providing fine-grained control over the sanitization process.

const sanitizeHtml = require('sanitize-html');
const dirtyHtml = '<a href="http://example.com" onclick="stealCookies()">Link</a>';
const cleanHtml = sanitizeHtml(dirtyHtml, {
  allowedTags: ['a'],
  allowedAttributes: {
    'a': ['href']
  }
});
console.log(cleanHtml); // Output: '<a href="http://example.com">Link</a>'

Transforming tags and attributes

This feature enables you to transform certain tags into other tags, or modify their attributes during the sanitization process.

const sanitizeHtml = require('sanitize-html');
const dirtyHtml = '<b>bold text</b>';
const cleanHtml = sanitizeHtml(dirtyHtml, {
  transformTags: {
    'b': sanitizeHtml.simpleTransform('strong')
  }
});
console.log(cleanHtml); // Output: '<strong>bold text</strong>'

Other packages similar to sanitize-html

dompurify

DOMPurify is a DOM-only XSS sanitizer for HTML, MathML, and SVG. It's similar to sanitize-html but works in a browser environment as well as server-side. It's also known for its speed and extensive configuration options.

xss

The xss package is another HTML sanitizer that aims to filter input from users to prevent XSS attacks. It provides a range of options for customization and is similar to sanitize-html in its goals, but it has a different API and set of defaults.

README

sanitize-html

sanitize-html provides a simple HTML sanitizer with a clear API.

sanitize-html is tolerant. It is well suited for cleaning up HTML fragments such as those created by ckeditor and other rich text editors. It is especially handy for removing unwanted CSS when copying and pasting from Word.

sanitize-html allows you to specify the tags you want to permit, and the permitted attributes for each of those tags.

If a tag is not permitted, the contents of the tag are not discarded. There are some exceptions to this, discussed below in the "Discarding the entire contents of a disallowed tag" section.

The syntax of poorly closed p and img elements is cleaned up.

href attributes are validated to ensure they only contain http, https, ftp and mailto URLs. Relative URLs are also allowed. Ditto for src attributes.

Allowing particular urls as a src to an iframe tag by filtering hostnames is also supported.

HTML comments are not preserved.

Requirements

sanitize-html is intended for use with Node. That's pretty much it. All of its npm dependencies are pure JavaScript. sanitize-html is built on the excellent htmlparser2 module.

How to use

Browser

Think first: why do you want to use it in the browser? Remember, servers must never trust browsers. You can't sanitize HTML for saving on the server anywhere else but on the server.

But, perhaps you'd like to display sanitized HTML immediately in the browser for preview. Or ask the browser to do the sanitization work on every page load. You can if you want to!

• Clone repository
• Run npm install and build / minify:

npm install
npm run minify

You'll find the minified and unminified versions of sanitize-html (with all its dependencies included) in the dist/ directory.

Use it in the browser:

<html>
    <body>
        <script type="text/javascript"  src="dist/sanitize-html.js"></script>
        <script type="text/javascript" src="demo.js"></script>
    </body>
</html>

var html = "<strong>hello world</strong>";
console.log(sanitizeHtml(html));
console.log(sanitizeHtml("<img src=x onerror=alert('img') />"));
console.log(sanitizeHtml("console.log('hello world')"));
console.log(sanitizeHtml("<script>alert('hello world')</script>"));

Node (Recommended)

Install module from console:

npm install sanitize-html

Import the module:

// In ESM
import sanitizeHtml from 'sanitize-html';

// Or in CJS
const sanitizeHtml = require('sanitize-html');

Use it in your JavaScript app:

const dirty = 'some really tacky HTML';
const clean = sanitizeHtml(dirty);

That will allow our default list of allowed tags and attributes through. It's a nice set, but probably not quite what you want. So:

// Allow only a super restricted set of tags and attributes
clean = sanitizeHtml(dirty, {
  allowedTags: [ 'b', 'i', 'em', 'strong', 'a' ],
  allowedAttributes: {
    'a': [ 'href' ]
  },
  allowedIframeHostnames: ['www.youtube.com']
});

Boom!

"I like your set but I want to add one more tag. Is there a convenient way?" Sure:

clean = sanitizeHtml(dirty, {
  allowedTags: sanitizeHtml.defaults.allowedTags.concat([ 'img' ])
});

If you do not specify allowedTags or allowedAttributes our default list is applied. So if you really want an empty list, specify one.

"What are the default options?"

allowedTags: [ 'h3', 'h4', 'h5', 'h6', 'blockquote', 'p', 'a', 'ul', 'ol',
  'nl', 'li', 'b', 'i', 'strong', 'em', 'strike', 'abbr', 'code', 'hr', 'br', 'div',
  'table', 'thead', 'caption', 'tbody', 'tr', 'th', 'td', 'pre', 'iframe' ],
disallowedTagsMode: 'discard',
allowedAttributes: {
  a: [ 'href', 'name', 'target' ],
  // We don't currently allow img itself by default, but this
  // would make sense if we did. You could add srcset here,
  // and if you do the URL is checked for safety
  img: [ 'src' ]
},
// Lots of these won't come up by default because we don't allow them
selfClosing: [ 'img', 'br', 'hr', 'area', 'base', 'basefont', 'input', 'link', 'meta' ],
// URL schemes we permit
allowedSchemes: [ 'http', 'https', 'ftp', 'mailto' ],
allowedSchemesByTag: {},
allowedSchemesAppliedToAttributes: [ 'href', 'src', 'cite' ],
allowProtocolRelative: true,
enforceHtmlBoundary: false

"What if I want to allow all tags or all attributes?"

Simple! instead of leaving allowedTags or allowedAttributes out of the options, set either one or both to false:

allowedTags: false,
allowedAttributes: false

"What if I don't want to allow any tags?"

Also simple! Set allowedTags to [] and allowedAttributes to {}.

allowedTags: [],
allowedAttributes: {}

"What if I want disallowed tags to be escaped rather than discarded?"

If you set disallowedTagsMode to discard (the default), disallowed tags are discarded. Any text content or subtags is still included, depending on whether the individual subtags are allowed.

If you set disallowedTagsMode to escape, the disallowed tags are escaped rather than discarded. Any text or subtags is handled normally.

If you set disallowedTagsMode to recursiveEscape, the disallowed tags are escaped rather than discarded, and the same treatment is applied to all subtags, whether otherwise allowed or not.

"What if I want to allow only specific values on some attributes?"

When configuring the attribute in allowedAttributes simply use an object with attribute name and an allowed values array. In the following example sandbox="allow-forms allow-modals allow-orientation-lock allow-pointer-lock allow-popups allow-popups-to-escape-sandbox allow-scripts" would become sandbox="allow-popups allow-scripts":

allowedAttributes: {
  iframe: [
    {
      name: 'sandbox',
      multiple: true,
      values: ['allow-popups', 'allow-same-origin', 'allow-scripts']
    }
  ]
}

With multiple: true, several allowed values may appear in the same attribute, separated by spaces. Otherwise the attribute must exactly match one and only one of the allowed values.

Wildcards for attributes

You can use the * wildcard to allow all attributes with a certain prefix:

allowedAttributes: {
  a: [ 'href', 'data-*' ]
}

Also you can use the * as name for a tag, to allow listed attributes to be valid for any tag:

allowedAttributes: {
  '*': [ 'href', 'align', 'alt', 'center', 'bgcolor' ]
}

Discarding text outside of <html></html> tags

Some text editing applications generate HTML to allow copying over to a web application. These can sometimes include undesireable control characters after terminating html tag. By default sanitize-html will not discard these characters, instead returning them in sanitized string. This behaviour can be modified using enforceHtmlBoundary option.

Setting this option to true will instruct sanitize-html to discard all characters outside of html tag boundaries -- before <html> and after </html> tags.

enforceHtmlBoundary: true

htmlparser2 Options

santizeHtml is built on htmlparser2. By default the only option passed down is decodeEntities: true You can set the options to pass by using the parser option.

clean = sanitizeHtml(dirty, {
  allowedTags: ['a'],
  parser: {
    lowerCaseTags: true
  }
});

See the [htmlparser2 wiki] (https://github.com/fb55/htmlparser2/wiki/Parser-options) for the full list of possible options.

Transformations

What if you want to add or change an attribute? What if you want to transform one tag to another? No problem, it's simple!

The easiest way (will change all ol tags to ul tags):

clean = sanitizeHtml(dirty, {
  transformTags: {
    'ol': 'ul',
  }
});

The most advanced usage:

clean = sanitizeHtml(dirty, {
  transformTags: {
    'ol': function(tagName, attribs) {
      // My own custom magic goes here

      return {
        tagName: 'ul',
        attribs: {
          class: 'foo'
        }
      };
    }
  }
});

You can specify the * wildcard instead of a tag name to transform all tags.

There is also a helper method which should be enough for simple cases in which you want to change the tag and/or add some attributes:

clean = sanitizeHtml(dirty, {
  transformTags: {
    'ol': sanitizeHtml.simpleTransform('ul', {class: 'foo'}),
  }
});

The simpleTransform helper method has 3 parameters:

simpleTransform(newTag, newAttributes, shouldMerge)

The last parameter (shouldMerge) is set to true by default. When true, simpleTransform will merge the current attributes with the new ones (newAttributes). When false, all existing attributes are discarded.

You can also add or modify the text contents of a tag:

clean = sanitizeHtml(dirty, {
  transformTags: {
    'a': function(tagName, attribs) {
      return {
        tagName: 'a',
        text: 'Some text'
      };
    }
  }
});

For example, you could transform a link element with missing anchor text:

<a href="http://somelink.com"></a>

To a link with anchor text:

<a href="http://somelink.com">Some text</a>

Filters

You can provide a filter function to remove unwanted tags. Let's suppose we need to remove empty a tags like:

<a href="page.html"></a>

We can do that with the following filter:

sanitizeHtml(
  '<p>This is <a href="http://www.linux.org"></a><br/>Linux</p>',
  {
    exclusiveFilter: function(frame) {
      return frame.tag === 'a' && !frame.text.trim();
    }
  }
);

The frame object supplied to the callback provides the following attributes:

• tag: The tag name, i.e. 'img'.
• attribs: The tag's attributes, i.e. { src: "/path/to/tux.png" }.
• text: The text content of the tag.
• mediaChildren: Immediate child tags that are likely to represent self-contained media (e.g., img, video, picture, iframe). See the mediaTags variable in src/index.js for the full list.
• tagPosition: The index of the tag's position in the result string.

You can also process all text content with a provided filter function. Let's say we want an ellipsis instead of three dots.

<p>some text...</p>

We can do that with the following filter:

sanitizeHtml(
  '<p>some text...</p>',
  {
    textFilter: function(text, tagName) {
      if (['a'].indexOf(tagName) > -1) return //Skip anchor tags

      return text.replace(/\.\.\./, '&hellip;');
    }
  }
);

Note that the text passed to the textFilter method is already escaped for safe display as HTML. You may add markup and use entity escape sequences in your textFilter.

Iframe Filters

If you would like to allow iframe tags but want to control the domains that are allowed through you can provide an array of hostnames and(or) array of domains that you would like to allow as iframe sources. This hostname is a property in the options object passed as an argument to the sanitize-html function.

These arrays will be checked against the html that is passed to the function and return only src urls that include the allowed hostnames or domains in the object. The url in the html that is passed must be formatted correctly (valid hostname) as an embedded iframe otherwise the module will strip out the src from the iframe.

Make sure to pass a valid hostname along with the domain you wish to allow, i.e.:

allowedIframeHostnames: ['www.youtube.com', 'player.vimeo.com'],
allowedIframeDomains: ['zoom.us']

You may also specify whether or not to allow relative URLs as iframe sources.

allowIframeRelativeUrls: true

Note that if unspecified, relative URLs will be allowed by default if no hostname or domain filter is provided but removed by default if a hostname or domain filter is provided.

Remember that the iframe tag must be allowed as well as the src attribute.

For example:

clean = sanitizeHtml('<p><iframe src="https://www.youtube.com/embed/nykIhs12345"></iframe><p>', {
  allowedTags: [ 'p', 'em', 'strong', 'iframe' ],
  allowedClasses: {
    'p': [ 'fancy', 'simple' ],
  },
  allowedAttributes: {
    'iframe': ['src']
  },
  allowedIframeHostnames: ['www.youtube.com', 'player.vimeo.com']
});

will pass through as safe whereas:

clean = sanitizeHtml('<p><iframe src="https://www.youtube.net/embed/nykIhs12345"></iframe><p>', {
  allowedTags: [ 'p', 'em', 'strong', 'iframe' ],
  allowedClasses: {
    'p': [ 'fancy', 'simple' ],
  },
  allowedAttributes: {
    'iframe': ['src']
  },
  allowedIframeHostnames: ['www.youtube.com', 'player.vimeo.com']
});

or

clean = sanitizeHtml('<p><iframe src="https://www.vimeo/video/12345"></iframe><p>', {
  allowedTags: [ 'p', 'em', 'strong', 'iframe' ],
  allowedClasses: {
    'p': [ 'fancy', 'simple' ],
  },
  allowedAttributes: {
    'iframe': ['src']
  },
  allowedIframeHostnames: ['www.youtube.com', 'player.vimeo.com']
});

will return an empty iframe tag.

If you want to allow any subdomain of any level you can provide the domain in allowedIframeDomains

clean = sanitizeHtml('<p><iframe src="https://us02web.zoom.us/embed/12345"></iframe><p>', {
  allowedTags: [ 'p', 'em', 'strong', 'iframe' ],
  allowedClasses: {
    'p': [ 'fancy', 'simple' ],
  },
  allowedAttributes: {
    'iframe': ['src']
  },
  allowedIframeHostnames: ['www.youtube.com', 'player.vimeo.com'],
  allowedIframeDomains: ['zoom.us']
});

will pass through as safe.

Allowed CSS Classes

If you wish to allow specific CSS classes on a particular element, you can do so with the allowedClasses option. Any other CSS classes are discarded.

This implies that the class attribute is allowed on that element.

// Allow only a restricted set of CSS classes and only on the p tag
clean = sanitizeHtml(dirty, {
  allowedTags: [ 'p', 'em', 'strong' ],
  allowedClasses: {
    'p': [ 'fancy', 'simple' ]
  }
});

Allowed CSS Styles

If you wish to allow specific CSS styles on a particular element, you can do that with the allowedStyles option. Simply declare your desired attributes as regular expression options within an array for the given attribute. Specific elements will inherit whitelisted attributes from the global (*) attribute. Any other CSS classes are discarded.

You must also use allowedAttributes to activate the style attribute for the relevant elements. Otherwise this feature will never come into play.

When constructing regular expressions, don't forget ^ and $. It's not enough to say "the string should contain this." It must also say "and only this."

URLs in inline styles are NOT filtered by any mechanism other than your regular expression.

clean = sanitizeHtml(dirty, {
  allowedTags: ['p'],
  allowedAttributes: {
    'p': ["style"],
  },
  allowedStyles: {
    '*': {
      // Match HEX and RGB
      'color': [/^#(0x)?[0-9a-f]+$/i, /^rgb\(\s*(\d{1,3})\s*,\s*(\d{1,3})\s*,\s*(\d{1,3})\s*\)$/],
      'text-align': [/^left$/, /^right$/, /^center$/],
      // Match any number with px, em, or %
      'font-size': [/^\d+(?:px|em|%)$/]
    },
    'p': {
      'font-size': [/^\d+rem$/]
    }
  }
});

Allowed URL schemes

By default we allow the following URL schemes in cases where href, src, etc. are allowed:

[ 'http', 'https', 'ftp', 'mailto' ]

You can override this if you want to:

sanitizeHtml(
  // teeny-tiny valid transparent GIF in a data URL
  '<img src="data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" />',
  {
    allowedTags: [ 'img', 'p' ],
    allowedSchemes: [ 'data', 'http' ]
  }
);

You can also allow a scheme for a particular tag only:

allowedSchemes: [ 'http', 'https' ],
allowedSchemesByTag: {
  img: [ 'data' ]
}

And you can forbid the use of protocol-relative URLs (starting with //) to access another site using the current protocol, which is allowed by default:

allowProtocolRelative: false

Discarding the entire contents of a disallowed tag

Normally, with a few exceptions, if a tag is not allowed, all of the text within it is preserved, and so are any allowed tags within it.

The exceptions are:

style, script, textarea, option

If you wish to replace this list, for instance to discard whatever is found inside a noscript tag, use the nonTextTags option:

nonTextTags: [ 'style', 'script', 'textarea', 'option', 'noscript' ]

Note that if you use this option you are responsible for stating the entire list. This gives you the power to retain the content of textarea, if you want to.

The content still gets escaped properly, with the exception of the script and style tags. Allowing either script or style leaves you open to XSS attacks. Don't do that unless you have good reason to trust their origin. sanitize-html will log a warning if these tags are allowed, which can be disabled with the allowVulnerableTags: true option.

Choose what to do with disallowed tags

Instead of discarding, or keeping text only, you may enable escaping of the entire content:

disallowedTagsMode: 'escape'

This will transform <disallowed>content</disallowed> to <disallowed>content</disallowed>

Valid values are: 'discard' (default), 'escape' (escape the tag) and 'recursiveEscape' (to escape the tag and all its content).

About P'unk Avenue and Apostrophe

sanitize-html was created at P'unk Avenue for use in ApostropheCMS, an open-source content management system built on node.js. If you like sanitize-html you should definitely check out apostrophecms.org.

Changelog

The changelog is now in a separate file for readability.

Support

Feel free to open issues on github.

Changelog

1.27.5 (2020-09-23):

• Updates README to include ES modules syntax.