dompurify

What is dompurify?

DOMPurify is a DOM-only, super-fast, uber-tolerant XSS sanitizer for HTML, MathML, and SVG. It helps prevent Cross-Site Scripting (XSS) attacks by sanitizing HTML content to ensure it's safe to insert into the DOM. It is written in JavaScript and works in all modern web browsers.

What are dompurify's main functionalities?

Sanitizing HTML strings

This feature allows you to sanitize HTML strings to prevent XSS attacks. The code sample demonstrates how to sanitize a string that contains a potentially malicious script. The result of this code would be a safe string with the malicious parts removed.

DOMPurify.sanitize('<img src=x onerror=alert(1)//>');

Configuring the sanitizer

DOMPurify can be configured to allow certain tags, attributes, or schemes. In the code sample, the sanitizer is configured to allow only 'img' tags and will strip out any other tags, including scripts or event handlers.

DOMPurify.sanitize('<img src=x onerror=alert(1)//>', {ALLOWED_TAGS: ['img']});

Hooking into sanitization

DOMPurify allows you to add hooks that can modify the content during the sanitization process. In the code sample, a hook is added that will be called after the attributes of all nodes have been sanitized, allowing for custom manipulation of the nodes.

DOMPurify.addHook('afterSanitizeAttributes', function(node) { /* manipulate node */ });

Other packages similar to dompurify

sanitize-html

sanitize-html is another HTML sanitizer that can clean up user-generated HTML, preventing XSS attacks. It is similar to DOMPurify but has a different API and set of defaults. It also allows for a high degree of customization in terms of what tags and attributes are allowed.

xss

xss is a package that aims to filter input from users to prevent XSS attacks. It is similar to DOMPurify but includes different options and is more focused on filtering input as opposed to sanitizing existing HTML content.

README

DOMPurify

DOMPurify is a DOM-only, super-fast, uber-tolerant XSS sanitizer for HTML, MathML and SVG.

It's also very simple to use and get started with. DOMPurify was started in February 2014 and, meanwhile, has reached version 2.3.1.

DOMPurify is written in JavaScript and works in all modern browsers (Safari (10+), Opera (15+), Internet Explorer (10+), Edge, Firefox and Chrome - as well as almost anything else using Blink or WebKit). It doesn't break on MSIE6 or other legacy browsers. It either uses a fall-back or simply does nothing.

Our automated tests cover 17 different browsers right now, more to come. We also cover Node.js v14.15.1, v15.4.0, running DOMPurify on jsdom. Older Node.js versions are known to work as well.

DOMPurify is written by security people who have vast background in web attacks and XSS. Fear not. For more details please also read about our Security Goals & Threat Model. Please, read it. Like, really.

What does it do?

DOMPurify sanitizes HTML and prevents XSS attacks. You can feed DOMPurify with string full of dirty HTML and it will return a string (unless configured otherwise) with clean HTML. DOMPurify will strip out everything that contains dangerous HTML and thereby prevent XSS attacks and other nastiness. It's also damn bloody fast. We use the technologies the browser provides and turn them into an XSS filter. The faster your browser, the faster DOMPurify will be.

How do I use it?

It's easy. Just include DOMPurify on your website.

Using the unminified development version

<script type="text/javascript" src="src/purify.js"></script>

Using the minified and tested production version (source-map available)

<script type="text/javascript" src="dist/purify.min.js"></script>

Afterwards you can sanitize strings by executing the following code:

let clean = DOMPurify.sanitize( dirty );

The resulting HTML can be written into a DOM element using innerHTML or the DOM using document.write(). That is fully up to you. Note that by default, we permit HTML, SVG and MathML. If you only need HTML, which might be a very common use-case, you can easily set that up as well:

let clean = DOMPurify.sanitize( dirty , {USE_PROFILES: {html: true}} );

Is there any foot-gun potential?

Well, please note, if you first sanitize HTML and then modify it afterwards, you might easily void the effects of sanitization. If you feed the sanitized markup to another library after sanitization, please be certain that the library doesn't mess around with the HTML on its own.

Okay, makes sense, let's move on

After sanitizing your markup, you can also have a look at the property DOMPurify.removed and find out, what elements and attributes were thrown out. Please do not use this property for making any security critical decisions. This is just a little helper for curious minds.

If you're using an AMD module loader like Require.js, you can load this script asynchronously as well:

import DOMPurify from 'dompurify';

var clean = DOMPurify.sanitize(dirty);

DOMPurify also works server-side with Node.js as well as client-side via Browserify or similar translators. At least Node.js 4.x or newer is required. Our support strives to follow the Node.js release cycle. DOMPurify intends to support any version being flagged as active. At the same time we phase out support for any version flagged as maintenance. DOMPurify might not break with all versions in maintenance immediately but stops to run tests against these older versions.

npm install dompurify

For JSDOM v10 or newer

const createDOMPurify = require('dompurify');
const { JSDOM } = require('jsdom');

const window = new JSDOM('').window;
const DOMPurify = createDOMPurify(window);

const clean = DOMPurify.sanitize(dirty);

For JSDOM versions older than v10

const createDOMPurify = require('dompurify');
const jsdom = require('jsdom').jsdom;

const window = jsdom('').defaultView;
const DOMPurify = createDOMPurify(window);

const clean = DOMPurify.sanitize(dirty);

Is there a demo?

Of course there is a demo! Play with DOMPurify

What if I find a security bug?

First of all, please immediately contact us via email so we can work on a fix. PGP key

Also, you probably qualify for a bug bounty! The fine folks over at Fastmail use DOMPurify for their services and added our library to their bug bounty scope. So, if you find a way to bypass or weaken DOMPurify, please also have a look at their website and the bug bounty info.

Some purification samples please?

How does purified markup look like? Well, the demo shows it for a big bunch of nasty elements. But let's also show some smaller examples!

DOMPurify.sanitize('<img src=x onerror=alert(1)//>'); // becomes <img src="x">
DOMPurify.sanitize('<svg><g/onload=alert(2)//<p>'); // becomes <svg><g></g></svg>
DOMPurify.sanitize('<p>abc<iframe//src=jAva&Tab;script:alert(3)>def</p>'); // becomes <p>abc</p>
DOMPurify.sanitize('<math><mi//xlink:href="data:x,<script>alert(4)</script>">'); // becomes <math><mi></mi></math>
DOMPurify.sanitize('<TABLE><tr><td>HELLO</tr></TABL>'); // becomes <table><tbody><tr><td>HELLO</td></tr></tbody></table>
DOMPurify.sanitize('<UL><li><A HREF=//google.com>click</UL>'); // becomes <ul><li><a href="//google.com">click</a></li></ul>

What is supported?

DOMPurify currently supports HTML5, SVG and MathML. DOMPurify per default allows CSS, HTML custom data attributes. DOMPurify also supports the Shadow DOM - and sanitizes DOM templates recursively. DOMPurify also allows you to sanitize HTML for being used with the jQuery $() and elm.html() API without any known problems.

What about older browsers like MSIE8?

DOMPurify offers a fall-back behavior for older MSIE browsers. It uses the MSIE-only toStaticHTML feature to sanitize. Note however that in this fall-back mode, pretty much none of the configuration flags shown below have any effect. You need to handle that yourself.

If not even toStaticHTML is supported, DOMPurify does nothing at all. It simply returns exactly the string that you fed it.

DOMPurify also exposes a property called isSupported, which tells you whether DOMPurify will be able to do its job.

What about DOMPurify and Trusted Types?

In version 1.0.9, support for Trusted Types API was added to DOMPurify. In version 2.0.0, a config flag was added to control DOMPurify's behavior regarding this.

When DOMPurify.sanitize is used in an environment where the Trusted Types API is available and RETURN_TRUSTED_TYPE is set to true, it tries to return a TrustedHTML value instead of a string (the behavior for RETURN_DOM, RETURN_DOM_FRAGMENT, and RETURN_DOM_IMPORT config options does not change).

Can I configure DOMPurify?

Yes. The included default configuration values are pretty good already - but you can of course override them. Check out the /demos folder to see a bunch of examples on how you can customize DOMPurify.

/**
 * General settings
 */

// strip {{ ... }} and <% ... %> to make output safe for template systems
// be careful please, this mode is not recommended for production usage.
// allowing template parsing in user-controlled HTML is not advised at all.
// only use this mode if there is really no alternative.
var clean = DOMPurify.sanitize(dirty, {SAFE_FOR_TEMPLATES: true});

/**
 * Control our allow-lists and block-lists
 */
// allow only <b> elements, very strict
var clean = DOMPurify.sanitize(dirty, {ALLOWED_TAGS: ['b']});

// allow only <b> and <q> with style attributes
var clean = DOMPurify.sanitize(dirty, {ALLOWED_TAGS: ['b', 'q'], ALLOWED_ATTR: ['style']});

// allow all safe HTML elements but neither SVG nor MathML
// note that the USE_PROFILES setting will override the ALLOWED_TAGS setting
// so don't use them together
var clean = DOMPurify.sanitize(dirty, {USE_PROFILES: {html: true}});

// allow all safe SVG elements and SVG Filters, no HTML or MathML
var clean = DOMPurify.sanitize(dirty, {USE_PROFILES: {svg: true, svgFilters: true}});

// allow all safe MathML elements and SVG, but no SVG Filters
var clean = DOMPurify.sanitize(dirty, {USE_PROFILES: {mathMl: true, svg: true}});

// change the default namespace from HTML to something different
var clean = DOMPurify.sanitize(dirty, {NAMESPACE: 'http://www.w3.org/2000/svg'});

// leave all safe HTML as it is and add <style> elements to block-list
var clean = DOMPurify.sanitize(dirty, {FORBID_TAGS: ['style']});

// leave all safe HTML as it is and add style attributes to block-list
var clean = DOMPurify.sanitize(dirty, {FORBID_ATTR: ['style']});

// extend the existing array of allowed tags and add <my-tag> to allow-list
var clean = DOMPurify.sanitize(dirty, {ADD_TAGS: ['my-tag']});

// extend the existing array of allowed attributes and add my-attr to allow-list
var clean = DOMPurify.sanitize(dirty, {ADD_ATTR: ['my-attr']});

// prohibit HTML5 data attributes, leave other safe HTML as is (default is true)
var clean = DOMPurify.sanitize(dirty, {ALLOW_DATA_ATTR: false});

/**
 * Control behavior relating to URI values
 */
// extend the existing array of elements that can use Data URIs
var clean = DOMPurify.sanitize(dirty, {ADD_DATA_URI_TAGS: ['a', 'area']});

// extend the existing array of elements that are safe for URI-like values (be careful, XSS risk)
var clean = DOMPurify.sanitize(dirty, {ADD_URI_SAFE_ATTR: ['my-attr']});

/**
 * Control permitted attribute values
 */
// allow external protocol handlers in URL attributes (default is false, be careful, XSS risk)
// by default only http, https, ftp, ftps, tel, mailto, callto, cid and xmpp are allowed.
var clean = DOMPurify.sanitize(dirty, {ALLOW_UNKNOWN_PROTOCOLS: true});

// allow specific protocols handlers in URL attributes via regex (default is false, be careful, XSS risk)
// by default only http, https, ftp, ftps, tel, mailto, callto, cid and xmpp are allowed.
// Default RegExp: /^(?:(?:(?:f|ht)tps?|mailto|tel|callto|cid|xmpp):|[^a-z]|[a-z+.\-]+(?:[^a-z+.\-:]|$))/i;
var clean = DOMPurify.sanitize(dirty, {ALLOWED_URI_REGEXP: /^(?:(?:(?:f|ht)tps?|mailto|tel|callto|cid|xmpp|xxx):|[^a-z]|[a-z+.\-]+(?:[^a-z+.\-:]|$))/i;});

/**
 * Influence the return-type
 *
 * Careful, this setting has foot-gun potential! If you set RETURN_DOM or RETURN_DOM_FRAGMENT to true, don't set RETURN_DOM_IMPORT to false!
 * By default, our settings are secure - we believe - but returning a DOM *and* manually setting RETURN_DOM_IMPORT to false will give you XSS in some situations.
 */
// return a DOM HTMLBodyElement instead of an HTML string (default is false)
var clean = DOMPurify.sanitize(dirty, {RETURN_DOM: true});

// return a DOM DocumentFragment instead of an HTML string (default is false)
var clean = DOMPurify.sanitize(dirty, {RETURN_DOM_FRAGMENT: true});

// return a DOM DocumentFragment instead of an HTML string (default is false)
// also import it into the current document (default is false).
// RETURN_DOM_IMPORT must be set if you would like to append
// the returned node to the current document (default is true)
var clean = DOMPurify.sanitize(dirty, {RETURN_DOM_FRAGMENT: true, RETURN_DOM_IMPORT: true});
document.body.appendChild(clean);

// use the RETURN_TRUSTED_TYPE flag to turn on Trusted Types support if available
var clean = DOMPurify.sanitize(dirty, {RETURN_TRUSTED_TYPE: true}); // will return a TrustedHTML object instead of a string if possible

/**
 * Influence how we sanitize
 */
// return entire document including <html> tags (default is false)
var clean = DOMPurify.sanitize(dirty, {WHOLE_DOCUMENT: true});

// disable DOM Clobbering protection on output (default is true, handle with care, minor XSS risks here)
var clean = DOMPurify.sanitize(dirty, {SANITIZE_DOM: false});

// keep an element's content when the element is removed (default is true)
var clean = DOMPurify.sanitize(dirty, {KEEP_CONTENT: false});

// glue elements like style, script or others to document.body and prevent unintuitive browser behavior in several edge-cases (default is false)
var clean = DOMPurify.sanitize(dirty, {FORCE_BODY: true});

/**
 * Influence where we sanitize
 */
// use the IN_PLACE mode to sanitize a node "in place", which is much faster depending on how you use DOMPurify
var dirty = document.createElement('a');
dirty.setAttribute('href', 'javascript:alert(1)');
var clean = DOMPurify.sanitize(dirty, {IN_PLACE: true}); // see https://github.com/cure53/DOMPurify/issues/288 for more info

There is even more examples here, showing how you can run, customize and configure DOMPurify to fit your needs.

Persistent Configuration

Instead of repeatedly passing the same configuration to DOMPurify.sanitize, you can use the DOMPurify.setConfig method. Your configuration will persist until your next call to DOMPurify.setConfig, or until you invoke DOMPurify.clearConfig to reset it. Remember that there is only one active configuration, which means once it is set, all extra configuration parameters passed to DOMPurify.sanitize are ignored.

Hooks

DOMPurify allows you to augment its functionality by attaching one or more functions with the DOMPurify.addHook method to one of the following hooks:

• beforeSanitizeElements
• uponSanitizeElement (No 's' - called for every element)
• afterSanitizeElements
• beforeSanitizeAttributes
• uponSanitizeAttribute
• afterSanitizeAttributes
• beforeSanitizeShadowDOM
• uponSanitizeShadowNode
• afterSanitizeShadowDOM

It passes the currently processed DOM node, when needed a literal with verified node and attribute data and the DOMPurify configuration to the callback. Check out the MentalJS hook demo to see how the API can be used nicely.

Example:

DOMPurify.addHook('beforeSanitizeElements', function (
  currentNode,
  hookEvent,
  config
) {
  // Do something with the current node and return it
  // You can also mutate hookEvent (i.e. set hookEvent.forceKeepAttr = true)
  return currentNode;
});

Continuous Integration

We are currently using Github Actions in combination with BrowserStack. This gives us the possibility to confirm for each and every commit that all is going according to plan in all supported browsers. Check out the build logs here: https://github.com/cure53/DOMPurify/actions

You can further run local tests by executing npm test. The tests work fine with Node.js v0.6.2 and jsdom@8.5.0.

All relevant commits will be signed with the key 0x24BB6BF4 for additional security (since 8th of April 2016).

Development and contributing

Installation (yarn i)

We support both yarn and npm@5.2 officially while providing lock-files for either dependency manager to provide reproducible installs and builds on either or. TravisCI itself is configured to install dependencies using yarn. When using an older version of npm we can not fully ensure the versions of installed dependencies which might lead to unanticipated problems.

Scripts

We rely on npm run-scripts for integrating with our tooling infrastructure. We use ESLint as a pre-commit hook to ensure code consistency. Moreover, to ease formatting we use prettier while building the /dist assets happens through rollup.

These are our npm scripts:

• npm run dev to start building while watching sources for changes
• npm run test to run our test suite via jsdom and karma
  • test:jsdom to only run tests through jsdom
  • test:karma to only run tests through karma
• npm run lint to lint the sources using ESLint (via xo)
• npm run format to format our sources using prettier to ease to pass ESLint
• npm run build to build our distribution assets minified and unminified as a UMD module
  • npm run build:umd to only build an unminified UMD module
  • npm run build:umd:min to only build a minified UMD module

Note: all run scripts triggered via npm run <script> can also be started using yarn <script>.

There are more npm scripts but they are mainly to integrate with CI or are meant to be "private" for instance to amend build distribution files with every commit.

Security Mailing List

We maintain a mailing list that notifies whenever a security-critical release of DOMPurify was published. This means, if someone found a bypass and we fixed it with a release (which always happens when a bypass was found) a mail will go out to that list. This usually happens within minutes or few hours after learning about a bypass. The list can be subscribed to here:

https://lists.ruhr-uni-bochum.de/mailman/listinfo/dompurify-security

Feature releases will not be announced to this list.

Who contributed?

Many people helped and help DOMPurify become what it is and need to be acknowledged here!

granlem 💸, oreoshake 💸, dcramer 💸,tdeekens ❤️, peernohell ❤️, NateScarlet, neilj, fhemberger, Joris-van-der-Wel, ydaniv, terjanq, filedescriptor, ConradIrwin, gibson042, choumx, 0xSobky, styfle, koto, tlau88, strugee, oparoz, mathiasbynens, edg2s, dnkolegov, dhardtke, wirehead, thorn0, styu, mozfreddyb, mikesamuel, jorangreef, jimmyhchan, jameydeorio, jameskraus, hyderali, hansottowirtz, hackvertor, freddyb, flavorjones, djfarrelly, devd, camerondunford, buu700, buildog, alabiaga, Vector919, Robbert, GreLI, FuzzySockets, ArtemBernatskyy, @garethheyes, @shafigullin, @mmrupp, @irsdl,ShikariSenpai, ansjdnakjdnajkd, @asutherland, @mathias, @cgvwzq, @robbertatwork, @giutro, @CmdEngineer_, @avr4mit and especially @securitymb ❤️ & @masatokinugawa ❤️

Testing powered by

And last but not least, thanks to BrowserStack Open-Source Program for supporting this project with their services for free and delivering excellent, dedicated and very professional support on top of that.