DOMPurify

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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 1.0.8.

DOMPurify is written in JavaScript and works in all modern browsers (Safari, 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 21 different browsers right now, more to come. We also cover Node.js v6.0.0, v8.0.0, v9.0.0 and v10.0.0, running DOMPurify on jsdom.

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 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:

var 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. But keep in mind, if you use the sanitized HTML with jQuery's very insecure elm.html() method, then the SAFE_FOR_JQUERY flag has to be set to make sure it's safe! Other than that, all is fine.

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.

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

require(['dompurify'], function(DOMPurify) {
    var clean = DOMPurify.sanitize(dirty);
});

DOMPurify also works server-side with node.js as well as client-side via Browserify or similar translators. Node.js 0.x is not supported; either io.js or Node.js 4.x or newer is required.

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 bypass?

If that happens, you probably qualify for a juicy 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 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'); // becomes <p>abcdef</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() methods but requires the SAFE_FOR_JQUERY flag for that - see below.

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.

Can I configure it?

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.

// make output safe for usage in jQuery's $()/html() method (default is false)
var clean = DOMPurify.sanitize(dirty, {SAFE_FOR_JQUERY: true});

// strip {{ ... }} and <% ... %> to make output safe for template systems
var clean = DOMPurify.sanitize(dirty, {SAFE_FOR_TEMPLATES: true});

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

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

// allow all safe HTML elements but neither SVG nor MathML
var clean = DOMPurify.sanitize(dirty, {USE_PROFILES: {html: true}});

// allow all safe SVG elements and SVG Filters
var clean = DOMPurify.sanitize(dirty, {USE_PROFILES: {svg: true, svgFilters: true}});

// allow all safe MathML elements and SVG
var clean = DOMPurify.sanitize(dirty, {USE_PROFILES: {mathMl: true, svg: true}});

// leave all as it is but forbid <style>
var clean = DOMPurify.sanitize(dirty, {FORBID_TAGS: ['style']});

// leave all as it is but forbid style attributes
var clean = DOMPurify.sanitize(dirty, {FORBID_ATTR: ['style']});

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

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

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

// allow external protocol handlers in URL attributes (default is false)
// 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 (default is false)
// 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;});

// 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
var clean = DOMPurify.sanitize(dirty, {RETURN_DOM_FRAGMENT: true, RETURN_DOM_IMPORT: true});
document.body.appendChild(clean);

// 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!)
var clean = DOMPurify.sanitize(dirty, {SANITIZE_DOM: false});

// discard 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});

// 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
• 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, data, config) {
    // Do something with the current node and return it
    return currentNode;
});

Continuous Integration

We are currently using Travis CI 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://travis-ci.org/cure53/DOMPurify

You can further run local tests by executing npm test. The tests work fine with Node.js v0.6.2 and [email protected].

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 [email protected] 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 out 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?

Several people need to be listed here!

@garethheyes and @filedescriptor for invaluable help, @shafigullin for breaking the library multiple times and thereby strengthening it, @mmrupp and @irsdl for doing the same. And lastly, thanks to @ShikariSenpai and @ansjdnakjdnajkd for spotting the massive Safari 10.1 bug in the first place.

Big thanks also go to @ydaniv, @asutherland, @mathias, @cgvwzq, @robbertatwork, @giutro and @fhemberger!

Further, thanks @neilj and @0xsobky for their code reviews and countless small optimizations, fixes and beautifications.

Big thanks also go to @tdeekens for doing all the hard work and getting us on track with Travis CI and BrowserStack. And thanks to @Joris-van-der-Wel for setting up DOMPurify for jsdom and creating the additional test suite. And again @tdeekens for his incredible efforts and contribution to refactor DOMPurify into using ES201x, proper build tools, better test coverage and much more!

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