Prevent DOM-based cross-site scripting vulnerabilities with Trusted Types

Krzysztof Kotowicz
Krzysztof Kotowicz
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DOM-based cross-site scripting (DOM XSS) happens when data from a user-controlled source (like a username, or a redirect URL taken from the URL fragment) reaches a sink, which is a function like eval() or a property setter like .innerHTML that can execute arbitrary JavaScript code.

DOM XSS is one of the most common web security vulnerabilities, and it's common for dev teams to accidentally introduce it in their apps. Trusted Types give you the tools to write, security review, and keep applications free of DOM XSS vulnerabilities by making dangerous web API functions secure by default. Trusted Types are available as a polyfill for browsers that don't yet support them.

Background

For many years DOM XSS has been one of the most prevalent and dangerous web security vulnerabilities.

There are two kinds of cross-site scripting. Some XSS vulnerabilities are caused by server-side code that insecurely creates the HTML code forming the website. Others have a root cause on the client, where the JavaScript code calls dangerous functions with user-controlled content.

To prevent server-side XSS, don't generate HTML by concatenating strings. Use safe contextual-autoescaping templating libraries instead, along with a nonce-based Content Security Policy for additional bug mitigation.

Now browsers can also help prevent client-side DOM-based XSS by using Trusted Types.

API introduction

Trusted Types work by locking down the following risky sink functions. You might already recognize some of them, because browser vendors and web frameworks already steer you away from using these features for security reasons.

Trusted Types require you to process the data before passing it to these sink functions. Using only a string fails, because the browser doesn't know if the data is trustworthy:

Don't
anElement.innerHTML  = location.href;
With Trusted Types enabled, the browser throws a TypeError and prevents use of a DOM XSS sink with a string.

To signify that the data was securely processed, create a special object - a Trusted Type.

Do
anElement.innerHTML = aTrustedHTML;
With Trusted Types enabled, the browser accepts a TrustedHTML object for sinks that expect HTML snippets. There are also TrustedScript and TrustedScriptURL objects for other sensitive sinks.

Trusted Types significantly reduce the DOM XSS attack surface of your application. It simplifies security reviews, and lets you enforce the type-based security checks done when compiling, linting, or bundling your code at runtime, in the browser.

How to use Trusted Types

Prepare for Content Security Policy violation reports

You can deploy a report collector, such as the open-source go-csp-collector, or use one of the commercial equivalents. You can also debug violations in the browser:

document.addEventListener('securitypolicyviolation',
    console.error.bind(console));

Add a report-only CSP header

Add the following HTTP Response header to documents that you want to migrate to Trusted Types:

Content-Security-Policy-Report-Only: require-trusted-types-for 'script'; report-uri //my-csp-endpoint.example

Now all the violations are reported to //my-csp-endpoint.example, but the website continues to work. The next section explains how //my-csp-endpoint.example works.

Identify Trusted Types violations

From now on, every time Trusted Types detect a violation, the browser sends a report to a configured report-uri. For example, when your application passes a string to innerHTML, the browser sends the following report:

{
"csp-report": {
    "document-uri": "https://my.url.example",
    "violated-directive": "require-trusted-types-for",
    "disposition": "report",
    "blocked-uri": "trusted-types-sink",
    "line-number": 39,
    "column-number": 12,
    "source-file": "https://my.url.example/script.js",
    "status-code": 0,
    "script-sample": "Element innerHTML <img src=x"
}
}

This says that in https://my.url.example/script.js on line 39, innerHTML was called with the string beginning with <img src=x. This information should help you narrow down which parts of code might be introducing DOM XSS and need to change.

Fix the violations

There are a couple of options for fixing a Trusted Type violation. You can remove the offending code, use a library, create a Trusted Type policy or, as a last resort, create a default policy.

Rewrite the offending code

It's possible that the non-conforming code isn't needed anymore, or can be rewritten without the functions that cause the violations:

Do
el.textContent = '';
const img = document.createElement('img');
img.src = 'xyz.jpg';
el.appendChild(img);
Don't
el.innerHTML = '';

Use a library

Some libraries already generate Trusted Types that you can pass to the sink functions. For example, you can use DOMPurify to sanitize an HTML snippet, removing XSS payloads.

import DOMPurify from 'dompurify';
el.innerHTML = DOMPurify.sanitize(html, {RETURN_TRUSTED_TYPE: true});

DOMPurify supports Trusted Types and returns sanitized HTML wrapped in a TrustedHTML object so that the browser doesn't generate a violation.

Create a Trusted Type policy

Sometimes you can't remove the code causing the violation, and there's no library to sanitize the value and create a Trusted Type for you. In those cases, you can create a Trusted Type object yourself.

First, create a policy. Policies are factories for Trusted Types that enforce certain security rules on their input:

if (window.trustedTypes && trustedTypes.createPolicy) { // Feature testing
  const escapeHTMLPolicy = trustedTypes.createPolicy('myEscapePolicy', {
    createHTML: string => string.replace(/\</g, '&lt;')
  });
}

This code creates a policy called myEscapePolicy that can produce TrustedHTML objects using its createHTML() function. The defined rules HTML-escape < characters to prevent the creation of new HTML elements.

Use the policy like this:

const escaped = escapeHTMLPolicy.createHTML('<img src=x onerror=alert(1)>');
console.log(escaped instanceof TrustedHTML);  // true
el.innerHTML = escaped;  // '&lt;img src=x onerror=alert(1)>'

Use a default policy

Sometimes you can't change the offending code, for example, if you're loading a third-party library from a CDN. In that case, use a default policy:

if (window.trustedTypes && trustedTypes.createPolicy) { // Feature testing
  trustedTypes.createPolicy('default', {
    createHTML: (string, sink) => DOMPurify.sanitize(string, {RETURN_TRUSTED_TYPE: true})
  });
}

The policy named default is used wherever a string is used in a sink that only accepts Trusted Type.

Switch to enforcing Content Security Policy

When your application no longer produces violations, you can start enforcing Trusted Types:

Content-Security-Policy: require-trusted-types-for 'script'; report-uri //my-csp-endpoint.example

Now, no matter how complex your web application is, the only thing that can introduce a DOM XSS vulnerability is the code in one of your policies, and you can lock that down even more by limiting policy creation.

Further reading