Trusted Types API

Client-side, or DOM-based, XSS attacks happen when data controlled by a user (such as that input into a form field) reaches a function that can execute that data. These functions are known as injection sinks. DOM-based XSS attacks happen when a user is able to write arbitrary JavaScript code and have it executed by one of these functions.

The Trusted Types API locks down risky injection sinks, requiring you to process the data before passing it to one of these functions. If you use a string, then the browser will throw a TypeError and prevent the use of the function.

Trusted Types works alongside Content-Security Policy with the trusted-types and require-trusted-types-for directives.

The Trusted Types API locks down injection sinks that can act as a vector for DOM-XSS attacks. An injection sink is any Web API function that should only be called with trusted, validated or sanitized input. Examples of injection sinks include:

• Functions that insert HTML into the document such as Element.innerHTML, Element.outerHTML, or Document.write().
• Functions that create a new same-origin Document with caller-controlled markup such as DOMParser.parseFromString().
• Functions that execute code such as eval().
• Setters for Element attributes that accept a URL of code to load or execute.

Trusted Types will force you to process the data before passing it to any injection sink rather than use a string. This ensures that the data is trustworthy.

A policy is a factory for Trusted Types. Web developers can specify a set of policies used for the creation of typed objects which form the trusted codebase for valid Trusted Type objects.

TrustedHTML

Represents a string to insert into an injection sink that will render it as HTML.

TrustedScript

Represents a string to insert into an injection sink that could lead to the script being executed.

TrustedScriptURL

Represents a string to insert into an injection sink that will parse it as a URL of an external script resource.

TrustedTypePolicy

Defines the functions used to create the above Trusted Type objects.

TrustedTypePolicyFactory

Creates policies and verifies that Trusted Type object instances were created via one of the policies.

In the below example we create a policy that will create TrustedHTML objects using TrustedTypePolicyFactory.createPolicy(). We can then use TrustedTypePolicy.createHTML() to create a sanitized HTML string to be inserted into the document.

The sanitized value can then be used with Element.innerHTML to ensure that no new HTML elements can be injected.

<div id="myDiv"></div>

const escapeHTMLPolicy = trustedTypes.createPolicy("myEscapePolicy", {
  createHTML: (string) =>
    string
      .replace(/&/g, "&")
      .replace(/</g, "&lt;")
      .replace(/"/g, "&quot;")
      .replace(/'/g, "&apos;"),
});

let el = document.getElementById("myDiv");
const escaped = escapeHTMLPolicy.createHTML("<img src=x onerror=alert(1)>");
console.log(escaped instanceof TrustedHTML); // true
el.innerHTML = escaped;

Read more about this example, and discover other ways to sanitize input in the article Prevent DOM-based cross-site scripting vulnerabilities with Trusted Types.

• Prevent DOM-based cross-site scripting vulnerabilities with Trusted Types
• Trusted Types polyfill (also available as an npm package)