Trusted Types API

The Trusted Types API gives web developers a way to lock down the insecure parts of the DOM API to prevent client-side Cross-site scripting (XSS) attacks.

Concepts and Usage

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.

Injection Sinks

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 santized input. Examples of injection sinks include:

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.

Trusted Type Policies

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.

Interfaces

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 an 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.
TrustedTypePolicyOptions
A dictionary that holds author-defined functions for converting string values into trusted values.

Examples

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, "<")
});

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.

Specifications

Specification Status Comment
Trusted Types Editor's Draft Initial definition.

Browser compatibility

See the compatibility data for each of the Trusted Types API interfaces.

Polyfill

A polyfill is available. The polyfill is also available as an npm package trusted-types.

See also