ShadowRoot: innerHTML property

Value

Getting the property returns a string containing the HTML serialization of the shadow root's descendants.

Setting the property accepts either a TrustedHTML object or a string. It parses this value as HTML and replaces all the element's descendants with the result. When set to the null value, that null value is converted to the empty string (""), so shadowRoot.innerHTML = null is equivalent to shadowRoot.innerHTML = "".

Exceptions

SyntaxError DOMException

Thrown if an attempt was made to set the value of innerHTML using a string which is not properly-formed HTML.

TypeError

Thrown if the property is set to a string when Trusted Types are enforced by a CSP and no default policy is defined.

Description

innerHTML gets a serialization of the nested child DOM elements within the shadow root, or sets HTML or XML that should be parsed to replace the DOM tree within the shadow root.

Note that some browsers serialize the < and > characters as < and > when they appear in attribute values (see Browser compatibility). This is to prevent a potential security vulnerability (mutation XSS) in which an attacker can craft input that bypasses a sanitization function, enabling a cross-site scripting (XSS) attack.

Security considerations

The innerHTML property is a possible vector for cross-site scripting (XSS) attacks, where potentially unsafe strings provided by a user are injected into the DOM without first being sanitized. While the property does prevent <script> elements from executing when they are injected, it is susceptible to many other ways that attackers can craft HTML to run malicious JavaScript. For example, the following example would execute the code in the error event handler, because the <img> src value is not a valid image URL:

const name = "<img src='x' onerror='alert(1)'>";
shadowRoot.innerHTML = name; // shows the alert

You can mitigate these issues by always assigning TrustedHTML objects instead of strings, and enforcing trusted types using the require-trusted-types-for CSP directive. This ensures that the input is passed through a transformation function, which has the chance to sanitize the input to remove potentially dangerous markup before it is injected.

Examples

Reading the HTML contents of an element

Reading innerHTML causes the user agent to serialize the shadow root's descendants.

Given the following HTML:

<div class="host">
  <template shadowrootmode="open">
    <p>My name is Joe</p>
  </template>
</div>

You can get and log the markup for the shadow root as shown:

const shadowHost = document.querySelector("#host");
const shadowRoot = shadowHost.shadowRoot;
const contents = shadowRoot.innerHTML;
console.log(contents); // "\n  <p>My name is Joe</p>\n"

Setting the innerHTML of a Shadow root

In this example we'll replace an element's DOM by assigning HTML to the element's innerHTML property. To mitigate the risk of XSS, we'll first create a TrustedHTML object from the string containing the HTML, and then assign that object to innerHTML.

Trusted types are not yet supported on all browsers, so first we define the trusted types tinyfill. This acts as a transparent replacement for the Trusted Types JavaScript API:

if (typeof trustedTypes === "undefined")
  trustedTypes = { createPolicy: (n, rules) => rules };

Next we create a TrustedTypePolicy that defines a createHTML() method for transforming an input string into TrustedHTML instances. Commonly, implementations of createHTML() use a library such as DOMPurify to sanitize the input as shown below:

const policy = trustedTypes.createPolicy("my-policy", {
  createHTML: (input) => DOMPurify.sanitize(input),
});

Then we use this policy object to create a TrustedHTML object from the potentially unsafe input string, and assign the result to the element:

// The potentially malicious string
const untrustedString = "<p>I might be XSS</p><img src='x' onerror='alert(1)'>";

// Create a TrustedHTML instance using the policy
const trustedHTML = policy.createHTML(untrustedString);

// Get the shadow root
const shadowHost = document.querySelector("#host");
const shadowRoot = shadowHost.shadowRoot;

// Inject the TrustedHTML (which contains a trusted string)
shadowRoot.innerHTML = trustedHTML;

Specifications

Browser compatibility