ShadowRoot: setHTML() method

input

A string defining HTML to be sanitized and injected into the shadow root.

options Optional

An options object with the following optional parameters:

sanitizer

A Sanitizer or SanitizerConfig object which defines what elements of the input will be allowed or removed, or the string "default" for the default sanitizer configuration. Note that generally a "Sanitizer is expected to be more efficient than a SanitizerConfig if the configuration is to reused. If not specified, the default sanitizer configuration is used.

None (undefined).

TypeError

This is thrown if options.sanitizer is passed a:

• non-normalized SanitizerConfig (one that includes both "allowed" and "removed" configuration settings).
• string that does not have the value "default".
• value that is not a Sanitizer, SanitizerConfig, or string.

The setHTML() method provides an XSS-safe method to parse and sanitize a string of HTML and use it to replace the existing tree in the Shadow DOM.

setHTML() removes any HTML entities that aren't allowed by the sanitizer configuration, and further removes any XSS-unsafe elements or attributes — whether or not they are allowed by the sanitizer configuration.

If no sanitizer configuration is specified in the options.sanitizer parameter, setHTML() is used with the default Sanitizer configuration. This configuration allows all elements and attributes that are considered XSS-safe, thereby disallowing entities that are considered unsafe. A custom sanitizer or sanitizer configuration can be specified to choose which elements, attributes, and comments are allowed or removed. Note that even if unsafe options are allowed by the sanitizer configuration, they will still be removed when using this method (which implicitly calls Sanitizer.removeUnsafe()).

setHTML() should be used instead of ShadowRoot.innerHTML for inserting untrusted strings of HTML into the shadow DOM. It should also be used instead of ShadowRoot.setHTMLUnsafe(), unless there is a specific need to allow unsafe elements and attributes.

Note that since this method always sanitizes input strings of XSS-unsafe entities, it is not secured or validated using the Trusted Types API.

This example shows some of the ways you can use setHTML() to sanitize and inject a string of HTML.

First we will create the ShadowRoot we want to target. This could be created programmatically using Element.attachShadow() but for this example we'll create the root declaratively.

<div id="host">
  <template shadowrootmode="open">
    <span>A span element in the shadow DOM</span>
  </template>
</div>

We can get a handle to the shadow root from the #host element like this:

const shadow = document.querySelector("#host").shadowRoot;

The code below shows how we can call setHTML() with a string and different sanitizers in order to filter and inject the HTML into the shadow root.

// Define unsanitized string of HTML
const unsanitizedString = "abc <script>alert(1)<" + "/script> def";

// setHTML() with default sanitizer
shadow.setHTML(unsanitizedString);

// Define custom Sanitizer and use in setHTML()
// This allows only elements: <div>, <p>, <span> (<script> is unsafe and will be removed)
const sanitizer1 = new Sanitizer({ elements: ["div", "p", "span", "script"] });
shadow.setHTML(unsanitizedString, { sanitizer: sanitizer1 });

// Define custom SanitizerConfig within setHTML()
// This removes elements <div>, <p>, <span>, <script>, and any other unsafe elements/attributes
shadow.setHTML(unsanitizedString, {
  sanitizer: { removeElements: ["div", "p", "span", "script"] },
});

This example provides a "live" demonstration of the method when called with different sanitizers. The code defines buttons that you can click to sanitize and inject a string of HTML using a default and a custom sanitizer, respectively. The original string and sanitized HTML are logged so you can inspect the results in each case.

HTML

The HTML defines two <button> elements for applying different sanitizers, another button to reset the example, and a <div> that contains the declarative shadow root.

<button id="buttonDefault" type="button">Default</button>
<button id="buttonAllowScript" type="button">allowScript</button>
<button id="reload" type="button">Reload</button>

<div id="host">
  <template shadowrootmode="open">
    <span>I am in the shadow DOM </span>
  </template>
</div>

<pre id="log"></pre>

#log {
  height: 220px;
  overflow: scroll;
  padding: 0.5rem;
  border: 1px solid black;
  margin: 5px;
}

JavaScript

const logElement = document.querySelector("#log");
function log(text) {
  logElement.textContent += text;
}

if ("Sanitizer" in window) {

First we define the handler for the reload button.

const reload = document.querySelector("#reload");
reload.addEventListener("click", () => document.location.reload());

Then we define the string to sanitize, which will be the same for all cases. This contains the <script> element and the onclick handler, both of which are considered XSS-unsafe. We also get variable shadow, which is our handle to the shadow root.

// Define unsafe string of HTML
const unsanitizedString = `
  <div>
    <p>Paragraph to inject into shadow DOM. <button onclick="alert('You clicked the button!')">Click me</button></p>
    <script src="path/to/a/module.js" type="module"><script>
  </div>
`;

const shadow = document.querySelector("#host").shadowRoot;

Next we define the click handler for the button that sets the shadow root with the default sanitizer. This should strip out all unsafe entities before inserting the string of HTML. Note that you can see exactly which elements are removed in the Sanitizer() constructor examples.

const defaultSanitizerButton = document.querySelector("#buttonDefault");
defaultSanitizerButton.addEventListener("click", () => {
  // Set the content of the element using the default sanitizer
  shadow.setHTML(unsanitizedString);

  // Log HTML before sanitization and after being injected
  logElement.textContent =
    "Default sanitizer: remove <script> element and onclick attribute\n\n";
  log(`\nunsanitized: ${unsanitizedString}`);
  log(`\nsanitized: ${shadow.innerHTML}`);
});

The next click handler sets the target HTML using a custom sanitizer that allows only <div>, <p>, and <script> elements. Note that because we're using the setHTML method, <script> will also be removed!

const allowScriptButton = document.querySelector("#buttonAllowScript");
allowScriptButton.addEventListener("click", () => {
  // Set the content of the element using a custom sanitizer
  const sanitizer1 = new Sanitizer({
    elements: ["div", "p", "script"],
  });
  shadow.setHTML(unsanitizedString, { sanitizer: sanitizer1 });

  // Log HTML before sanitization and after being injected
  logElement.textContent =
    "Sanitizer: {elements: ['div', 'p', 'script']}\n Script removed even though allowed\n";
  log(`\nunsanitized: ${unsanitizedString}`);
  log(`\nsanitized: ${shadow.innerHTML}`);
});

} else {
  log("The HTML Sanitizer API is NOT supported in this browser.");
  // Provide fallback or alternative behavior
}

Results

Click the "Default" and "allowScript" buttons to see the effects of the default and custom sanitizer, respectively. Note that because we are using a same sanitization method, in both cases the <script> element and onclick handler are removed, even if explicitly allowed by the sanitizer.

• ShadowRoot.setHTMLUnsafe()
• Element.setHTML() and Element.setHTMLUnsafe()
• Document.parseHTML() and Document.parseHTMLUnsafe()
• HTML Sanitizer API