Document: parseHTMLUnsafe() static method

Syntax

Document.parseHTMLUnsafe(input)
Document.parseHTMLUnsafe(input, options)

Parameters

input

A TrustedHTML instance or a string defining HTML to be parsed.

options Optional

An options object with the following optional parameters:

sanitizer Optional

A Sanitizer or SanitizerConfig object which defines what elements of the input will be allowed or removed. This can also be a string with the value "default", which applies a Sanitizer with the (XSS-safe) default sanitizer configuration. If not specified, no sanitizer is used.

Note that if you're using the same configuration multiple times, it's expected to be more efficient to use a Sanitizer and modify it when you need to.

Return value

A Document.

Exceptions

TypeError

This is thrown if:

• html is passed a string when Trusted Types are enforced by a CSP and no default policy is defined.
• options.sanitizer is passed a:
  • SanitizerConfig that isn't valid. For example, a configuration 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.

Description

The parseHTMLUnsafe() static method can be used to create a new Document instance, optionally filter out unwanted elements and attributes. The resulting Document will have a content type of "text/html", a character set of UTF-8, and a URL of "about:blank".

The input HTML may include declarative shadow roots. If the string of HTML defines more than one declarative shadow root in a particular shadow host then only the first ShadowRoot is created — subsequent declarations are parsed as <template> elements within that shadow root.

parseHTMLUnsafe() doesn't perform any sanitization by default. If no sanitizer is passed as a parameter, all HTML entities in the input will be injected.

Security considerations

The suffix "Unsafe" in the method name indicates that it does not enforce removal of all XSS-unsafe HTML entities (unlike Document.parseHTML()). While it can do so if used with an appropriate sanitizer, it doesn't have to use an effective sanitizer, or any sanitizer at all! The method is therefore 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.

You should mitigate this risk by always passing 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 (such as <script> elements and event handler attributes), before it is injected.

Using TrustedHTML makes it possible to audit and check that sanitization code is effective in just a few places, rather than scattered across all your injection sinks. You should not need to pass a sanitizer to the method when using TrustedHTML.

If for any reason you can't use TrustedHTML (or even better, setHTML()) then the next safest option is to use setHTMLUnsafe() with the XSS-safe default sanitizer configuration.

Specifications

Browser compatibility

See also

• Document.parseHTML()
• Element.setHTML() and Element.setHTMLUnsafe()
• ShadowRoot.setHTML() and ShadowRoot.setHTMLUnsafe()
• DOMParser.parseFromString() for parsing HTML or XML into a DOM tree
• HTML Sanitizer API