chrome-untrusted:// FAQ

What is “untrustworthy content”?

In this context, untrustworthy content is content that comes from untrustworthy sources, e.g. an image downloaded from the internet, a PDF file provided by the user, etc. Code is also considered “content” in this case.

In general, content coming from the network is considered untrustworthy, regardless of the source and transport protocol.

Examples of trustworthy content include, the contents of chrome://version which are populated entirely within the browser process, the contents of chrome://about which is a hardcoded list of URLs, etc.

What is chrome-untrusted://?

It is a new scheme which can be used to serve resources bundled with Chrome and that process untrustworthy content. It has the usual protections provided to chrome://, e.g. process isolation, but it won’t be default-granted extra capabilities that are default-granted to chrome://.

The -untrusted suffix indicates that the WebUI processes untrustworthy content. For example, rendering an image provided by users, parsing a PDF file, etc.

The -untrusted suffix does not mean the web page is designed to do malicious things, or users should not trust it. Instead, the -untrusted suffix is to signal to us, Chromium developers, that this page will process untrustworthy content, and should be assumed to be compromised, much like an ordinary renderer process.

Why do we need chrome-untrusted://?

Separate fully trusted WebUIs and untrustworthy ones

chrome-untrusted:// acts as a technical and semantic boundary between fully-trusted WebUIs and untrustworthy WebUIs.

Technical because developers can use chrome-untrusted:// to separate their WebUIs into two origins e.g. chrome://media-app and chrome-untrusted://media-app with access to different capabilities, resources, etc.

Semantic because it indicates to chromium developers and security reviewers that a WebUI is meant to process untrustworthy content and shouldn’t be granted dangerous capabilities.

chrome:// is too powerful to process untrustworthy content

Historically, chrome:// pages have been built with the assumption that they are an extension to the browser process, so chrome:// web pages are granted special capabilities not granted to ordinary web pages. For example, all chrome:// pages can use Web APIs like camera and mic without requesting permission.

Some WebUIs would like to be able to process untrustworthy content, but granting these capabilities to a chrome:// page would violate the rule of 2: running in an privileged context:

  • a chrome:// page is considered an extension to the browser process
  • the renderer is written in an unsafe programming language (C++).

By using chrome-untrusted:// we put the untrustworthy content into a sandboxed and non-privileged environment (an ordinary renderer, with no dangerous capabilities). This brings us back to safety, a compromised chrome-untrusted:// page is no worse than an ordinary web page.

chrome-untrusted:// re-uses a lot of the code that backs chrome:// pages, so it doesn’t impose a big maintenance burden; even then, our hope is to one day remove all default granted capabilities based on the chrome:// scheme to the point that the difference between chrome:// and chrome-untrusted:// WebUIs is just a semantic one (see previous point).

When is it appropriate to use chrome-untrusted://?

chrome-untrusted:// is usually used for implementing privilege separation so that processing untrustworthy content e.g. parsing JSON, displaying an image, running code from the network, etc. is done in an unprivileged context.

Today, the main use case is when we want to have code that ships with Chrome work with untrustworthy content that comes over the network.

Can I use $js_library_from_url?

Yes. “Content” in this context also includes code.

Do we grant any extra capabilities to chrome-untrusted://?

Yes, but not by default and with some caveats.

Any team that requires extra capabilities granted to chrome-untrusted:// should consult with the security team to ensure they are non-dangerous. In this context, we consider non-dangerous any API that we would expose to the renderer process, e.g. UMA.

We recommend using Mojo to expose APIs to chrome-untrusted://. Mojo for chrome-untrusted:// works similarly to how it works for chrome:// with a few key differences:

  • Unlike chrome:// pages, chrome-untrusted:// pages don't get access to all renderer exposed Mojo interfaces by default. Use PopulateChromeWebUIFrameInterfaceBrokers to expose WebUI specific interfaces to your WebUI. See this CL for example.
  • The exposed interface has a different threat model: a compromised chrome-untrusted:// page could try to exploit the interface (e.g. sending malformed messages, closing the Mojo pipe).

When exposing extra capabilities to chrome-untrusted://, keep in mind:

  • Don‘t grant any capabilities that we wouldn’t grant to a regular renderer. For example, don't expose unrestricted access to Bluetooth devices, but expose a method that opens a browser-controlled dialog where the user chooses a device.
  • What you received (from the WebUI page) is untrustworthy. You must sanitize and verify its content before processing.
  • What you send (to the WebUI page) could be exfiltrated to the Web. Don't send sensitive information (e.g. user credentials). Only send what you actually need.
  • The difference in Mojo interface lifetimes could lead to use-after-free bugs (e.g. a page reloads itself when it shouldn't). We recommend you create and reinitialize the interface on each page load (using WebUIPrimaryPageChanged), and have the JavaScript bind the interface on page load.

We also recommend using Mojo to communicate between parent and child frames whenever possible. See this CL for example.

You should only use postMessage() when transferring objects unsupported by Mojo. For example, Media App uses postMessage() to pass a read-only FileSystemHandle file handle to chrome-untrusted://media-app from its parent chrome://media-app.

We encourage teams to engage with SECURITY_OWNERS early and get the reviews required.

Can chrome-untrusted:// be the main document or does it need to be embedded in a chrome:// page?

Yes, chrome-untrusted:// can be the main document, although the most common case is for a chrome:// page to embed a chrome-untrusted:// subframe.

That said, the chrome-untrusted:// scheme is an implementation detail of the WebUI and should never be shown to users. This should be factored into account when deciding whether or not to use chrome-untrusted:// as the main document.

How do I use chrome-untrusted://?

Create a standalone chrome-untrusted:// WebUI

  1. Create a class overriding ui::WebUIConfig and another one overriding ui::UntrustedWebUIController

WebUIConfig contains properties for the chrome-untrusted:// page i.e. the host and scheme. In the future, this might also contain other properties like permissions or resources.

UntrustedWebUIController register the resources for the page.

const char kUntrustedExampleHost[] = "untrusted-example";
const char kUntrustedExampleURL[] = "chrome-untrusted://untrusted-example";

class UntrustedExampleUIConfig : public ui::WebUIConfig {
 public:
  UntrustedExampleUIConfig()
    // Set scheme and host.
    : WebUIConfig(content::kChromeUIUntrustedScheme, kUntrustedExampleHost) {}
  ~UntrustedExampleUIConfig() override = default;

  std::unique_ptr<content::WebUIController> CreateWebUIController(
      content::WebUI* web_ui) override {
    return std::make_unique<UntrustedExampleUI>(web_ui);
  }
};

class UntrustedExampleUI : public ui::UntrustedWebUIController {
 public:
  UntrustedExampleUI::UntrustedExampleUI(content::WebUI* web_ui)
    : ui::UntrustedWebUIController(web_ui) {

    // Create a URLDataSource and add resources.
    auto* untrusted_source =
      content::WebUIDataSource::Create(kUntrustedExampleURL);
    untrusted_source->AddResourcePath(...);

    // Register the URLDataSource
    auto* browser_context = web_ui->GetWebContents()->GetBrowserContext();
    content::WebUIDataSource::Add(browser_context, untrusted_source);
  }

  UntrustedExampleUI(const UntrustedExampleUI&) = delete;
  UntrustedExampleUI& operator=(const UntrustedExampleUI&) = delete;

  UntrustedExampleUI::~UntrustedExampleUI() = default;
};
  1. Register the WebUIConfig

Add the WebUIConfig to the list of WebUIConfigs in ChromeUntrustedWebUIControllerFactory.

register_config(std::make_unique<chromeos::UntrustedExampleUIConfig>());
  1. If needed, implement and register the necessary Mojo interfaces. See this CL for example.

Embed chrome-untrusted:// in chrome:// WebUIs

Developers can embed chrome-untrusted:// iframes in chrome:// pages. Example CL.

The general steps are:

  1. Create a WebUIConfig and UntrustedWebUIController to register the resources for the chrome-untrusted:// page.
  2. Allow the chrome:// WebUI to embed the corresponding chrome-untrusted:// WebUI.
untrusted_data_source->AddFrameAncestor(kWebUIPageURL)
  1. Make chrome-untrusted:// requestable by the main chrome:// WebUI.
web_ui->AddRequestableScheme(content::kChromeUIUntrustedScheme)
  1. Allow the chrome:// WebUI to embed chrome-untrusted://.
trusted_data_source->OverrideContentSecurityPolicy(
    “frame-src ” + kUntrustedExampleURL);
  1. Add communication mechanism to chrome-untrusted:// frames. For example, using Mojo, or postMessage the JavaScript object is not supported by Mojo.