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 ofchrome://version which are populated entirely within the browser process, the contents ofchrome://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 tochrome://, e.g. process isolation, but it won’t be default-granted extra capabilities that are default-granted tochrome://.

The-untrusted suffix indicates that theWebUI 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 usechrome-untrusted:// to separate their WebUIs into two origins e.g.chrome://media-app andchrome-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, sochrome:// web pages are granted special capabilities not granted to ordinary web pages. For example, allchrome:// 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 achrome:// page would violate therule of 2:

• achrome:// page is considered an extension to the browser process
• the renderer is written in an unsafe programming language (C++).
• running in an privileged context:

By usingchrome-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 compromisedchrome-untrusted:// page is no worse than an ordinary web page.

chrome-untrusted:// re-uses a lot of the code that backschrome:// 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 thechrome:// scheme to the point that the difference betweenchrome:// andchrome-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 tochrome-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 tochrome-untrusted://. Mojo forchrome-untrusted:// works similarly to how it works forchrome:// with a few key differences:

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

When exposing extra capabilities tochrome-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 (usingWebUIPrimaryPageChanged), and have the JavaScript bind the interface on page load.

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

You should only usepostMessage() when transferring objects unsupported by Mojo. For example, Media App usespostMessage() to pass a read-onlyFileSystemHandle file handle tochrome-untrusted://media-app from its parentchrome://media-app.

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

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

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

That said, thechrome-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 usechrome-untrusted:// as the main document.

How do I use chrome-untrusted://?

Create a standalone chrome-untrusted:// WebUI

1. Create a class overridingui::WebUIConfig and another one overridingui::UntrustedWebUIController

WebUIConfig contains properties for thechrome-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.

constchar kUntrustedExampleHost[]="untrusted-example";constchar kUntrustedExampleURL[]="chrome-untrusted://untrusted-example";classUntrustedExampleUIConfig:public content::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);}};classUntrustedExampleUI: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::CreateAndAdd(          web_ui->GetWebContents()->GetBrowserContext(), kUntrustedExampleURL);    untrusted_source->AddResourcePath(...);}UntrustedExampleUI(constUntrustedExampleUI&)=delete;UntrustedExampleUI&operator=(constUntrustedExampleUI&)=delete;UntrustedExampleUI::~UntrustedExampleUI()=default;};

2. Register the WebUIConfig

Add theWebUIConfig to the appropriate list of WebUIConfigs inchrome_untrusted_web_ui_configs.

register_config(std::make_unique<chromeos::UntrustedExampleUIConfig>());

3. If needed, implement and register the necessary Mojo interfaces. Seethis CL for example.

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

Developers can embedchrome-untrusted:// iframes inchrome:// pages.Example CL.

The general steps are:

1. Create a WebUIConfig and UntrustedWebUIController to register the resources for thechrome-untrusted:// page.
2. Allow thechrome:// WebUI to embed the correspondingchrome-untrusted:// WebUI.

untrusted_data_source->AddFrameAncestor(kWebUIPageURL)

3. Makechrome-untrusted:// requestable by the mainchrome:// WebUI.

web_ui->AddRequestableScheme(content::kChromeUIUntrustedScheme)

4. Allow thechrome:// WebUI to embed chrome-untrusted://.

trusted_data_source->OverrideContentSecurityPolicy(“frame-src”+ kUntrustedExampleURL);

5. Add communication mechanism tochrome-untrusted:// frames. For example,using Mojo, orpostMessage the JavaScript object is not supported by Mojo.