Right-To-Left Override Twitter Name

Its rare to find devs anticipating Unicode control characters showing up in user input. And the most fun when unanticipated is the Right-To-Left Override character U+202E. Unicode characters have an implicit direction so that for example by default Hebrew characters are rendered from right to left, and English characters are rendered left to right. The override characters force an explicit direction for all the text that follows.

I chose my Twitter display name to include the HTML encoding of the Right-To-Left Override character #x202E; as a sort of joke or shout out to my favorite Unicode control character. I did not anticipate that some Twitter clients in some of their UI would fail to encode it correctly. There's no way I can remove that from my display name now.

Try it on Amazon.

How about pages that want to tell you about the U+202E. 

Scrollbars in EdgeHtml WebView and Edge browser

The scrollbars in UWP WebView and in Edge have different default behavior leading to many emails to my team. (Everything I talk about here is for the EdgeHtml based WebView and Edge browser and does not apply to the Chromium based Edge browser and WebView2).

There is a Edge only -ms-overflow-style CSS property that controls scroll behavior. We have a different default for this in the WebView as compared to the Edge browser. If you want the appearance of the scrollbar in the WebView to match the browser then you must explicitly set that CSS property. The Edge browser default is scrollbar which gives us a Windows desktop styled non-auto-hiding scrollbar. The WebView default is -ms-autohiding-scrollbar which gives a sort of compromise between desktop and UWP app scrollbar behavior. In this configuration it is auto-hiding. When used with the mouse you'll get Windows desktop styled scrollbars and when used with touch you'll get the UWP styled scrollbars.

Since WebViews are intended to be used in apps this style is the default in order to better match the app's scrollbars. However this difference between the browser and WebView has led to confusion.

Here’s an -ms-overflow-style JSFiddle showing the difference between the two styles. Try it in the Edge browser and in WebView. An easy way to try it in the Edge WebView is using the JavaScript Browser.

Edge browser and JavaScript UWP app security model comparison

There are two main differences in terms of security between a JavaScript UWP app and the Edge browser:

Process Model

A JavaScript UWP app has one process (technically not true with background tasks and other edge cases but ignoring that for the moment) that runs in the corresponding appcontainer defined by the app's appx manifest. This one process is where edgehtml is loaded and is rendering HTML, talking to the network, and executing script. Specifically, the UWP main UI thread is the one where your script is running and calling into WinRT.

In the Edge browser there is a browser process running in the same appcontainer defined by its appx manifest, but there are also tab processes. These tab processes are running in restricted app containers that have fewer appx capabilities. The browser process has XAML loaded and coordinates between tabs and handles some (non-WinRT) brokering from the tab processes. The tab processes load edgehtml and that is where they render HTML, talk to the network and execute script.

There is no way to configure the JavaScript UWP app's process model but using WebViews you can approximate it. You can create out of process WebViews and to some extent configure their capabilities, although not to the same extent as the browser. The WebView processes in this case are similar to the browser's tab processes. See the MSWebViewProcess object for configuring out of process WebView creation. I also implemented out of proc WebView tabs in my JSBrowser fork.

ApplicationContentUriRules

The ApplicationContentUriRules (ACUR) section of the appx manifest lets an application define what URIs are considered app code. See a previous post for the list of ACUR effects.

Notably app code is able to access WinRT APIs. Because of this, DOM security restrictions are loosended to match what is possible with WinRT.

Privileged DOM APIs like geolocation, camera, mic etc require a user prompt in the browser before use. App code does not show the same browser prompt. There still may be an OS prompt – the same prompt that applies to any UWP app, but that’s usually per app not per origin.

App code also gets to use XMLHttpRequest or fetch to access cross origin content. Because UWP apps have separate state, cross origin here might not mean much to an attacker unless your app also has the user login to Facebook or some other interesting cross origin target.

Changing the User Agent string in UWP WebView

There's no perfect way to change the user agent string for the UWP WebView (x-ms-webview in HTML, Windows.UI.Xaml.Controls.WebView in XAML, and Windows.Web.UI.Interop.WebViewControl in Win32) but there are two imperfect methods folks end up using.

The first is to call UrlMkSetSessionOption. This is an old public API that allows you to configure various arcane options including one that is the default user agent string for requests running through urlmon. This API is allowed by the Microsoft Store for UWP apps. The change it applies is process wide which has two potential drawbacks. If you want to be able to have different UA strings set for different requests from a WebView that's not really possible with this solution. The other drawback is if you're using out of process WebView, you need to ensure you're calling into UrlMkSetSessionOption in the WebView's process. You'll need to write third party WinRT that calls UrlMkSetSessionOption, create the out of proc WebView, navigate it to some trusted local page, use AddWebAllowedObject or provide that URI WinRT access, and call into your third party WinRT. You'll need to do that for any new WebView process you create.

The second less generally applicable solution is to use NavigateWithHttpRequestMessage and set the User-Agent HTTP header. In this case you get to control the scope of the user agent string changes but has the limitations that not all sub resource downloads will use this user agent string and for navigations you don't initiate you have to manually intercept and re-request being careful to transfer over all POST body state and HTTP headers correctly. That last part is not actually possible for iframes.

GoBack/GoForward in Win10 UWP WebView

The GoBack and GoForward methods on the UWP WebView (x-ms-webview in HTML, Windows.UI.Xaml.Controls.WebView in XAML, and Windows.Web.UI.Interop.WebViewControl in Win32) act the same as the Back and Forward buttons in the Edge browser. They don't necessarily change the top level document of the WebView. If inside the webview an iframe navigates then that navigation will be recorded in the forward/back history and the GoBack / GoForward call may result in navigating that iframe. This makes sense as an end user using the Edge browser since if I click a link to navigate one place and then hit Back I expect to sort of undo that most recent navigation regardless of if that navigation happened in an iframe or the top level document.

If that doesn't make sense for your application and you want to navigate forward or back ignoring iframe navigates, unfortunately there's no perfect workaround.

One workaround could be to try calling GoBack and then checking if a FrameNavigationStarting event fires or a NavigationStarting event fires. If a frame navigates then try calling GoBack again. There could be async races in this case since other navigates could come in and send you the wrong signal and interrupt your multi step GoBack operation.

You could also try keeping track of all top level document navigations and manually navigate back to the URIs you care about. However, GoBack and GoForward also restore some amount of user state (form fills etc) in addition to navigating. Manually calling navigate will not give this same behavior.

Windows.Web.UI.Interop.WebViewControl localhost access

If you're developing with the new Windows.Web.UI.Interop.WebViewControl you may have noticed you cannot navigate to localhost HTTP servers. This is because the WebViewControl's WebView process is a UWP process. All UWP processes by default cannot use the loopback adapter as a security precaution. For development purposes you can allow localhost access using the checknetisolation command line tool on the WebViewControl's package just as you can for any other UWP app. The command should be the following:

checknetisolation loopbackexempt -a -n=Microsoft.Win32WebViewHost_cw5n1h2txyewy

As a warning checknetisolation is not good on errors. If you attempt to add a package but get its package family name wrong, checknetisolation just says OK:

C:\Users\davris>checknetisolation LoopbackExempt -a -n=Microsoft.BingWeather_4.21.2492.0_x86__8wekyb3d8bbwe
OK.

And if you then list the result of the add with the bad name you'll see the following:

[1] -----------------------------------------------------------------
    Name: AppContainer NOT FOUND
    SID:  S-1-15-...

There's also a UI tool for modifying loopback exemption for packages available on GitHub and also one available with Fiddler.

As an additional note, I mentioned above you can try this for development. Do not do this in shipping products as this turns off the security protection for any consumer of the WebViewControl.

Win10 PWA Terminology

Folks familiar with JavaScript UWP apps in Win10 have often been confused by what PWAs in Win10 actually are. TLDR: PWAs in Win10 are simply JavaScript UWP apps. The main difference between these JS UWP Apps and our non-PWA JS UWP apps are our target end developer audience, and how we get Win10 PWAs into the Microsoft Store. See this Win10 blog post on PWAs on Win10 for related info.

Web App

On the web a subset of web sites are web apps. These are web sites that have app like behavior - that is a user might call it an app like Outlook, Maps or Gmail. And they may also have a W3C app manifest.

A subset of web apps are progressive web apps. Progressive web apps are web apps that have a W3C app manifest and a service worker. Various OSes are beginning to support PWAs as first class apps on their platform. This is true for Win10 as well in which PWAs are run as a WWA.

Windows Web App

In Win10 a WWA (Windows Web App) is an unofficial term for a JavaScript UWP app. These are UWP apps so they have an AppxManifest.xml, they are packaged in an Appx package, they run in an App Container, they use WinRT APIs, and are installed via the Microsoft Store. Specific to WWAs though, is that the AppxManifest.xml specifies a StartPage attribute identifying some HTML content to be used as the app. When the app is activated the OS will create a WWAHost.exe process that hosts the HTML content using the EdgeHtml rendering engine.

Packaged vs Hosted Web App

Within that we have a notion of a packaged web app and an HWA (hosted web app). There's no real technical distinction for the end developer between these two. The only real difference is whether the StartPage identifies remote HTML content on the web (HWA), or packaged HTML content from the app's appx package (packaged web app). An end developer may create an app that is a mix of these as well, with HTML content in the package and HTML content from the web. These terms are more like ends on a continuum and identifying two different developer scenarios since the underlying technical aspect is pretty much identical.

Win10 PWA

Win10 PWAs are simply HWAs that specify a StartPage of a URI for a PWA on the web. These are still JavaScript UWP apps with all the same behavior and abilities as other UWP apps. We have two ways of getting PWAs into the Microsoft Store as Win10 PWAs. The first is PWA Builder which is a tool that helps PWA end developers create and submit to the Microsoft Store a Win10 PWA appx package. The second is a crawler that runs over the web looking for PWAs which we convert and submit to the Store using an automated PWA Builder-like tool to create a Win10 PWA from PWAs on the web (see Welcoming PWAs to Win10 for more info). In both cases the conversion involves examining the PWAs W3C app manifest and producing a corresponding AppxManifest.xml. Not all features supported by AppxManifest.xml are also available in the W3c app manifest. But the result of PWA Builder can be a working starting point for end developers who can then update the AppxManifest.xml as they like to support features like share targets or others not available in W3C app manifests.