[Note: This article also appears as a guest post on the SANS Application Security blog]
Popular web browsers today do not allow arbitrary websites to modify the text displayed in the address bar or to hide the address bar (some browsers may allow popups to hide the address bar but in such cases the URL is then displayed in the title of the window). The reasoning behind this behavior is quite simple: if browsers can be influenced by arbitrary web applications to hide the URL or to modify how it is displayed, then malicious web applications can spoof User Interface elements to display arbitrary URLs thus tricking the user to thinking he or she is browsing a trusted site.
I'd like to call your attention to the behavior of Safari on the iPhone via a proof of concept demo. If you have an iPhone, browse to the following demo and keep an eye out on the address bar:
For those who do not have an iPhone available, here is a video:
And here are two images detailing the issue:
Figure: Image on left illustrates the page rendered which displays the 'fake' URL bar while the real URL bar is hidden above. Image on right illustrates the real URL bar that is visible once the user scrolls up.
Notice that the address bar stays visible while the page renders, but immediately disappears as soon as it is rendered. Perhaps this may give the user some time to notice but it is not a reasonably reliable control (and I don't think Apple intended it to be).
Since I used Bank of America's mobile site for the demo, it makes sense to point out that Bank of America (like many other institutions who are a frequent target of phishing attacks) advises their customers to watch the browser address bar in their Online Banking Security and Technical FAQ. However, when you go to Bank of America's (mobile) site using Safari on the iPhone, the very address bar they recommend their customers watch for, disappears from sight.
I did contact Apple about this issue and they let me know they are aware of the implications but do not know when and how they will address the issue.
I have two main thoughts on this behavior, outlined below:
1. Precious screen real estate on mobile devices. This is most likely the primary reason why the address bar disappears upon page load on the iPhone. Note that on the iPhone, this only happens for websites that follow directives in HTML to advertise themselves as mobile sites (see the source of the index.html in the demo site above for example).
Since the address bar in Safari occupies considerable real estate, perhaps Apple may consider displaying or scrolling the current domain name right below the universal status bar (i.e. below the carrier and time stamp). Positioning the current domain context in a location that is unalterable by the rendered web content can provide the users similar indication that browsers such as IE and Chrome provide by highlighting the current domain being rendered.
2. The consequences of full screen apps in iOS using UIWebView. Desktop operating systems most often launch the default web browser of choice when a http or https handler is invoked (this is most often the case even though the operating systems provide interface elements that can be used to render web content within the applications).
However, in the case of iOS, since most applications are full-screen, it is in the interest of the application designers to keep the users immersed within their application instead of yanking the user out into Safari to render web content. Given this situation, it becomes vital for iOS to provide consistency so the user can be ultimately assured what domain the web content is being rendered.
To render web content within applications, all developers have to do is invoke the UIWebView class. It is as simple as invoking a line of code such as [webView loadRequest:requestObj]; where requestObj contains the URL to render.
Figure: Twitter App rendering web content on the iPad.
The screenshot above illustrates web-content rendered by the fantastic Twitter app on the iPad. To create this screen-shot, I launched the Twitter app on the iPad and selected a tweet from @appleinsider and clicked on the URL http://dlvr.it/9D81j in the tweet. Notice that the URL of the actual page being rendered is no where to be seen.
In such cases, it is clear that developers of iOS applications need to make sure they clearly display the ultimate domain from which they are rending web content. A welcome addition to this would be default behavior on part of UIWebView to display the current domain context in a designated and consistent location.
Given how rampant phishing and malware attempts are these days, I hope Apple chooses to not allow arbitrary web applications to scroll the real Safari address bar out of view. In the case of applications that utilize UIWebView, I recommend a designated screen location label only accessible by iOS that displays the domain from where the web content is being rendered when serving requests via calls to UIWebView. That said, I do realize how precious real estate is on mobile devices and if Apple choses to come up with a better way of addressing this issue, I'd welcome and look forward to that as well.