Mozilla's creative lead for Firefox, Aza Raskin, has developed novel phishing attack that Firefox engineers will need address.

Raskin calls the attack "tabnapping" because it can replace the content of a Web page that's open in an inactive browser tab -- and thus isn't visible at the moment it's being changed -- with a page designed to capture personal information.

While the user of a Web browser is looking at an active tab, malicious JavaScript code on one of the hidden, inactive tabs that happens to be open can replace the loaded page with new content, such as a fake Gmail login page.

As Raskin demonstrates on his Web site, the code also replaces the favicon -- the tiny graphic element that shows up on tabs and to the left of the browser address bar -- of the inactive tab page with the Gmail favicon.

The result is a phishing page that is very easy to mistake for a legitimate Gmail login page.

"As the user scans their many open tabs, the favicon and title act as a strong visual cue -- memory is malleable and moldable and the user will most likely simply think they left a Gmail tab open," explains Raskin in a blog post. "When they click back to the fake Gmail tab, they’ll see the standard Gmail login page, assume they’ve been logged out, and provide their credentials to log in. The attack preys on the perceived immutability of tabs."

The attack works in current versions of Firefox, but not current versions of Chrome or Safari. It's certainly avoidable, if the user is alert. But such attacks rely on the fact that users are often not wary enough.

Raskin says there are many ways the attack can be made more effective. He adds that the attack shows the need for improvements on traditional password-based authentication, such as the Firefox Account Manager that Mozilla is developing.

Update, 5/26/10: Aza Raskin wrote to say that Chrome wasn't initially vulnerable because of a bug. Now that the bug is fixed, it is vulnerable, he claims.

Jerry Bryant, group manager of response communications at Microsoft, also offered this lengthy statement:

Safety online is about defense-in-depth. Internet Explorer 8 includes world-class technologies such as the SmartScreen Filter and Domain Highlighting. These technologies, along with the Lock icon, help block the malicious pages required for this kind of attack, and highlight that such pages are not ones the user should trust.

Before entering personal information on any Web site, users should always check that the Lock icon is present in the address bar and that the Web address of the page is one they’d expect given the service they think they are using. Domain Name helps users do this by highlighting in black the actual domain of the page they’re visiting.

Behind the scenes, the SmartScreen Filter also plays a role in combating this sort of hijacking attempt. SmartScreen successfully blocks millions of views of malicious pages each month and would help protect the user in this situation. Some stories indicated that Internet Explorer on XP was susceptible to the available Proof of Concept code released on a Web page by Mozilla. Those stories are not complete. Since the site/code is not malicious, it did not trigger the SmartScreen filter which would protect these users against this PoC.

When understanding the real world risk of situations like this, it is really important to consider the defense-in-depth protections offered by Internet Explorer.