Mozilla has uncovered a new attack tactic dubbed 'tabnapping' that dupes web users into giving up passwords by secretly changing already-open browser tabs.
All of the major browsers on Windows and Mac OS X are vulnerable to the attack.
Aza Raskin, Firefox's creative lead, spelled out the scenario, which is striking in its assumption: Most people keep multiple tabs open, often for long periods.
Raskin's technique requires that identity thieves trick users into visiting a malicious or compromised site - no problem in today's spam- and scam-infected online world.
"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," said Raskin, referring to his example of a spoofed Google Gmail log-in.
"When they click back to the fake Gmail tab, they'll see the standard Gmail log-in page, assume they've been logged out, and provide their credentials to log in."
There's no need for the attacker to change the actual URL that shows in the browser's address bar, since the tactic banks on the trust that tabs can't suddenly mutate. "The attack preys on the perceived immutability of tabs," Raskin said.
Raskin also laid out several ways hackers could boost tabnapping's sneakiness, ranging from sniffing out sites that the victim actually visits - put up a fake Facebook log-in, say, rather than simply betting that the user opens Gmail - to changing the text on the bogus page.
"You can mention that the session has timed out and the user needs to re-authenticate," Raskin said.
"This happens often on bank web sites, which makes them even more susceptible to this kind of attack."
PC Advisor's sister title Computerworld ran Raskin's proof-of-concept - his blog post explaining the attack includes the necessary code - and found that Chrome, Firefox, Opera and Safari in Mac OS X 10.6 all showed the fake Gmail tab and contents.
But some browsers were more susceptible than others.
In both Windows and Mac OS X, for instance, Raskin's code changed only Firefox's 'favicon', the small icon that typically shows a miniature site logo.
In other browsers, the favicon for Raskin's blog remained, though the label and content was that of Gmail.
Google's Chrome seemed especially resilient to the tactic. On the Mac, Raskin's trick sometimes changed the tab, often did not.
Computerworld was not able to nail down the specific situations when Chrome fell victim, however.
Raskin did not reply to questions about what steps Firefox and other browser makers might take to stymie such attacks.
In his blog, Raskin touted ongoing work on a new username/password tool called 'Account Manager' that is tentatively slated to show up in Firefox 4 , the ambitious upgrade Mozilla plans to release this November.
Andrew Storms, director of security operations at nCircle Security, doubted whether there is an easy fix.
"I can't think of anything off hand that could be done," Storms said.
"That's the part of the new dynamic nature of web browsing. You can alter the look/feel of the experience for both good and bad."
Jerry Bryant, a group manager with the Microsoft Security Response Center (MSRC), said his team is looking into Raskin's claims, but hinted that Microsoft wouldn't be patching IE anytime soon.
"I wouldn't classify this as a 'vulnerability' though," Bryant said.
When Microsoft declines to name an issue a security vulnerability, it generally means that if a fix does come, it won't appear until a service pack or next major upgrade is released.
In the case of Internet Explorer, that would be IE9, which remains in the early development stage.