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Real, honest to goodness browser functionality can end up tricking users into believing a trusted website has asked them to download a file, which is actually coming from a rogue server.

It all starts with a button on a page that, when clicked, opens the official Flash Player download website in a second tab and switches the browser’s focus to it, said Google Security Engineer Michal Zalewski. After a few seconds, the original page serves a file called flash11_updater.exe from Zalewski’s server, which causes the browser to display a download dialog.

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However, because this happens while the active tab is the one with the official Flash Player website loaded into it and an URL in the address bar, it appears as if the download came from the Adobe website.

“In a way this is a social engineer’s holy grail,” said Emmanuel Carabott, security research manager at security vendor GFI Software. “What a social engineer is trying to do is getting you to trust what they are saying. The more authentic they can make it seem the more successful the attack will be.”

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There have been social engineering attacks in the past that tricked users into downloading malicious files by passing them as Flash Player updates. A lot of these attacks used spoofed pages that mimicked Adobe’s official Flash Player site.

Zalewski’s method removes the need for spoofed pages, therefore potentially increasing the success rate of such attacks by making the trick more credible.

“All the top three browsers [Internet Explorer, Mozilla Firefox and Google Chrome] are currently vulnerable to this attack,” Zalewski said.

Internet Explorer and Firefox do specify the hostname from where the file will download inside the download dialog. However, Zalewski feels these indicators are inadequate and as long as the download prompt attaches to the wrong browser window, users will fail to notice them.

Zalewski said browser vendors have been aware of this issue since the beginning of April, but none of them is rushing to address it. Microsoft does not plan to fix it via a security patch for any of the current versions of Internet Explorer, Mozilla didn’t commit to a fix either and, while the Chrome developers agreed that it needs addressing, they haven’t set a specific date for it yet, Zalewski said.

“I think these responses are fine, given the sorry state of browser UI security in general; although in good conscience, I can’t dismiss the problem as completely insignificant,” Zalewski said.

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