Google patches Chrome 'carpet bomb' bug

It's 'enough for the time being' but not a full fix, researcher says

Google has patched its Chrome browser to block a months-old bug that can be used to trick people into downloading and launching malicious code.

The fix has not been pushed out to most users, however.

The security researcher who reported the vulnerability, which involves a combination of the "carpet bomb" bug with another flaw disclosed in August, called the fix "enough for the time being," but said Google's patch wasn't the final word.

Google plugged the hole in a developer-only version of Chrome that has not yet been sent to all users via the browser's update mechanism. Chrome users, however, can reset the browser to receive all updates, including the developer editions, with the Channel Chooser plug-in .

According to a Google blog , Chrome 0.3.154.3, which was released last week, changes the browser's download behavior for executable files, such as .exe, .dll, and .bat files on Windows.

"These files are now downloaded to 'unconfirmed_*.download' files," said Mark Larson, Chrome program manager, in the blog post. "In the browser, you're asked if you want to accept the download. Only after you click Save is the 'unconfirmed_*.download' file converted to the real file name. Unconfirmed downloads are deleted when Google Chrome exits."

Last month, Israeli security researcher Aviv Raff demonstrated how hackers could create a new "blended threat" -- so-named because it relies on multiple vulnerabilities -- to attack Chrome. Raff's proof-of-concept code used an auto-download vulnerability (aka 'carpet bomb') along with a user interface design flaw and an issue with Java.

Chrome contributed to the vulnerability by making downloaded files appear as buttons at the bottom of the browser's frame, Raff said then.

Tuesday, after examining the 0.3.154.3 developer build, Raff proclaimed the fix sufficient for the short term, but nothing more. "The fix is not good enough. [But] it's enough for the time being, until other small issues might popup and be used to exploit the auto download problem," Raff said in an interview conducted via instant messaging. "The best solution was if they just won't download the files until the user approves, or download them to a random directory..., as it's done with other browsers, like Internet Explorer's Temporary Internet Files folder or Firefox's random profile directory."

On the plus side, Raff said, Chrome shows the full filename -- the "unconfirmed_*.download" that Google's Larson described -- so that users can see if the file is, in fact, an executable and potentially dangerous.

But Chrome still has holes. "Even if [Google assigns executables] a random filename, it might still be possible to predict the downloaded filename," Raff said. "They delete the automatically downloaded files only after the user shuts down the browser. What happens if the browser crashes? The malicious files might still exist after the crash."

The best solution would be for Google to prevent any files from downloading through Chrome without user permission. "I think that downloading any file without user interaction to a predictable location, [for example] the default download directory, is still bad," Raff argued. "Even if the extension is not an executable, there might be other ways to execute those files. For example, through the Windows command line you can execute any file with a PE header, even if they have a different extension."

Chrome accounted for less than 1 percent of the browser market share during its first month of availability, according to data from Net Applications.

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Gregg Keizer

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