Windows 7 RC ignores file extension security risk

No change in the design that lets hackers easily mask malicious files

Windows 7 Release Candidate (RC) continues a long-running Microsoft practice that puts users at risk, a security researcher said Wednesday.

The new operating system's Windows Explorer file manager still misleads users about the true extension of a file, said Patrik Runald, chief research advisor at Helsinki-based F-Secure Corp. Rather than reveal the full extension for a filename, Windows Explorer hides the extension for known file types, giving hackers a way to disguise malware by using those file types' extensions and icons.

Windows Explorer, for example, will show the .txt icon and display "attack.txt" as the filename for a Trojan horse that's actually been named "attack.txt.exe" by the hacker. The practice goes back to at least Windows NT, and has been criticized in the still-popular Windows XP and the newer Windows Vista.

"People typically look at the icon to know what the file is," said Runald. "If it looks like a Word doc or a PDF file, there's an implicit trust in it, and users are more likely to click on those files, even if they are actually an executable."

Windows, Runald continued, is smart enough to know the true nature of the file, and will, for instance, run an .exe even if the filename shows as "attack.txt" in Explorer.

"This has been used for years by virus writers -- maybe less than it used to be, since most attacks now are drive-by downloads [using browser vulnerabilities], and not e-mail attachments," Runald noted. "But you still see it."

Microsoft should show the true filename in Explorer, urged Runald. "Bottom line, it's a still bad idea not to."

Windows 7 RC launched yesterday, and will be available for download until at least through the end of July.

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Gregg Keizer

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