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by Alexandra Krasne

(IDG) -- Market research calls Microsoft's Internet Explorer 5 the most widely used browser, but if you haven't loaded a patch for its latest security vulnerability, your privacy--and your files--may be at stake.

Download Behavior, the most recent Internet Explorer 5 bug, distinguishes itself from past exploits with the unsettling chance of allowing a hacker to gain read-only access to your local and intranet files.

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And, although the mischievous Download Behavior bug was found September 28 by Bulgarian hacker Georgi Guninski, Microsoft didn't post its fix until October 8.

"Microsoft responded pretty quickly to this," says Brad Shimmin of BugNet, a bug tracking company. "But it's a real eye-opener for users and Microsoft. There are plenty of bugs that are an inconvenience for the user, but it's the security bugs that are the most notable and inconvenient."

Before posting a fix, Microsoft told IE5 users to turn off Active Scripting for protection. But this workaround has the significant drawback of eliminating many capabilities of sites you'd visit, like image rollovers and page formatting.

But Download Behavior is less harmful that a previous bug, ImportExportFavorites, for which Microsoft posted a patch in September, says one Microsoft official. ImportExportFavorites can sometimes allow a hacker to run code on your machine.

Download Behavior exposure

Logically, you should only be able to download a file from a site if that file is housed on the site's own domain. But a nosy site administrator could take advantage of the recently discovered flaw to pilfer your files.

Normally, it shouldn't be possible for someone to access data on your PC from the Web because the two aren't on the same network, according to KeyLabs, an independent PC testing facility that tests for BugNet. But the "Download Behavior" function may let a malicious server trick your PC into thinking that a downloaded application is on the same server, allowing an administrator to gain access to your private local files.

KeyLabs tested both IE5 and Netscape Communications' Communicator 4.61 recently. The KeyLabs facility found that Netscape's browser is impervious to this kind of attack, but the lab finds that IE5 allows full file system access.

Microsoft posted its fix three days later. Officials say this delivers a new, and safe, version of Download Behavior, and eliminates the security problem. Also, no customers have reported being affected by the bug, according to Microsoft.

KeyLabs will test the new patch for effectiveness this week, Shimmin says.

Until then, rest assured that any browser below IE5 is unaffected by this vulnerability. Nor are IE5 for Windows 3.1 and Windows NT 3.5, and IE5 for Macintosh. The update isn't ready yet for IE5 for UNIX.