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Microsoft issued a security patch Monday that fixes six holes in its Internet Explorer Web browser, just as a nasty Internet worm that threatens to overwrite certain PC files continued its assault on users.

The "cumulative" patch made available for download Monday fixes holes in versions 5.01, 5.5, and 6.0 of Internet Explorer. It addresses six vulnerabilities discovered in the past few months, one of which could allow a malicious hacker to run code on another user's computer, Microsoft says. Microsoft gave the vulnerabilities a "critical" rating, and advised users to download the patch from its Web site immediately.

One of the holes could disguise the name of a file posted on a Web site so that a user might be tricked into opening or saving the file without knowing it is unsafe. Other vulnerabilities could allow an attacker to run code or view certain files on a user's PC, or run a script even when the user has disabled scripting, Microsoft says.

The patch and a technical description of the flaws are on Microsoft's Web site.

Older Flaws and Fixes

In addition, the cumulative patch includes a handful of previously released fixes for other holes found in the browser. One of those fixes is designed to block a worm known as "Klez," which has threatened some Internet Explorer users since it first circulated via e-mail in October.

Several variants of the Klez worm have emerged, including a recent one known as "Klez.e," which poses the most serious threat, according to Kaspersky Labs, a security research company based in Moscow. Last week, Kaspersky Labs released a software tool designed to detect and delete the Klez.e worm from infected computers.

Klez.e affects Internet Explorer versions 5.01 and 5.5, and strikes at a vulnerability in the Web browser first identified in March of last year. "This is a relatively old vulnerability," says Vincent Weafer, senior director of the security response team at security software company Symantec. Microsoft has issued patches for that hole in the past.

Automatic Launch

Klez.e can automatically launch itself when a user views an e-mail infected with the worm. Those e-mails use one of 20 words or phrases in the subject line, and contain either no text or random text in the body of the message, Kaspersky says. The words and phrases are listed on Kapsersky's Web site.

If a user opens an infected e-mail in Outlook or Outlook Express the worm tries to redirect itself as an e-mail attachment to everyone listed in the user's address book. It also attempts to overwrite certain PC files including text, HTML, and MPEG files.

More threatening, Klez.e is triggered to overwrite all the files on a user's computer on the sixth day of every odd-numbered month, with the next "hot date" being March 6.

The worm also tries to disable antivirus software on a user's PC, though Symantec says its Norton Antivirus tools are protected. For antivirus programs that remain susceptible, the worm could render the user interface unviewable or interfere with the "real-time scanner" that searches for viruses.

Way of the Worms

The same vulnerability in Internet Explorer that allows for Klez.e to do its harm was partly responsible for the Nimda virus that emerged last year, Weafer says. "We've had other worms in the past that exploited the same vulnerability," he says.

Symantec has received about 1,000 notifications of Klez.e from home and corporate users since Jan. 18, when the harmful variant surfaced. Those submissions have now "leveled off," Weafer says, but Symantec still regards the worm as a "moderate" threat. By comparison, the company has received about 5,000 submissions of the Magistr worm and about 8,000 submissions of the Badtrans worm, he says.

Klez.e was among the top 10 most frequently occurring viruses this month, according to antivirus protection company Sophos, in Wakefield, Massachusetts.

Kapersky Lab's tool for detecting the Klez.e worm, and a list of the 20 words and phrases that appear in the subject line of infected emails, are on its Web site.

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