Security software

Hackers Exploit Excel 'Zero-Day' Flaw

For the second time in the past five days, security researchers are warning that hackers are exploiting a critical unpatched vulnerability in widely-used software.

Attackers are exploiting a "zero-day," or unfixed, flaw in Microsoft Corp.'s popular Excel spreadsheet, using the bug to hijack select systems in Asia, many of them in government offices and high-profile corporations, said Vincent Weafer, vice president of Symantec Corp.'s security response group.

Hackers have been using another unpatched vulnerability in Adobe Reader for several weeks in a similar fashion, although now that the exploit code has gone public, experts expect to see attacks quickly increase.

The newest vulnerability, which is in all supported versions of Excel, including the latest -- Excel in Office 2007 on Windows and in Office 2008 for the Mac -- is in the program's file format, said Weafer.

"This is very similar to the Adobe [Reader] vulnerability we found earlier in that it's being used as a targeted threat," said Weafer. He said Symantec's researchers first came across attack code yesterday, and reported their findings to Microsoft the same day.

Today, Microsoft issued a security advisory with more information about the bug; that's typically a first step toward releasing a patch when a vulnerability goes public.

Microsoft spokesman Bill Sisk downplayed the threat to most users, repeating Weafer's comment that attacks have been seen in only limited numbers. But he promised that the company would patch the problem. "Microsoft is currently working to develop a security update for Microsoft Office that addresses this vulnerability and will release it after it has completed testing," he said in an e-mail.

According to Microsoft's advisory, Excel 2000, 2002, 2003 and 2007 on Windows, and Excel 2004 and 2008 on Mac OS X, are affected by the vulnerability.

Until a patch is produced, Microsoft said users could protect themselves by blocking Excel files from opening, a process that requires editing the Windows registry, normally a chore that's beyond the ability of most users. Alternately, users can run Excel 2003 documents through the Microsoft Office Isolated Conversion Environment (MOICE), a tool the company launched in 2007 that converts those files into the more-secure Office 2007 formats to strip out possible exploit code.

It's not clear how effective MOICE will be in stymieing attacks, however, since the exploit now circulating was crafted with Excel 2007 in mind, said Weafer. According to additional analysis by Symantec, the exploit works on PCs running that version of Excel but fails against earlier editions.

Hackers are using the Excel bug to deliver a Trojan horse to targeted machines, added Weaver. The Trojan acts as a downloader that is capable of retrieving and installing additional malware on the hijacked computer.

Weafer declined to draw a line between the recent zero-day dots, noting that attacks -- particularly targeted attacks like those triggering the Excel and Adobe Reader vulnerabilities -- often come in waves. But he was less hesitant to speculate on the near future.

"As soon as you talk about an [unpatched] vulnerability, people start looking at it for use in broad-based attacks," he said.

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