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Microsoft Works Around Excel Bug

As Microsoft issues workarounds for last week's zero-day Excel attack, yet another hole is found in the spreadsheet program UPDATED 5:30 PM

Microsoft today issued some workarounds for the Excel vulnerability discovered last week, but security researchers say they've found another zero-day bug that threatens the popular spreadsheet program.

Secunia reported a new vulnerability it found in Excel 2003 SP2 that can cause a "stack-based buffer overflow" via hyperlinks in Excel documents. The source of the problem is an error in Excel's hlink.dll file, which is activated when a user clicks on a malicious hyperlink in an Excel document.

Secunia hasn't found any live exploits of the vulnerability yet, but the security vendor warns IT managers to be on the lookout because it's an easy flaw to exploit. Microsoft, meanwhile, says this appears to be a new vulnerability in Windows that's exploited when a user clicks on a hyperlink in an Office document. The company isn't aware of any attacks or impacts on any customers at this time, a Microsoft spokesperson says, but stay tuned for another security advisory or update to address it.

The new discovery is a step backward for Microsoft, which just issued an official security advisory early this morning about last week's highly publicized Excel vulnerability, which was caused by a memory corruption error in Excel's "repair mode." Microsoft offered workarounds for last week's Excel vulnerability designed to hold users over until its next regularly scheduled patch update.

Secunia classifies last week's Excel vulnerability as "extremely critical" and the new one "highly critical."

Although the flaw discovered last week in Excel's repair mode hasn't been exploited in a big way yet, Microsoft says it's investigating reports of zero-day attacks that take advantage of the hole. If successful, the exploit can steal a user's credentials. The attack uses a Trojan called "Trojan.Mdropper.J" that comes in an Excel spreadsheet file called "okN.xls." Once the Trojan is let loose, it adds another malware program that closes Excel and tries to place code into Internet Explorer to get past firewalls and do its dirty work.

One theory is that, because they are so similar, last week's Excel exploit is related to the Word trojan attack of two weeks ago. (See Microsoft Prepares to Patch Things Up). Jonathan Singer, an analyst with the Yankee Group, says the Excel exploit appears to be more of a targeted attack to get into a specific enterprise. "It's nothing to panic about."

Microsoft is currently working on a patch for Excel's repair mode problem, but in the meantime recommends these workarounds (details can be found in the Microsoft Security Bulletin at the above URL):

  • Disable Excel repair mode
  • Block all Excel email types at the corporate email gateway
  • Block the ability to open Excel documents from Outlook
  • Don't open or save Excel files from untrusted sources

Microsoft's Excel 2003, Excel Viewer 2003, Excel 2002, Excel 2000, Excel 2004 for Mac, and Excel v.X for Mac all come with this hole and are subject to the bug.

The attack can occur either via the Web or email, Microsoft said. On the Web, an attacker would host a site containing a malicious Excel file and a user would have to actually go to that site to get infected. In an email attack, a user receives a corrupted Excel file attachment and opens it.

— Kelly Jackson Higgins, Senior Editor, Dark Reading

Companies mentioned in this article:

  • Microsoft Corp. (Nasdaq: MSFT)
  • Secunia
  • Yankee Group Research Inc.
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