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Microsoft Warns Of Zero-Day Excel Exploit

The vulnerability in Excel could allow an attacker to execute malicious code, if a user opens a specially crafted Excel file.

Microsoft on Tuesday issued two Security Advisories, one warning about a zero-day Excel 2007 vulnerability, the other addressing with a problem that prevented the disabling of Windows Autorun.

Microsoft said that it is investigating public reports of a vulnerability in Excel that could allow an attacker to execute malicious code, if a user opens a specially crafted Excel file. Attempts to exploit this vulnerability appear to be limited in scope, the company said.

The following Microsoft software packages appear to be affected: Microsoft Office 2000, Microsoft Office 2002, Microsoft Office 2003, Microsoft Office 2007, Microsoft Office 2004 for Mac, Microsoft Office 2008 for Mac, and Open XML File Format Converter for Mac.

Symantec engineers in Japan yesterday identified some suspicious samples submitted by their customers, all of which contained suspect Microsoft Office Excel 2007 spreadsheets. "Further analysis showed that these files were exploiting a vulnerability in Excel that allowed them to drop and execute a binary onto the file system," said Symantec researcher Patrick Fitzgerald in a blog post.

According to Fitzgerald, the vulnerability hasn't been seen before. "It turns out that this vulnerability exists in the old Excel binary .xls format and not the new .xlsx format. Opening the malicious spreadsheet triggers the vulnerability," he wrote. "This causes the shellcode to execute and then drops two files on the system -- the malicious binary mentioned earlier and another valid Excel document. The shellcode then executes the dropped file and opens the valid Excel document to mask the fact that Excel has just crashed. This helps to decrease suspicion when the affected spreadsheet is opened."

The company has designated the malware as Trojan.Mdropper.AC.

The malware author or authors have employed some basic techniques to evade detection, Fitzgerald explained, such as weak encryption and code obfuscation.

Microsoft said it expects to release a fix for the Excel vulnerability once it has been tested.

Is your vulnerability management program ready for 2009? InformationWeek has published an independent analysis of this topic. Download the report here (registration required).

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