Another Microsoft's Office critical vulnerability has been found making it the fifth unpatched Office flaw since December last year.

Microsoft issued a security advisory last Friday confirming the security issue bas been indeed spotted. The problem affects all versions of Microsoft Office 2000, Office XP, Office 2003, and Office 2004 for Mac OS X and is being actively exploited today with an Excel zero-day attack. According to secunia vulnerability database this issue is “extremely critical” for system's security.

Office users are advised to use Microsoft Office document with increased caution until appropriate patches become available. However, Office 2007 version is not affected by this issue.