"Microsoft has warned that an unpatched vulnerability in older versions of Excel is being actively exploited by hackers. The bug, which has become the subject of targeted attacks, affects older versions of Excel. Newer versions Excel 2003 SP3 and Excel 2007 are said to be immune. Details of the vulnerability beyond what packages might be affected remain sketchy at best.
Microsoft techies are investigating the flaw, which may become the topic of a future patch." – Content courtesy of Hackers go after Excel | The Register