A critical flaw in Excel, which is currently being exploited by hackers, will not be fixed by Microsoft in the next patch Tuesday.

Redmond said it will deliver three security updates on Tuesday, one of them ranked as "critical," but will not fix an Excel flaw even though hackers have been using it to access company computers.

Last week, Microsoft downplayed that threat, saying that the company's security experts had seen only a small number of attacks. Security experts at Symantec said that the vulnerability is a file format bug in all supported versions, including the latest version Excel 2007 on Windows and Excel 2008 for the Mac.

While it has been exploited, Microsoft has probably got bigger fish to fry, the security experts said.