The flaw is due to a memory corruption error that occurs when handling or repairing a document containing overly long styles, the French Security Incident Response Team said in an advisory.
The flaw, which affects Excel 2000, 2002 and 2003 and Office 2000, XP and 2003, "could be exploited by attackers to execute arbitrary commands by convincing a user to open and repair a specially crafted Excel file," the advisory said.
A Microsoft representative said the company is investigating reports of a new vulnerability in Excel and was not aware of any attacks related to it.
"In order for this attack to be carried out, a user must first open a malicious Excel document that is sent as an e-mail attachment or otherwise provided to them by an attacker," the representative said in an e-mail. "Opening the Excel document out of e-mail will prompt the user to be careful about opening the attachment."
The vulnerability affects only users of Japanese, Korean or Chinese language versions of Excel, the Microsoft representative said.
Excel hackers have been busy. On June 16, experts warned about. About two weeks ago, that could crash the program after a malicious file is opened.