Microsoft is warning users of a flaw in Excel which is already being targeted by hackers.
The company has received reports of attacks attempting to exploit the vulnerability via email.
In order to launch the attack, users must manually launch the malicious file.
A successful exploit could allow the attacker to access the machine with the same rights as the current user.
The flaw affects Excel 2003 Service Pack 2, Excel Viewer 2003, Excel 2002, Microsoft Office Excel 2000 and Excel 2004 for Mac.
Excel 2003 Service Pack 3, Excel 2007 and Excel 2008 for Mac are not believed to be affected by the vulnerability. The Microsoft Office Isolated Conversion Environment protects against the attack.
Microsoft said that the attacks were launched against specific targets, rather than random users.
Because the attack only grants the permissions of the current user, Microsoft said that administrators can minimise the risk by restricting user rights.
The US Computer Emergency Response Team urged users to mitigate the risk of attack by not opening suspicious email attachments.
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