Microsoft has warned of more "zero-day" attacks in its Office productivity suite, but says the current wave is "very limited". 

In a statement on its website, the software giant warns users: "Microsoft is investigating new public reports of very limited Microsoft Excel “zero-day” attacks using a vulnerability in Microsoft Office 2000, Microsoft Office XP, Microsoft Office 2003, and Microsoft Office 2004 for Mac.
"In order for this attack to be carried out, a user must first open a malicious Office file attached to an E-mail or otherwise provided to them by an attacker.
"While we are currently only aware that Excel is the current attack vector, other Office applications are potentially vulnerable."