Microsoft has warned that hackers could exploit a "vulnerability" in its operating system to gain user rights to the affected computers.

It said attackers could exploit this by requesting users to preview or open a specially crafted email or web content.

Microsoft said it was "aware of targeted attacks" and was investigating.

The issue affects Microsoft Windows Vista, Windows Server 2008, Microsoft Office 2003 - 2010, and Microsoft Lync.

Recent versions of Microsoft Windows and Office are not affected by the issue - which centres on a graphics component. Details of which products are at risk are listed on the firm's site.

Microsoft said it would take appropriate action to address the issue, which "may include providing a security update through our monthly release process or providing an out-of-cycle security update, depending on customer needs".

In the meantime, it has advised customers to apply workarounds - a setting or configuration change that "does not correct the underlying issue but would help block known attack vectors before a security update is available".

BBC News