Microsoft has warned of a second problem with Word, leaving users vulnerable to criminal attacks. The new vulnerability follows the onset of a similar attack on Word last week.

The software vendor said that hackers were sending maliciously crafted Word attachments to victims. Microsoft added that while these attacks were not widespread, they were dangerous because the attacker could run unauthorised software on the victim's PC if the attachment were opened.

Over the past year, hackers have increasingly looked to Microsoft's Office suite as a source of new bugs. Attacks based on flaws in Excel and PowerPoint have been reported as well.

Exploits of this latest flaw appear to be even rarer than those based on last week's Word bug. "The vulnerability is being exploited on a very, very limited and targeted basis," Microsoft said in its alert.

Neither problem is expected to be fixed in Tuesday's software patches from Microsoft, which will address flaws in Windows and Visual Studio.

This second bug affects Word 2000, 2002, 2003 and the Word Viewer 2003, which are all vulnerable to last week's bug as well. The most recent version of the software, Word 2007, which was released to business users in late November, is not affected, Microsoft said.