Microsoft's Office software has been hit by another zero-day attack, one that is being taken very seriously by the software company.

The unpatched vulnerability potentially gives attackers complete control over the victim's PC.

The flaw is a memory corruption bug, and is found in many versions of Word, including two for Mac. Microsoft Word 2000, 2002, 2003, Word Vieewr 2003, Word 2004 for Mac, and Works 2004, 2005, and 2006 are all vulnerable.

The attack is carried out when an unsuspecting person opens a malicious document to which a string of characters has been added that then corrupt the computer's memory so that the hacker can run software on the system.

Hackers can post the malicious file to websites, or send it in a direct email. Microsoft is therefore warning users not to open suspect files or open files received unexpectedly from trusted sources.

This type of targeted Trojan horse is considered more serious that myriad other worms and viruses that spread via email attacks.