Microsoft on Wednesday issued security bulletins for five new software vulnerabilities, including a flaw in Visual Basic for Applications that the company rated as critical. The company has posted patches for each of the flaws on its Web site.

Four of the problems affect Microsoft's Office desktop software. The critical flaw in Visual Basic for Applications could be exploited by a hacker to execute code on a targeted PC, according to Microsoft. Visual Basic for Applications is used in many of the individual Office applications, as well as in some of the business applications sold by Microsoft Business Solutions.

Two of the other affected programs--a flaw in Microsoft Word and a buffer overrun in the WordPerfect converter--were rated as important, while a glitch in Access's so-called snapshot viewer was considered moderate. There are two ways an attacker might exploit the Office vulnerabilities--through an open document or by crafting a rogue HTML message that gets opened by an unsuspecting user.

"The way to prevent those situations is to not open attachments you weren't expecting. And we urge everyone to apply the patches," says Stephen Toulouse, security program manager with Microsoft's Security Response Center.

Last and, according to Microsoft, of least significance is a hole in NetBIOS that a hacker could use to view information on a Windows PC or server. At worst, Toulouse says, a hacker might see "fragmented and random" data in system memory. Affected platforms include Windows XP, Windows 2000, Windows NT 4.0 Server, and Windows Server 2003.