A security bug in a network service that ships as part of Windows NT 4.0, Windows 2000 and Windows XP can open systems up to attack, Microsoft warned in its first security bulletin of the year, issued on Wednesday and rated "critical."A security bug in a network service that ships as part of Windows NT 4.0, Windows 2000 and Windows XP can open systems up to attack, Microsoft warned in its\u00a0first security bulletin\u00a0of the year, issued on Wednesday and rated "critical."The Redmond, Wash., software vendor also issued the second and third bulletins of 2003 at the same time. Bulletin\u00a0MS03-002\u00a0details a flaw in Content Management Server 2001 rated "important" and\u00a0MS03-003\u00a0offers a patch for a "moderate" vulnerability in Outlook 2002.The "critical" flaw lies in the Microsoft Locator service, software used to map easy-to-remember logical names of systems on a company's network, such as a print server, to the actual network addresses, Microsoft said in security bulletin\u00a0MS03-001.By default, the Locator service is enabled only on Windows 2000 and Windows NT 4.0 server configurations used as domain controllers. It is not enabled on Windows NT 4.0 workstations or servers, on Windows 2000 workstations or servers, or on Windows XP, Microsoft said.An attacker could take over a vulnerable system by sending a malformed request to the Locator service, Microsoft said. However, a firewall set to block external NetBIOS traffic would prevent attacks from the Internet, the company said.Nevertheless, Microsoft urges users of all the affected operating systems to apply the available software patch. Administrators of Windows 2000 and Windows NT 4.0 domain controllers should apply the patch immediately, Microsoft said.Users can check to see whether the Locator service is running on their system by typing "net start" at the command line. The Locator service is running if the entry "Remote Procedure Call (RPC) Locator" appears in the displayed the list of services.Bulletin MS03-002 details a serious flaw in Content Management Server 2001, a product used to build and manage Web sites. It contains a flaw that could allow an attacker to intercept data that an Internet user shares with a site created using the Microsoft software and alter the data shown to the user, the software maker said.To exploit the flaw, an attacker would have to make a user visit the target site through a link that adds malicious script. This could be done by sending that link in an e-mail, Microsoft said. Content Management Server 2002 is not affected, according to the vendor.Finally, MS03-003 discusses a flaw in the way Outlook 2002 handles V1 Exchange Server Security Certificates for e-mail encryption. As a result of this flaw, Outlook does not correctly encrypt mail when such certificates are selected and sends plain text messages instead of secured messages, Microsoft warned.The scope of this vulnerability is limited because V1 Exchange Server Security certificates are not commonly used, according to Microsoft. Outlook 2002 by default uses the widely used S\/MIME certificates, the company said.Microsoft changed the way it rates security issues late last year. Under the new system, fewer bulletins get the "critical" stamp. Only vulnerabilities that could be exploited to allow malicious Internet worms to spread without user action are now rated critical. Many issues that were previously rated critical are now "important," a new category in the rating system. These "important" vulnerabilities could still expose user data or threaten system resources.