Microsoft revealed the first vulnerability in Windows Server 2003 that the software maker has deemed "critical," its most serious security-flaw designation. The three previous flaws in Windows Server 2003 were all rated "moderate."

The vulnerability, discovered by The Last Stage of Delirium Research Group, also affects Windows NT 4.0, NT Terminal Services Edition, Windows 2000, and Windows XP. According to Microsoft bulletin MS03-026, the flaw is in the way the remote procedure call used by Windows incorrectly handles messages sent over TCP/IP. It creates the potential for a buffer overflow. If successful, an attacker could take any action on a victimized system, including destroying data or running any application.

Microsoft has touted Windows Server 2003 as its most secure operating system so far. But experts say that doesn't mean it has zero vulnerabilities. "The level of complexity of software has reached a point where it is absurd not to expect software flaws," says Pete Lindstrom, research director with Spire Security.

"This is not a shocker, and there'll be more in the future," he says.

Two other software vulnerabilities affecting Microsoft software were disclosed late Wednesday afternoon. The first, described in bulletin MS03-028, is for Microsoft Internet Security and Acceleration Server 2000, Microsoft's firewall application. The vulnerability is a cross-site scripting flaw found in some ISA server-generated warning pages. The flaw makes it possible for an attacker to run code on a victim's system.

The third flaw, detailed in security bulletin MS03-027, was found in the Windows shell within Windows XP. This flaw could also let attackers run code of their choice on a vulnerable system.