Microsoft fills NT 4.0 security hole

Microsoft Corp. has plugged the “Named Pipes over RPC” security flaw, the first hole sprung by Windows NT 4.0 since the release of the company’s latest pack of fixes, Service Pack 4.

The hole allowed hackers to provoke a denial-of-service attack on an NT 4.0 system by opening multiple named pipe connections to Remote Procedure Call (RPC) services and sending random data.

The security breach was discovered and reported to Microsoft by Mnemonix, an information security specialist and self-described NT hacker who works for a company in the United Kingdom. It affects Windows NT Server and Workstation, both the standard and Enterprise editions, as well as Windows Terminal Server.

Microsoft security engineers said a hacker could exploit the way NT 4.0 deals with invalidly named pipe RPC connections. Although different system services could be hacked, two of the services typically targeted are the SPOOLSS and LSASS system service processes, according to Microsoft officials.

“When the RPC service attempts to close the invalid connections, the service consumes all CPU resources and memory use grows considerably, which may result in the system hanging,” Microsoft said in a statement on its Security Advisor Web site. “This is a denial of service vulnerability only; there is no risk of compromise or loss of data from the attacked system.”

Late in November, Microsoft posted a Knowledge Base article about the security hole to its Web site (, along with patches for all of the systems affected except for Terminal Server. That fix will be posted as soon as it is available, officials said.

However, officials said the fixes, although fully supported, had not yet been regression tested and warned users against applying them unless their systems are specifically vulnerable to the attack.

Otherwise, Microsoft recommended that users wait for the next NT service pack, which will include a fully regression-tested version of the patch. Microsoft has not indicated when a fifth service pack for NT 4.0 will be released.

Service Pack 4 was released in mid-October. More than a year in the making, it includes year 2000 fixes, support for the euro currency, and an array of other patches, fixes and updates.

An explanation of the security flaw is posted at