Microsoft has admitted that clients on its networks can be crashed by remote and intranet users because of a problem with the TCP/IP stack for both NT and Windows 95.
Although Microsoft has posted a fix for NT version 3.5x and 4.0, it is still working on producing a solution for Windows 95-based networks. This means such systems are vulnerable to attacks from inside and outside the corporation.
According to researchers at the web site http://www.ba.be/security, a problem with Microsoft Netbios is to blame. The person who owns the site, who declined to be named, said: "If you commit to the Netbios port, Windows 95 blows up the network, Windows for Workgroup exits Windows and Windows NT crashes."
He claimed that although Microsoft has posted what it claims is a fix for the Windows-wide problem on its site, that may not be sufficient for network administrators.
The problem, he claimed, centres around out-of-bound data on ports allocated to Netbios. "It's very easy to use a program called Win Nuke to crash every machine on an intranet that uses Netbios. System administrators should close off the Netbios port to prevent the problem."
It is also claimed that every Microsoft network is in danger, even those accessed using dial-up. He called Microsoft's fix "quick and dirty", adding: "They claim to have a fix for NT versions 3.5x and 4.0 but are still working on Windows 95. There have been problems with Netbios before."
David Bridger, NT server product manager at Microsoft UK, admitted there was a problem. However, he said the fault was not at the Netbios level but with the TCP/IP stack.
"A hacker with detailed knowledge of the TCP/IP protocol can send an out-of-bound packet to specify a port and create a denial of service error.
The impact on the client is to hang their systems," Bridger said.
He claimed the problem would not matter much to those using applications like Office 97, which have auto-recovery built in, because even if the system crashed the data would be safe.
He also said that as far as he was aware, Windows for Workgroups was not affected. Bridger was unable to say when a fix for Windows 95 would be ready nor could he say whether other, non-Microsoft TCP/IP stacks, were vulnerable to attack.
"This is a problem we want to fix. We were going to include it in Server Pack 3.0 for NT (out this week), but instead we're posting it now," Bridger said. The fix for NT, he added, is available from the Microsoft web site.
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