Enabling NetBT to Open IP Ports Exclusively

View products that this article applies to.

This article was previously published under Q241041

SYMPTOMS

A user-mode program can listen to TCP port 139 as well as UDP ports 137 and 138. Because these ports are used by Windows NT services, it is a Trusted Computer System Evaluation Criteria (TCSEC) C2 requirement that an unprivileged user-mode program should not be able to listen to these ports used by Windows NT services, regardless of the cryptographic protection applied to the Windows NT service traffic using these ports.

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CAUSE

This problem occurs because Netbt.sys (NetBIOS over TCP/IP) is responsible for opening the following TCP and UDP ports that subsequently are used by Windows NT services:

UDP Port 137
UDP Port 138
TCP Port 139

For additional information about Windows NT services that use these ports, click the article number below to view the article in the Microsoft Knowledge Base:

150543 (http://support.microsoft.com/kb/150543/EN-US/ ) WinNT, Terminal Server, and Exchange Services Use TCP/IP Ports

As explained in the Windows NT 4.0 Device Development Kit (DDK) for network driver development, Netbt.sys (as a TDI client) begins communicating with its local-node transport by opening a file object that represents a transport address. Netbt.sys calls ZwCreateFile, passing the address specification in the extended attributes (EA) buffer parameter to ZwCreateFile.

In Windows NT 4.0 Service Pack 6a and earlier, Netbt.sys allows file share access to file objects representing the aforementioned TCP and UDP ports when calling ZwCreateFile. Subsequently, a user-mode program installed on a computer running Windows NT 4.0 Service Pack 6a and earlier can gain access to a share and listen to the TCP and UDP ports. The Post-SP6a hotfix, C2 Update, allows the option to change the Netbt.sys behavior so that it does not allow file share access to files objects representing the aforementioned TCP and UDP ports when calling ZwCreateFile. The option is selected through the use of the following registry key and key value:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NetBT\Parameters
</B>
EnablePortLocking:REG_DWORD

Default: 0 - NetBt allows file share access
	 1 - NetBt disallows file share access

When EnablePortLocking is set to 1, Netbt.sys passes in zero (ULONG) in the ShareAccess parameter of ZwCreateFile. After the application of Netbt-fix and the setting of the REG_DWORD value to 1 for EnablePortLocking, a user-mode program cannot gain share access and listen to the TCP and UDP ports.

NOTE: NETBT in Windows 2000 does not support this option and the above registry key and value. The default behaviour in Windows 2000 is to disallow file share access to all ports created by NETBT.

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RESOLUTION

Windows NT 4.0

To resolve this problem, obtain the individual package referenced below or obtain the Windows NT 4.0 Security Rollup Package. For additional information on the SRP, click the article number below to view the article in the Microsoft Knowledge Base:

299444 (http://support.microsoft.com/kb/299444/EN-US/ ) Post-Windows NT 4.0 Service Pack 6a Security Rollup Package (SRP)

The individual hotfix is available for download from the Microsoft Download Center or Microsoft's FTP site. Click the file names below to download the appropriate file: