Piggybacked data on a TCP Acknowledgement (ACK) package may bypass the WFP inspection process in Windows Vista

The TCP protocol uses a three-way handshake to establish a TCP connection. The last interaction in the three-way handshake is a TCP Acknowledgement (ACK) package. However, in Windows Vista, the Windows Filtering Platform (WFP) inspection occurs only after the three-way handshake is completed. Therefore, any data that is piggybacked on the ACK package may bypass the WFP inspection process.

Note A payload can legitimately piggyback on the ACK package.

This issue affects socket applications that use NetBIOS communication on a Windows Vista-based computer.

A hotfix is available to resolve this issue. After you apply this hotfix, the WFP inspection also occurs on the ACK package.

Hotfix information

A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.

If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, contact Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.

Prerequisites

To apply this hotfix, the computer must be running one of the following operating systems:

• Windows Vista
• Windows Vista Service Pack 1 (SP1)
• Windows Server 2008

Restart requirement

You must restart the computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace any other previously released hotfixes.

Registry information

To use this hotfix, you do not have to make any changes to the registry.

File information

The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.

Windows Vista, x86-based versions

Windows Vista SP1 and Windows Server 2008, x86-based versions

Windows Vista, x64-based versions

Windows Vista SP1 and Windows Server 2008, x64-based versions

Windows Vista SP1 and Windows Server 2008, Itanium-based versions

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

For more information about Windows Filtering Platform, visit the following Microsoft Web site:For more information about stream inspection, visit the following Microsoft Web site: For more information, click the following article number to view the article in the Microsoft Knowledge Base: