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Under specific circumstances, an attacker may be able to misuse an FTP server that is accessed through Microsoft Internet Security and Acceleration Server (ISA Server). The attacker may use the FTP server to open connections to other servers. This problem may occur if the FTP Server service is vulnerable to FTP Bounce attacks.
Note CERT Advisory CA-1997-27 describes FTP Bounce attacks. For more information, see the "References" section of this article.
By default, the FTP Server service in Microsoft Internet Information Server (IIS), versions 4.0, 5.0, and 6.0, is not vulnerable to this type of attack. However, the FTP Server service may be vulnerable if it has been configured to permit transfers to third-party IP addresses by setting the
registry key in IIS 4.0 and IIS 5.0, or the
registry key in IIS 6.0.
For more information about the registry locations of EnablePortAttack and EnableDataConnTo3rdIP, click the following article number to view the article in the Microsoft Knowledge Base:
294679 How to enable external client computers access to a File Transfer Protocol server
This problem occurs because the FTP Application filter in ISA Server does not verify whether the IP address that is passed in the PORT command agrees with the IP addresses that are being used in the current FTP session.
You must install ISA Server Service Pack 1 (SP1) before you install the following hotfix.
For more information about how to obtain the latest ISA Server service pack, click the following article number to view the article in the Microsoft Knowledge Base:
313139 How to obtain the latest Internet Security and Acceleration Server 2000 service pack
A supported hotfix is now available from Microsoft, but it is only intended to correct the problem that is described in this article. Only apply it to systems that are experiencing this specific problem. This hotfix may receive additional testing. Therefore, if you are not severely affected by this problem, Microsoft recommends that you wait for the next ISA Server 2000 service pack that contains this hotfix.
To obtain this hotfix, visit the following Microsoft Web site:
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 tool in Control Panel.
Date Time Version Size File name ----------------------------------------------------- 25-Nov-2003 11:21 3.0.1200.297 19,728 Ftpfltr.dll
This hotfix also applies to the French, German, Spanish, and Japanese versions of ISA Server.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
After you apply this hotfix, ISA server will compare the address that is parsed in the PORT command to the address of the current FTP session. If the two addresses do not match, the PORT command is denied and the user receives the following error message:
500 invalid PORT command
For more information about the terminology that is used to describe Microsoft product updates, click the following article number to view the article in the Microsoft Knowledge Base:
824684 Description of the standard terminology that is used to describe Microsoft software updates
For more information about the FTP Bounce attack, click the following article number to view the article in the Microsoft Knowledge Base:
185378 IIS 4.0: FTP Bounce attack and CERT Advisory CA-97.27
For more information, visit the following World Wide Web Consortium (W3C) Web site:
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