Consider the following scenario:
You have a server that is running Microsoft Internet Security and Acceleration (ISA) 2006.
Internal clients use ISA 2006 as their outgoing firewall.
Internal clients use Socks V4 to connect to an Internet resource through the ISA 2006 server.
A Socks V4 internal client has to download a large file from the Internet resource, for example, by using FTP protocol.
In this scenario, the large file that is downloaded by the client is corrupted.
There is a problem in the Socks V4 application filter that is included together with ISA 2006. This issue does not occur when you use a third-party Socks V5 application filter.
To resolve this problem, install the ISA Server 2006 hotfix rollup package that is described in the following Microsoft Knowledge Base article:
2616326 Description of the ISA Server 2006 hotfix package: September 2011