A vulnerability allows remote attackers to extract IP addresses visited through the SOCKS4 Proxy on vulnerable ISA Server installations. Authentication is not required to exploit this vulnerability. This specific flaw exists when an empty packet is sent to the SOCKS4. The server will return a packet containing the last IP address it proxied to.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/59548/ZDI-07-053.txt
Source: https://packetstormsecurity.com/files/59548/Zero-Day-Initiative-Advisory-07-053.html