When you connect your Windows NT-UNIX network to the Internet, security weaknesses can result. One weakness is vulnerability to sniffer attacks. A sniffer is an application that can trap and filter packets, undermining the integrity of data you send across the Internet. With sniffers, it doesn't matter which protocol (e.g., TCP/IP, IPX/SPX, or even NetBEUI) you use; if you have unencrypted data on the same subnetwork as the sniffer, the hacker has access to your data.
Sniffers are readily available. For example, hackers can use the UNIX tool, IP-Watcher, as a sniffing device. With this utility, they can intercept a packet and forward a completely different one in its place. NETCAT is another sniffer that both hackers and systems administrators often use. Application developers originally created NETCAT for UNIX but have now adapted it for NT.