To prevent any random person from logging in to your system, telnet runs a simple authentication protocol. When it first receives an incoming connection, telnet prompts for a username and password. This information is then compared to information stored in the system user database, and if it matches, the remote user is granted access to the system. There are two big problems with this authentication protocol. One is that the username and password are sent across the network without any attempt to disguise them from prying eyes.
Packet sniffers are small programs that examine every item of traffic on a network.
They are widely available, and difficult to detect. You might think that you have nothing to worry about because your network is protected behind a firewall and you don't allow incoming connections, but you're wrong. One of your coworkers might be running a packet sniffer for fun, for insurance, or for shadier purposes. Or a coworker might be 100 percent honest, but have the misfortune to have downloaded a trojan horse while at home and brought it into the company network on his or her laptop. If you think you're safe from packet sniffing because you run switched Ethernet hubs, again you're wrong.