Nefarious computer users have a variety of tools at their disposal enabling them to disrupt, intercept, and re-route network traffic in an effort to gain access to a system. In general terms, these threats can be categorized as follows:
Interception of communication between two systems — In this scenario, the attacker can be somewhere on the network between the communicating parties, copying any information passed between them. The attacker may intercept and keep the information, or alter the information and send it on to the intended recipient.
This attack can be mounted through the use of a packet sniffer — a common network utility.
Impersonation of a particular host — Using this strategy, an attacker's system is configured to pose as the intended recipient of a transmission. If this strategy works, the user's system remains unaware that it is communicating with the wrong host.
This attack can be mounted through techniques known as DNS poisoning[6] or IP spoofing[7].
Both techniques intercept potentially sensitive information and, if the interception is made for hostile reasons, the results can be disastrous.
If SSH is used for remote shell login and file copying, these security threats can be greatly diminished. This is because the SSH client and server use digital signatures to verify their identity. Additionally, all communication between the client and server systems is encrypted. Attempts to spoof the identity of either side of a communication does not work, since each packet is encrypted using a key known only by the local and remote systems.