Denial of Service

• Although a denial of service attack is not classified as a real hack, it is one of the most common attacks against companies and governments. In a DoS attack, the attacker floods the target server with requests, overwhelming it to the point that it is no longer capable of servicing requests. The server appears down to legitimate users attempting to access it. One of the most common DoS attacks is called the "Ping of Death," where the attacker sends one or more "ping forever" commands to the target.
  
  

Man in the Middle

• Some attackers are able to insert their own computer into a connection between two other computers, making the hacker the "man in the middle." From his vantage point, the hacker is able to receive commands from one computer, modify them or just steal the information, and forward a response to the other computer.
  
  

Replay

• A replay attack involves a hacker who records a session between two computers. The hacker then uses the recorded information to log on to another computer, impersonating the original participant in the session. The hacker might also replay a monetary transaction, or a transaction involving access to confidential data.
  
  

Social Engineering

• Of all the various hacker methods, social engineering is by far the most effective. A technical hacker may spend hours trying to break into a network or server, not always succeeding. The social engineer does some research, and calls and asks for a password using a manager's name or some other piece of critical information. Human nature is subject to the weaknesses of always wanting to help someone, and fear of losing a job. Social engineers exploit human weaknesses to gain easy entry into technical areas.