Spoofing – malicious users attacks the system by altering the source IP address.
Denial of Service (DoS) – involve using so much important resources that legitimate users are denied service.
Distributed Denial of Service (DDoS) – uses many sending computers in clogging the network with so much traffic.
Unauthorized access – accessing information or resources in an unauthorized fashion. Examples are exploiting protocol implementations bugs, malware, botnets.