From Computer Crimes: Examples of Network Security attacks Tango Group Internal Technology Seminars -- April 23 99. byRoman Markowski
gain access to an account
default or ease to guess passwords; social engineering
exploit a vulnerabilities in an operating systems - Unix services are particularly vulnerable (sendmail /IMAP/POP, NFS, NIS, DNS/BIND, FTP, TFTP, SYSLOG)
use "crack" to break more user passwords
obtain superuser privileges
install "back doors" (Trojan Horses)
install sniffers (packet and password grabbers) to obtain more password and site information