How malicious hackers attack
September 01, 20061. Log-on credential or password guessing/cracking
2. Buffer overflow
3. Application or OS vulnerability
4. Application or OS misconfiguration
5. Data malformation -- SQL injection, XSS, and so on
6. Sniffing/eavesdropping
7. Client-side attack
8. Social engineering
Source: http://www.infoworld.com/