The Top 20 Most Critical Internet Security Vulnerabilities

Top Vulnerabilities in Windows Systems
W1. Windows Services
W2. Internet Explorer
W3. Windows Libraries
W4. Microsoft Office and Outlook Express
W5. Windows Configuration Weaknesses

Top Vulnerabilities in Cross-Platform Applications
C1. Backup Software
C2. Anti-virus Software
C3. PHP-based Applications
C4. Database Software
C5. File Sharing Applications
C6. DNS Software
C7. Media Players
C8. Instant Messaging Applications
C9. Mozilla and Firefox Browsers
C10. Other Cross-platform Applications

Top Vulnerabilities in UNIX Systems
U1. UNIX Configuration Weaknesses
U2. Mac OS X

Top Vulnerabilities in Networking Products
N1. Cisco IOS and non-IOS Products
N2. Juniper, CheckPoint and Symantec Products
N3. Cisco Devices Configuration Weaknesses


Source: http://www.sans.org/top20