The 2010 CWE/SANS Top 25 Most Dangerous Programming Errors is a list of the most widespread and critical programming errors that can lead to serious software vulnerabilities. They are often easy to find, and easy to exploit. They are dangerous because they will frequently allow attackers to completely take over the software, steal data, or prevent the software from working at all. Read more here: mitre.org: http://cwe.mitre.org/top25/ PDF: http://cwe.mitre.org/top25/archive/2010/2010_cwe_sans_top25.pdf The 1st place: Failure to Preserve Web Page Structure (‘Cross-site Scripting’) The 2nd place: Sanitization of Special Elements used in an SQL Command (‘SQL Injection’) The 3rd place: Buffer Copy without Checking Size of Input (‘Classic Buffer Overflow’) My personal opinion: I am astonished that is ONLY on place 3. I would set it on place 1.