source: http://www.securityfocus.com/bid/13219/info

Knowledge Base Module is affected by an SQL injection vulnerability. This issue is due to a failure in the application to properly sanitize user-supplied input before using it in an SQL query.

Successful exploitation could result in a compromise of the application, disclosure or modification of data, or may permit an attacker to exploit vulnerabilities in the underlying database implementation. 

http://www.example.com/kb.php?mode=cat&cat=0+UNION+SELECT+0,0,0,0,0,0+FROM+phpbb_users+WHERE+1=0