SerendipityNZ (aka SimpleBoxes) Serene Bach 2.20R and earlier, and 3.00 beta023 and earlier 3.x versions, uses a predictable session id, which makes it easier for remote attackers to hijack sessions via a modified id.
http://www.securityfocus.com/bid/35254
http://serenebach.net/log/sb221R.html
http://secunia.com/advisories/35335