Moodle 1.8.x and 1.9.x before 1.9.8 does not enable the "Regenerate session id during login" setting by default, which makes it easier for remote attackers to conduct session fixation attacks.
http://www.vupen.com/english/advisories/2010/1107
http://lists.opensuse.org/opensuse-security-announce/2010-05/msg00001.html