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.
CWE-287 - Improper Authentication
Improper (or broken) authentication attacks are widespread, and have accounted for many of the worst data breaches in recent years. Improper authentication attacks are a class of vulnerabilities where an attacker impersonates a legitimate user by exploiting weaknesses in either session management or credential management to gain access to the user’s account. This can result in disclosure of sensitive information, and can lead to system compromise, theft, identity theft, and fraud.