includes/functions.php in Craig Knudsen WebCalendar before 1.0.5 does not protect the noSet variable from external modification, which allows remote attackers to set arbitrary global variables via a URL with modified values in the noSet parameter, which leads to resultant vulnerabilities that probably include remote file inclusion and other issues.
WebCalendar 1.0.1 to 1.0.3 generates different error messages depending on whether or not a username is valid, which allows remote attackers to enumerate valid usernames.