Anything-Digital.com identified a few low-level security issues affecting JCal Pro for Joomla! 1.0.x (versions 1.5.3, 1.5.3.1 and 1.5.3.2 only). A free patch is available to fix these bugs (login required), though they recommend that all  users of JCal Pro for Joomla! 1.0.x upgrade to the latest version  1.5.3.5.