This indicates an attempt to exploit a vulnerability in Mac OS X iCal.
This vulnerability is due to null-pointer dereference errors while parsing malformed .ics files, which leads to abnormal termination of the iCal application.
iCal 3.0.1 running on Mac OS X 10.5.1; previous versions may also be affected.
Denial of Service: Remote attackers can crash vulnerable systems.
Apple security updates are available via the Software Update mechanism:
Apple security updates are also available for manual download: http://www.apple.com/support/downloads/