The Exchange Collaboration Data Objects (EXCDO) functionality in Microsoft Exchange Server 2000 SP3, 2003 SP1 and SP2, and 2007 allows remote attackers to cause a denial of service (crash) via an Internet Calendar (iCal) file containing multiple X-MICROSOFT-CDO-MODPROPS (MODPROPS) properties in which the second MODPROPS is longer than the first, which triggers a NULL pointer dereference and an unhandled exception.