Authentication is not required to exploit this vulnerability.
The specific flaw exists within the gwwww1.dll module responsible for parsing VCALENDAR data within e-mail messages. When the code encounters a RRULE variable it allocates memory for 0x800 bytes for the variable's contents, a list of numbers. It then proceeds to copy the numbers from the request while there are numbers to parse. By specifying a large amount of comma-separated values within an RRULE, an attacker can overflow the buffer and execute arbitrary code under the context of the SYSTEM user.
