From the Cisco Security Advisory (http://www.cisco.com/warp/public/707/cisco-sa-20080814-webex.shtml):
A buffer overflow vulnerability exists in an ActiveX control used by the WebEx Meeting Manager. Exploitation of this vulnerability could allow a remote attacker to execute arbitrary code on the user client machine. The WebEx Meeting Manager is a client-side program that is provided by the Cisco WebEx meeting service. [...]