An activex control installed by Cisco WebEx Meeting Manager is vulnerable to remote code execution or denial of service. The activex control, atucfobj.dll, is installed when a user connects to a WebEx meeting service. When users connect to an upgraded meeting service, the client side activex is automatically upgraded. Exploit code for this vulnerability has been publicly released.
As an aside, the interesting part of this vulnerability, according to a post from NANOG, is that even if you have cleaned the install of the client off your machine and have the latest version, if you connect to a meeting service that is NOT up to date, you could then become vulnerable again.
The full vulnerability details can be found at http://www.cisco.com/warp/public/707/cisco-sa-20080814-webex.shtml