Cisco WebEx Meeting Center Original Release Base allows remote attackers to obtain sensitive information about username validity by (1) attending or (2) hosting a meeting, aka Bug ID CSCux84312.
http://www.securitytracker.com/id/1035977
http://www.securityfocus.com/bid/90908
http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20160526-wmc