Outlook Web App (OWA) in Microsoft Exchange Server 2013 SP1 and Cumulative Update 6 does not properly validate redirection tokens, which allows remote attackers to redirect users to arbitrary web sites and spoof the origin of e-mail messages via unspecified vectors, aka "Exchange URL Redirection Vulnerability."
https://docs.microsoft.com/en-us/security-updates/securitybulletins/2014/ms14-075