CVE 2016-3378
Open redirect vulnerability in Microsoft Exchange Server 2013 SP1, 2013 Cumulative Update 12, 2013 Cumulative Update 13, 2016 Cumulative Update 1, and 2016 Cumulative Update 2 allows remote attackers to redirect users to arbitrary web sites and conduct phishing attacks via a crafted URL, aka "Microsoft Exchange Open Redirect Vulnerability."
See the
CVE page on Mitre.org
for more details.