CVE-2015-3900 rubygems: DNS hijacking vulnerability in api_endpoint()

RubyGems 2.0.x before 2.0.16, 2.2.x before 2.2.4, and 2.4.x before 2.4.7
does not validate the hostname when fetching gems or making API requests, which
allows remote attackers to redirect requests to arbitrary domains via a crafted
DNS SRV record, aka a “DNS hijack attack.” A flaw was found in a way rubygems verified
the API endpoint hostname retrieved through a DNS SRV record. A man-in-the-middle
attacker could use this flaw to force a client to download content from an untrusted
domain.