Jonathan Claudius reports:
RubyGems provides the ability of a domain to direct clients to a separate host that is used to fetch gems and make API calls against. This mechanism is implemented via DNS, specifically a SRV record _rubygems._tcp under the original requested domain. RubyGems did not validate the hostname returned in the SRV record before sending requests to it. This left clients open to a DNS hijack attack, whereby an attacker could return a SRV of their choosing and get the client to use it.