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, specificly 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. As discussed upstream, CVE-2015-4020 is due to an incomplete fix for CVE-2015-3900 , which allowed redirection to an arbitrary gem server in any security domain.