RubyGems version 2.6.12 and earlier is vulnerable to a DNS hijacking vulnerability that allows a MITM attacker to force the RubyGems client to download and install gems from a server that the attacker controls.
https://www.debian.org/security/2017/dsa-3966
https://usn.ubuntu.com/3685-1/
https://usn.ubuntu.com/3553-1/
https://security.gentoo.org/glsa/201710-01
https://lists.debian.org/debian-lts-announce/2018/07/msg00012.html
https://hackerone.com/reports/218088
https://github.com/rubygems/rubygems/commit/8d91516fb7037ecfb27622f605dc40245e0f8d32
https://access.redhat.com/errata/RHSA-2018:0585
https://access.redhat.com/errata/RHSA-2018:0583
https://access.redhat.com/errata/RHSA-2018:0378
https://access.redhat.com/errata/RHSA-2017:3485
http://www.securitytracker.com/id/1039249