RubyGems versions between 2.0.0 and 2.6.13 are vulnerable to a possible remote code execution vulnerability. YAML deserialization of gem specifications can bypass class white lists. Specially crafted serialized objects can possibly be used to escalate to remote code execution.
https://www.debian.org/security/2017/dsa-4031
https://usn.ubuntu.com/3685-1/
https://usn.ubuntu.com/3553-1/
https://lists.debian.org/debian-lts-announce/2018/07/msg00012.html
https://hackerone.com/reports/274990
https://github.com/rubygems/rubygems/commit/510b1638ac9bba3ceb7a5d73135dafff9e5bab49
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.securityfocus.com/bid/101275
http://blog.rubygems.org/2017/10/09/unsafe-object-deserialization-vulnerability.html