Ruby through 2.4.7, 2.5.x through 2.5.6, and 2.6.x through 2.6.4 allows code injection if the first argument (aka the "command" argument) to Shell#[] or Shell#test in lib/shell.rb is untrusted data. An attacker can exploit this to call an arbitrary Ruby method.
https://www.ruby-lang.org/ja/news/2019/10/01/ruby-2-6-5-released/
https://www.ruby-lang.org/ja/news/2019/10/01/ruby-2-5-7-released/
https://www.ruby-lang.org/ja/news/2019/10/01/ruby-2-4-8-released/
https://www.ruby-lang.org/ja/news/2019/10/01/code-injection-shell-test-cve-2019-16255/
https://www.oracle.com/security-alerts/cpujan2020.html
https://www.debian.org/security/2019/dsa-4587
https://security.gentoo.org/glsa/202003-06
https://seclists.org/bugtraq/2019/Dec/32
https://seclists.org/bugtraq/2019/Dec/31
https://lists.debian.org/debian-lts-announce/2023/04/msg00033.html
https://lists.debian.org/debian-lts-announce/2020/08/msg00027.html
https://lists.debian.org/debian-lts-announce/2019/12/msg00009.html
https://lists.debian.org/debian-lts-announce/2019/11/msg00025.html
https://hackerone.com/reports/327512
http://lists.opensuse.org/opensuse-security-announce/2020-03/msg00041.html