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.
http://lists.opensuse.org/opensuse-security-announce/2020-03/msg00041.html
https://hackerone.com/reports/327512
https://lists.debian.org/debian-lts-announce/2019/11/msg00025.html
https://lists.debian.org/debian-lts-announce/2019/12/msg00009.html
https://lists.debian.org/debian-lts-announce/2020/08/msg00027.html
https://seclists.org/bugtraq/2019/Dec/31
https://seclists.org/bugtraq/2019/Dec/32
https://security.gentoo.org/glsa/202003-06
https://www.debian.org/security/2019/dsa-4587
https://www.oracle.com/security-alerts/cpujan2020.html
https://www.ruby-lang.org/ja/news/2019/10/01/code-injection-shell-test-cve-2019-16255/
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/ruby-2-5-7-released/
https://www.ruby-lang.org/ja/news/2019/10/01/ruby-2-6-5-released/
Source: MITRE
Published: 2019-11-26
Updated: 2020-08-24
Type: CWE-94
Base Score: 6.8
Vector: AV:N/AC:M/Au:N/C:P/I:P/A:P
Impact Score: 6.4
Exploitability Score: 8.6
Severity: MEDIUM
Base Score: 8.1
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Impact Score: 5.9
Exploitability Score: 2.2
Severity: HIGH