In Ruby before 2.2.10, 2.3.x before 2.3.7, 2.4.x before 2.4.4, 2.5.x before 2.5.1, and 2.6.0-preview1, an attacker can pass a large HTTP request with a crafted header to WEBrick server or a crafted body to WEBrick server/handler and cause a denial of service (memory consumption).
https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-5-1-released/
https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-4-4-released/
https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-3-7-released/
https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-2-10-released/
https://www.ruby-lang.org/en/news/2018/03/28/large-request-dos-in-webrick-cve-2018-8777/
https://www.debian.org/security/2018/dsa-4259
https://usn.ubuntu.com/3685-1/
https://lists.debian.org/debian-lts-announce/2018/07/msg00012.html
https://lists.debian.org/debian-lts-announce/2018/04/msg00024.html
https://lists.debian.org/debian-lts-announce/2018/04/msg00023.html
https://access.redhat.com/errata/RHSA-2020:0663
https://access.redhat.com/errata/RHSA-2020:0591
https://access.redhat.com/errata/RHSA-2020:0542
https://access.redhat.com/errata/RHSA-2019:2028
https://access.redhat.com/errata/RHSA-2018:3731
https://access.redhat.com/errata/RHSA-2018:3730
https://access.redhat.com/errata/RHSA-2018:3729
http://www.securitytracker.com/id/1042004
http://www.securityfocus.com/bid/103683
http://lists.opensuse.org/opensuse-security-announce/2019-07/msg00036.html