The rest-client gem 1.6.10 through 1.6.13 for Ruby, as distributed on RubyGems.org, included a code-execution backdoor inserted by a third party.
Users of an affected version should consider downgrading to the last non-affected version of 1.6.9, or upgrading to 1.7.x.
Additionally, a set of other minor gems have been partially or completely yanked and are included in this advisory.
These include cron_parser, coin_base, blockchain_wallet, awesome-bot, doge-coin, capistrano-colors, bitcoin_vanity, lita_coin, coming-soon, and omniauth_amazon.

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-15224
• rest-client/rest-client#713
• rubygems/rubygems.org#2097
• https://github.com/rubygems/rubygems.org/wiki/Gems-yanked-and-accounts-locked#19-aug-2019
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/blockchain_wallet/CVE-2019-15224.yml
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/coin_base/CVE-2019-15224.yml
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/doge-coin/CVE-2019-15224.yml
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/lita_coin/CVE-2019-15224.yml
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/omniauth_amazon/CVE-2019-15224.yml
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rest-client/CVE-2019-15224.yml