SSH Credentials Plugin allowed the creation of SSH credentials with keys "From a file on Jenkins master". Credentials Binding Plugin 1.13 and newer allows binding SSH credentials to environment variables. In combination, these two features allow users with the permission to configure a job to read arbitrary files on the Jenkins master by creating an SSH credential referencing an arbitrary file on the Jenkins master, and binding it to an environment variable in a job.
External References: https://jenkins.io/security/advisory/2018-06-25/
Created jenkins-ssh-credentials-plugin tracking bugs for this issue: Affects: fedora-all [bug 1596123]