A sandbox bypass vulnerability exists in Pipeline: Groovy Plugin 2.59 and earlier in groovy-sandbox/src/main/java/org/kohsuke/groovy/sandbox/SandboxTransformer.java, groovy-cps/lib/src/main/java/com/cloudbees/groovy/cps/SandboxCpsTransformer.java that allows attackers with Job/Configure permission, or unauthorized attackers with SCM commit privileges and corresponding pipelines based on Jenkinsfiles set up in Jenkins, to execute arbitrary code on the Jenkins master JVM
Arbitrary code execution due to incomplete sandbox protection: Constructors, instance variable initializers, and instance initializers in Pipeline scripts were not subject to sandbox protection, and could therefore execute arbitrary code. This could be exploited e.g. by regular Jenkins users with the permission to configure Pipelines in Jenkins, or by trusted committers to repositories containing Jenkinsfiles.