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Welcome to the third installment of our Jenkins Groovy Sandbox series. In this demo you’ll learn how the sandbox enforces security by whitelisting and blacklisting Groovy methods, and how administrators can approve blocked signatures via In-process Script Approval.

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Table of Contents

  1. Understanding Whitelists & Blacklists
  2. Inspecting the Whitelist
  3. Inspecting the Blacklist
  4. Adding Blacklisted Calls to a Pipeline
  5. First Build: getInstance Blocked
  6. Approving Signatures in Jenkins
  7. Second Build: getProperty Blocked
  8. Final Build: Success!
  9. References

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Understanding Whitelists & Blacklists

Jenkins uses the script-security plugin to sandbox Groovy scripts.
Under src/main/resources/org/jenkinsci/plugins/scriptsecurity/sandbox/whitelists you’ll find files that list allowed methods. The same folder contains a blacklist file defining methods that are blocked by default and require admin approval.
List TypePurposeSample Entry
WhitelistAPI methods and signatures allowed in sandboxmethod hudson.model.Run getFullDisplayName
BlacklistMethods blocked unless approved via Script Approvalmethod java.io.Reader read

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Inspecting the Whitelist

Browse the Jenkins script-security-plugin repository on GitHub and open the whitelists folder:
The image shows a GitHub repository page for the "script-security-plugin" by "jenkinsci," displaying a file named "jenkins-whitelist" with a list of Jenkins API methods. The interface includes navigation options and a code viewer.
An excerpt from jenkins-whitelist: 
staticField hudson.model.Result ABORTED
staticField hudson.model.Result FAILURE
staticMethod hudson.model.Result fromString java.lang.String
method hudson.model.Run getFullDisplayName
method hudson.model.User getId
...
staticField jenkins.model.Jenkins VERSION

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Inspecting the Blacklist

The blacklist file in the same directory lists methods that are disallowed by default: 
method groovy.lang.Closure ncurry int java.lang.Object
new java.io.PrintWriter java.lang.String
method java.io.Reader read char[] int int
...
staticMethod groovy.xml.XmlUtil escapeXml java.lang.String
The image shows a GitHub repository page displaying a file named "blacklist" from the "script-security-plugin" project. The file contains a list of Java methods and classes related to security restrictions in Jenkins.
Any invocation of these methods in a sandboxed pipeline will be rejected unless approved.

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Adding Blacklisted Calls to a Pipeline

Let’s modify a declarative pipeline to call two blacklisted methods: 
pipeline {
  agent any
  stages {
    stage('Topic') {
      steps {
        echo 'Exploring Groovy Sandbox'
      }
    }
    stage('Get Hudson Instance') {
      steps {
        script {
          // This staticMethod is blacklisted by default
          def hudson = hudson.model.Hudson.getInstance()
          println "Hudson Instance: ${hudson}"
        }
      }
    }
    stage('Get System Property') {
      steps {
        script {
          // This method is also blacklisted by default
          def userName = java.lang.System.getProperty("user.name")
          println "System Property: ${userName}"
        }
      }
    }
  }
}
Make sure Use Groovy Sandbox is checked in your pipeline configuration before running the build.
The image shows a configuration screen for a Jenkins pipeline, displaying a Groovy script with stages to get a Hudson instance and a system property. There are options to approve the script and use the Groovy sandbox.

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First Build: getInstance Blocked

The Topic stage will pass, but Get Hudson Instance fails due to the blacklist:
The image shows a Jenkins dashboard displaying the status of a pipeline named "groovy-sandbox-test," with various stages and their completion statuses.
Console output:
The image shows a Jenkins console output with a Groovy script execution, highlighting a permission error related to using a static method. The sidebar includes options like "Open Blue Ocean" and "Pipeline Overview."
Scripts not permitted to use staticMethod hudson.model.Hudson getInstance

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Approving Signatures in Jenkins

Click the error link or navigate to Manage Jenkins → In-process Script Approval to review pending signatures:
The image shows a Jenkins script approval interface with options to approve or deny script signatures, and a warning about potential security vulnerabilities.
Approving method signatures grants scripts additional privileges. Review each request carefully.
Approve the hudson.model.Hudson getInstance signature, then rerun the build.

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Second Build: getProperty Blocked

After approving, Get Hudson Instance now succeeds but Get System Property fails:
The image shows a Jenkins console output with a script error message indicating that a script is not permitted to use a specific Java method. It suggests that administrators can decide whether to approve or reject the signature.
Return to Script Approval and approve the java.lang.System getProperty signature:
The image shows a Jenkins interface on the "Script Approval" page, displaying no pending script approvals and a list of already approved signatures.

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Final Build: Success!

Run the pipeline one last time. All stages should complete without errors:
The image shows a Jenkins console interface with a dropdown menu open, displaying options like "Configure" and "Delete Pipeline." The console output includes a script error message related to permission issues with Java methods.
Running on Jenkins in /var/lib/jenkins/workspace/groovy-sandbox-test
[Pipeline] {
[Pipeline] stage
[Pipeline] { (Topic)
[Pipeline] echo
Exploring Groovy Sandbox
[Pipeline] }
[Pipeline] // stage
[Pipeline] stage
[Pipeline] { (Get Hudson Instance)
[Pipeline] script
[Pipeline] {
[Pipeline] echo
Hudson Instance: hudson.model.Hudson@623e629d
[Pipeline] }
[Pipeline] // script
[Pipeline] stage
[Pipeline] { (Get System Property)
[Pipeline] script
[Pipeline] {
[Pipeline] echo
System Property: jenkins
[Pipeline] }
[Pipeline] // script

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References

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