Cron Jobs and PATH Variable Exploitation for Privilege Escalation

In the pursuit of privilege escalation, understanding how scheduled tasks and system environment variables can be manipulated is crucial. This module delves into exploiting misconfigured cron jobs and the PATH variable to gain higher privileges on a target system, a common technique tested in certifications like OSCP.

Understanding Cron Jobs

Cron is a time-based job scheduler in Unix-like operating systems. It allows users to schedule commands or scripts to run periodically at fixed times, dates, or intervals. These scheduled tasks are defined in configuration files called crontabs.

Identifying Vulnerable Cron Jobs

The first step in exploiting cron jobs is to identify them. We look for scripts that are executed regularly and have permissions that allow modification by the current user, or scripts that call other executables in a way that can be manipulated.

What is the primary goal when looking for vulnerable cron jobs?

To find cron jobs that run with elevated privileges and can be influenced by a lower-privileged user.

Common indicators of vulnerable cron jobs include scripts owned by root that are writable by other users, or scripts that execute commands without specifying their full path.

Exploiting Cron Jobs: The PATH Variable

The PATH environment variable is a fundamental concept in Unix-like systems. It's a list of directories that the shell searches through when you type a command. If a cron job executes a command without specifying its full path (e.g., just script.sh instead of /usr/local/bin/script.sh), the system will look for script.sh in the directories listed in the PATH variable, in order.

When a cron job executes a command like mycommand, the operating system searches for mycommand in each directory listed in the PATH variable sequentially. If a malicious executable with the same name is placed in a directory that appears earlier in the PATH than the legitimate command's location, the malicious version will be executed instead. This is known as PATH hijacking or PATH variable exploitation.

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This vulnerability is particularly potent when a cron job runs with root privileges. If a lower-privileged user can control or influence the PATH variable used by a root-owned cron job, they can trick the system into executing their malicious script with root privileges.

Practical Exploitation Steps

Here's a typical workflow for exploiting cron jobs and the PATH variable:

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Identify Cron Jobs: Use commands like crontab -l (for the current user) and check system crontabs (e.g., /etc/crontab, /etc/cron.d/, /etc/cron.hourly/, etc.).
Check Permissions: Determine if any scripts run by cron are writable by your current user.
Analyze Script: Examine the script for commands executed without full paths.
Manipulate PATH: If the script relies on the PATH, try to influence it. This can sometimes be done by modifying the PATH variable in a user's profile files that are sourced by the cron job, or by placing a malicious executable in a directory that appears earlier in the PATH.
Place Malicious Executable: Create a malicious script (e.g., a reverse shell) and place it in a directory that will be searched first due to your PATH manipulation.
Wait for Execution: The cron job will eventually run, executing your malicious script with the privileges of the user the cron job is assigned to (often root).

Mitigation Strategies

To prevent such attacks, system administrators should:

Vulnerability	Mitigation
Writable Cron Scripts	Ensure cron scripts are owned by root and not writable by other users.
PATH Variable Exploitation	Always use absolute paths for executables within cron jobs. Regularly review and sanitize the PATH environment variable.
Unnecessary Cron Jobs	Remove or disable any cron jobs that are not essential.
Principle of Least Privilege	Run cron jobs with the minimum necessary privileges.

Learning Resources

Cron Job Exploitation - HackTricks(documentation)

A comprehensive guide on identifying and exploiting cron job vulnerabilities, including PATH variable manipulation.

Privilege Escalation - PATH Variable Exploitation(blog)

Explains how the PATH environment variable can be exploited for privilege escalation with practical examples.

OSCP - Privilege Escalation - Cron Jobs(documentation)

Official Offensive Security documentation that touches upon privilege escalation techniques, including cron job exploitation.

Linux Privilege Escalation - PATH Variable(blog)

A step-by-step tutorial on how to exploit the PATH variable for privilege escalation on Linux systems.

Cron Job Vulnerabilities and Exploitation(blog)

Discusses common cron job misconfigurations and how they can be leveraged by attackers.

Understanding the PATH Environment Variable(documentation)

Provides a foundational understanding of the PATH environment variable and how it works in Linux.

GTFOBins - Cron(documentation)

A curated list of Unix binaries that can be exploited for privilege escalation, including detailed examples for cron.

Privilege Escalation on Linux - A Deep Dive(blog)

A comprehensive resource covering various Linux privilege escalation techniques, with sections on cron jobs and environment variables.

Exploiting Cron Jobs for Privilege Escalation(video)

A video tutorial demonstrating how to exploit cron jobs for privilege escalation in a practical lab environment.

Linux PATH Variable Explained(video)

A visual explanation of the Linux PATH variable and its significance in command execution.