T1053.003 - Scheduled Task/Job: Cron

Description from ATT&CK (opens in a new tab)

Adversaries may abuse the cron utility to perform task scheduling for initial or recurring execution of malicious code.(Citation: 20 macOS Common Tools and Techniques) The cron utility is a time-based job scheduler for Unix-like operating systems. The crontab file contains the schedule of cron entries to be run and the specified times for execution. Any crontab files are stored in operating system-specific file paths.

An adversary may use cron in Linux or Unix environments to execute programs at system startup or on a scheduled basis for Persistence (opens in a new tab). In ESXi environments, cron jobs must be created directly via the crontab file (e.g., /var/spool/cron/crontabs/root).(Citation: CloudSEK ESXiArgs 2023)

Atomic Tests

• Atomic Test #1 - Cron - Replace crontab with referenced file

• Atomic Test #2 - Cron - Add script to all cron subfolders

• Atomic Test #3 - Cron - Add script to /etc/cron.d folder

• Atomic Test #4 - Cron - Add script to /var/spool/cron/crontabs/ folder

Atomic Test #1 - Cron - Replace crontab with referenced file

This test replaces the current user's crontab file with the contents of the referenced file. This technique was used by numerous IoT automated exploitation attacks.

Supported Platforms: Linux, macOS

auto_generated_guid: 435057fb-74b1-410e-9403-d81baf194f75

Inputs:

Attack Commands: Run with sh!

crontab -l > /tmp/notevil
echo "* * * * * #{command}" > #{tmp_cron} && crontab #{tmp_cron}

Cleanup Commands:

crontab /tmp/notevil

Atomic Test #2 - Cron - Add script to all cron subfolders

This test adds a script to /etc/cron.hourly, /etc/cron.daily, /etc/cron.monthly and /etc/cron.weekly folders configured to execute on a schedule. This technique was used by the threat actor Rocke during the exploitation of Linux web servers.

Supported Platforms: macOS, Linux

auto_generated_guid: b7d42afa-9086-4c8a-b7b0-8ea3faa6ebb0

Inputs:

Attack Commands: Run with bash! Elevation Required (e.g. root or admin)

echo "#{command}" > /etc/cron.daily/#{cron_script_name}
echo "#{command}" > /etc/cron.hourly/#{cron_script_name}
echo "#{command}" > /etc/cron.monthly/#{cron_script_name}
echo "#{command}" > /etc/cron.weekly/#{cron_script_name}

Cleanup Commands:

rm /etc/cron.daily/#{cron_script_name} -f
rm /etc/cron.hourly/#{cron_script_name} -f
rm /etc/cron.monthly/#{cron_script_name} -f
rm /etc/cron.weekly/#{cron_script_name} -f

Atomic Test #3 - Cron - Add script to /etc/cron.d folder

This test adds a script to /etc/cron.d folder configured to execute on a schedule.

Supported Platforms: Linux

auto_generated_guid: 078e69eb-d9fb-450e-b9d0-2e118217c846

Inputs:

Attack Commands: Run with sh! Elevation Required (e.g. root or admin)

echo "#{command}" > /etc/cron.d/#{cron_script_name}

Cleanup Commands:

rm /etc/cron.d/#{cron_script_name} -f

Atomic Test #4 - Cron - Add script to /var/spool/cron/crontabs/ folder

This test adds a script to a /var/spool/cron/crontabs folder configured to execute on a schedule. This technique was used by the threat actor Rocke during the exploitation of Linux web servers.

Supported Platforms: Linux

auto_generated_guid: 2d943c18-e74a-44bf-936f-25ade6cccab4

Inputs:

Attack Commands: Run with bash! Elevation Required (e.g. root or admin)

echo "#{command}" >> /var/spool/cron/crontabs/#{cron_script_name}

Cleanup Commands:

rm /var/spool/cron/crontabs/#{cron_script_name} -f