T1053.006 Systemd Timers

> Sub-technique of: T1053

High-Level Description

Adversaries may abuse systemd timers to perform task scheduling for initial or recurring execution of malicious code. Systemd timers are unit files with file extension <code>.timer</code> that control services. Timers can be set to run on a calendar event or after a time span relative to a starting point. They can be used as an alternative to Cron in Linux environments. Systemd timers may be activated remotely via the <code>systemctl</code> command line utility, which operates over SSH.

Each <code>.timer</code> file must have a corresponding <code>.service</code> file with the same name, e.g., <code>example.timer</code> and <code>example.service</code>. <code>.service</code> files are Systemd Service unit files that are managed by the systemd system and service manager. Privileged timers are written to <code>/etc/systemd/system/</code> and <code>/usr/lib/systemd/system</code> while user level are written to <code>~/.config/systemd/user/</code>.

An adversary may use systemd timers to execute malicious code at system startup or on a scheduled basis for persistence. Timers installed using privileged paths may be used to maintain root level persistence. Adversaries may also install user level timers to achieve user level persistence.

Kill Chain Phase

• Execution (TA0002)
• Persistence (TA0003)
• Privilege Escalation (TA0004)

Platforms: Linux

What to Check

• ] Identify if Systemd Timers technique is applicable to target environment
• ] Check Linux systems for indicators of Systemd Timers
• ] Verify mitigations are bypassed or absent (3 known mitigations)
• ] Assess detection coverage (1 detection strategies)

How to Test

Atomic Red Team Tests

The following tests are from Atomic Red Team and provide actionable ways to test this technique:

Atomic Test 1: Create Systemd Service and Timer

This test creates Systemd service and timer then starts and enables the Systemd timer

Supported Platforms: linux Elevation Required: Yes

bash
echo "[Unit]" > #{path_to_systemd_service}
echo "Description=Atomic Red Team Systemd Timer Service" >> #{path_to_systemd_service}
echo "[Service]" >> #{path_to_systemd_service}
echo "Type=simple" >> #{path_to_systemd_service}
echo "ExecStart=/bin/touch /tmp/art-systemd-timer-marker" >> #{path_to_systemd_service}
echo "[Install]" >> #{path_to_systemd_service}
echo "WantedBy=multi-user.target" >> #{path_to_systemd_service}
echo "[Unit]" > #{path_to_systemd_timer}
echo "Description=Executes Atomic Red Team Systemd Timer Service" >> #{path_to_systemd_timer}
echo "Requires=#{systemd_service_name}" >> #{path_to_systemd_timer}
echo "[Timer]" >> #{path_to_systemd_timer}
echo "Unit=#{systemd_service_name}" >> #{path_to_systemd_timer}
echo "OnCalendar=*-*-* *:*:00" >> #{path_to_systemd_timer}
echo "[Install]" >> #{path_to_systemd_timer}
echo "WantedBy=timers.target" >> #{path_to_systemd_timer}
systemctl start #{systemd_timer_name}
systemctl enable #{systemd_timer_name}
systemctl daemon-reload

Atomic Test 2: Create a user level transient systemd service and timer

Schedule a user level transient task (will not survive a reboot) without having to create the .timer or .service files by using the systemd-run command.

Supported Platforms: linux

bash
systemd-run --user --unit=Atomic-Red-Team --on-calendar '*:0/1' /bin/sh -c 'echo "$(date) $(whoami)" >>/tmp/log'

Dependencies:

• Check if systemd-run exists on the machine

Atomic Test 3: Create a system level transient systemd service and timer

Schedule a system level transient task (will not survive a reboot) without having to create the .timer or .service files by using the systemd-run command.

Supported Platforms: linux Elevation Required: Yes

bash
systemd-run --unit=Atomic-Red-Team --on-calendar '*:0/1' /bin/sh -c 'echo "$(date) $(whoami)" >>/tmp/log'

Dependencies:

• Check if systemd-run exists on the machine

Manual Testing

If Atomic Red Team tests are not applicable, manually verify the technique by:

1. Identify Attack Surface: Determine if the target environment is susceptible to Systemd Timers by examining the target platforms (Linux).

1. Assess Existing Defenses: Review whether mitigations for T1053.006 are in place. If defenses are absent or misconfigured, this technique may be exploitable.

1. Execute Test: Use tools and methods described in the MITRE ATT&CK page and external references below.

Remediation Guide

M1022 Restrict File and Directory Permissions

Restrict read/write access to systemd <code>.timer</code> unit files to only select privileged users who have a legitimate need to manage system services.

M1018 User Account Management

Limit user access to system utilities such as 'systemctl' or 'systemd-run' to users who have a legitimate need.

M1026 Privileged Account Management

Limit access to the root account and prevent users from creating and/or modifying systemd timer unit files.

Detection

Behavioral Detection of Systemd Timer Abuse for Scheduled Execution

Risk Assessment

| Finding | Severity | Impact | | ----------------------------------- | -------- | --------- | | Systemd Timers technique applicable | High | Execution |

CWE Categories

| CWE ID | Title | | ------ | -------------------------------------- | | CWE-94 | Improper Control of Generation of Code |

References

• Systemd Remote Control
• archlinux Systemd Timers Aug 2020
• gist Arch package compromise 10JUL2018
• Arch Linux Package Systemd Compromise BleepingComputer 10JUL2018
• acroread package compromised Arch Linux Mail 8JUL2018
• Falcon Sandbox smp: 28553b3a9d
• Linux man-pages: systemd January 2014
• Atomic Red Team - T1053.006
• MITRE ATT&CK - T1053.006