T1053.006: Systemd Timers

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 .timer 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 systemctl command line utility, which operates over SSH.

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

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.

Positive Technologies products that cover the technique

MaxPatrol SIEM knowledge base

unix_mitre_attck_persistence: PT-CR-1664: Unix_Systemd_Timers: Persistence on a host was gained using systemd timer files. Attackers can use a timer to execute malicious code at system startup or based on a schedule to gain persistence and escalate privileges.

Detection

ID	DS0022	Data source and component	File: File Modification	Description	Monitor for changes made to systemd timer unit files for unexpected modification events within the /etc/systemd/system, /usr/lib/systemd/system/, and ~/.config/systemd/user/ directories, as well as associated symbolic links

ID	DS0009	Data source and component	Process: Process Creation	Description	Monitor for newly constructed processes and/or command-lines that will have a parent process of ‘systemd’, a parent process ID of 1, and will usually execute as the ‘root’ user.

ID	DS0017	Data source and component	Command: Command Execution	Description	Monitor executed commands and arguments the 'systemd-run' utility as it may be used to create timers

ID	DS0003	Data source and component	Scheduled Job: Scheduled Job Creation	Description	Suspicious systemd timers can also be identified by comparing results against a trusted system baseline. Malicious systemd timers may be detected by using the systemctl utility to examine system wide timers: systemctl list-timers –all. Analyze the contents of corresponding .service files present on the file system and ensure that they refer to legitimate, expected executables.

ID	Data source and component	Description
DS0022	File: File Modification	Monitor for changes made to systemd timer unit files for unexpected modification events within the /etc/systemd/system, /usr/lib/systemd/system/, and ~/.config/systemd/user/ directories, as well as associated symbolic links
DS0009	Process: Process Creation	Monitor for newly constructed processes and/or command-lines that will have a parent process of ‘systemd’, a parent process ID of 1, and will usually execute as the ‘root’ user.
DS0017	Command: Command Execution	Monitor executed commands and arguments the 'systemd-run' utility as it may be used to create timers
DS0003	Scheduled Job: Scheduled Job Creation	Suspicious systemd timers can also be identified by comparing results against a trusted system baseline. Malicious systemd timers may be detected by using the systemctl utility to examine system wide timers: systemctl list-timers –all. Analyze the contents of corresponding .service files present on the file system and ensure that they refer to legitimate, expected executables.

Mitigation

ID	M1018	Name	User Account Management	Description	Limit user access to system utilities such as 'systemctl' or 'systemd-run' to users who have a legitimate need.

ID	M1026	Name	Privileged Account Management	Description	Limit access to the root account and prevent users from creating and/or modifying systemd timer unit files.

ID	M1022	Name	Restrict File and Directory Permissions	Description	Restrict read/write access to systemd `.timer` unit files to only select privileged users who have a legitimate need to manage system services.

ID	Name	Description
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.
M1022	Restrict File and Directory Permissions	Restrict read/write access to systemd `.timer` unit files to only select privileged users who have a legitimate need to manage system services.