Adversaries may create or modify systemd services to repeatedly execute malicious payloads as part of persistence. Systemd is a system and service manager commonly used for managing background daemon processes (also known as services) and other system resources.(Citation: Linux man-pages: systemd January 2014) Systemd is the default initialization (init) system on many Linux distributions replacing legacy init systems, including SysVinit and Upstart, while remaining backwards compatible.  

Systemd utilizes unit configuration files with the `.service` file extension to encode information about a service's process. By default, system level unit files are stored in the `/systemd/system` directory of the root owned directories (`/`). User level unit files are stored in the `/systemd/user` directories of the user owned directories (`$HOME`).(Citation: lambert systemd 2022) 

Inside the `.service` unit files, the following directives are used to execute commands:(Citation: freedesktop systemd.service)  

* `ExecStart`, `ExecStartPre`, and `ExecStartPost` directives execute when a service is started manually by `systemctl` or on system start if the service is set to automatically start.
* `ExecReload` directive executes when a service restarts. 
* `ExecStop`, `ExecStopPre`, and `ExecStopPost` directives execute when a service is stopped.  

Adversaries have created new service files, altered the commands a `.service` file’s directive executes, and modified the user directive a `.service` file executes as, which could result in privilege escalation. Adversaries may also place symbolic links in these directories, enabling systemd to find these payloads regardless of where they reside on the filesystem.(Citation: Anomali Rocke March 2019)(Citation: airwalk backdoor unix systems)(Citation: Rapid7 Service Persistence 22JUNE2016) 

The .service file’s User directive can be used to run service as a specific user, which could result in privilege escalation based on specific user/group permissions. 
Systemd Service
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1543/002  external_id: T1543.002     source_name: airwalk backdoor unix systems  description: airwalk. (2023, January 1). A guide to backdooring Unix systems. Retrieved May 31, 2023.  url: http://www.ouah.org/backdoors.html     source_name: Anomali Rocke March 2019  description: Anomali Labs. (2019, March 15). Rocke Evolves Its Arsenal With a New Malware Family Written in Golang. Retrieved April 24, 2019.  url: https://www.anomali.com/blog/rocke-evolves-its-arsenal-with-a-new-malware-family-written-in-golang     source_name: freedesktop systemd.service  description: Free Desktop. (n.d.). systemd.service — Service unit configuration. Retrieved March 20, 2023.  url: https://www.freedesktop.org/software/systemd/man/systemd.service.html     source_name: Linux man-pages: systemd January 2014  description: Linux man-pages. (2014, January). systemd(1) - Linux manual page. Retrieved April 23, 2019.  url: http://man7.org/linux/man-pages/man1/systemd.1.html     source_name: Berba hunting linux systemd  description: Pepe Berba. (2022, January 30). Hunting for Persistence in Linux (Part 3): Systemd, Timers, and Cron. Retrieved March 20, 2023.  url: https://pberba.github.io/security/2022/01/30/linux-threat-hunting-for-persistence-systemd-timers-cron/     source_name: Rapid7 Service Persistence 22JUNE2016  description: Rapid7. (2016, June 22). Service Persistence. Retrieved April 23, 2019.  url: https://www.rapid7.com/db/modules/exploit/linux/local/service_persistence     source_name: lambert systemd 2022  description: Tony Lambert. (2022, November 13). ATT&CK T1501: Understanding systemd service persistence. Retrieved March 20, 2023.  url: https://redcanary.com/blog/attck-t1501-understanding-systemd-service-persistence/     