Unix Shell

What the technique is

Adversaries may abuse Unix shell commands and scripts for execution. Unix shells are the primary command prompt on Linux, macOS, and ESXi systems, though many variations of the Unix shell exist (e.g. sh, ash, bash, zsh, etc.) depending on the specific OS or distribution. Unix shells can control every aspect of a system, with certain commands requiring elevated privileges. Unix shells also support scripts that enable sequential execution of commands as well as other typical programming operations such as conditionals and loops. Common uses of shell scripts include long or repetitive tasks, or the need to run the same set of commands on multiple systems. Adversaries may abuse Unix shells to execute various commands or payloads. Interactive shells may be accessed through command and control channels or during lateral movement such as with SSH. Adversaries may also leverage shell scripts to deliver and execute multiple commands on victims or as part of payloads used for persistence. Some systems, such as embedded devices, lightweight Linux distributions, and ESXi servers, may leverage stripped-down Unix shells via Busybox, a small executable that contains a variety of tools, including a simple shell.

What MITRE says you would watch

• AN1081

  Detects bash, sh, zsh, or BusyBox shell execution initiated via remote sessions, unauthorized users, or embedded within secondary script interpreters. Focus is on chained behavior: shell > suspicious commands > network discovery or persistence indicators.

• AN1082

  Identifies use of sh/bash/zsh in suspicious context, such as user scripts launched from non-standard apps (e.g., Preview.app), embedded in LaunchDaemons, or executed outside Terminal.app. Looks for misuse in Automator, LaunchAgents, or NSAppleScript-executed shell.

• AN1083

  Detects BusyBox or Ash shell execution from unauthorized logins or remote connections. Focus is on rare shell invocations from DCUI, SSH sessions, or remote management paths. Also watches for payload droppers or persistence artifacts using shell.