Event Triggered Execution: Trap

Adversaries may establish persistence by executing malicious content triggered by an interrupt signal. The trap command allows programs and shells to specify commands that will be executed upon receiving interrupt signals. A common situation is a script allowing for graceful termination and handling of common keyboard interrupts like ctrl+c and ctrl+d.

Adversaries can use this to register code to be executed when the shell encounters specific interrupts as a persistence mechanism. Trap commands are of the following format trap 'command list' signals where "command list" will be executed when "signals" are received.[1][2]

ID: T1546.005
Sub-technique of:  T1546
Tactics: Privilege Escalation, Persistence
Platforms: Linux, macOS
Permissions Required: Administrator, User
Data Sources: File monitoring, Process command-line parameters, Process monitoring
Version: 1.0
Created: 24 January 2020
Last Modified: 24 March 2020

Mitigations

This type of attack technique cannot be easily mitigated with preventive controls since it is based on the abuse of system features.

Detection

Trap commands must be registered for the shell or programs, so they appear in files. Monitoring files for suspicious or overly broad trap commands can narrow down suspicious behavior during an investigation. Monitor for suspicious processes executed through trap interrupts.

References