Linux Restricted Shell Breakout via flock Shell evasion

Identifies Linux binary flock abuse to break out from restricted environments by spawning an interactive system shell. The flock utility allows users to manage advisory file locks in shell scripts or on the command line. The activity of spawning a shell is not a standard use of this binary by a user or system administrator. It indicates a potentially malicious actor attempting to improve the capabilities or stability of their access.

Rule type: eql
Rule indices:

• logs-endpoint.events.*

Rule Severity: medium
Risk Score: 47
Runs every:
Searches indices from: now-9m
Maximum alerts per execution: 100
References:

• https://gtfobins.github.io/gtfobins/flock/

Tags:

• Elastic
• Host
• Linux
• Threat Detection
• Execution
• GTFOBins

Version: 100
Rule authors:

• Elastic

Rule license: Elastic License v2

process where event.type == "start" and process.parent.name == "flock" and process.parent.args == "-u" and process.parent.args == "/" and process.parent.args in ("/bin/sh", "/bin/bash", "/bin/dash", "sh", "bash", "dash") and process.name in ("bash", "dash", "sh")
		

Framework: MITRE ATT&CK

• Tactic:

  • Name: Execution
  • Id: TA0002
  • Reference URL: https://attack.mitre.org/tactics/TA0002/

• Technique:

  • Name: Command and Scripting Interpreter
  • Id: T1059
  • Reference URL: https://attack.mitre.org/techniques/T1059/

• Sub Technique:

  • Name: Unix Shell
  • Id: T1059.004
  • Reference URL: https://attack.mitre.org/techniques/T1059/004/