Linux Restricted Shell Breakout via c89/c99 Shell evasion

Elastic Stack Serverless Security

Identifies Linux binary c89/c99 abuse to break out from restricted environments by spawning an interactive system shell.The c89/c99 utility is an interface to the standard C compilation system and the activity of spawing 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.*

Severity: medium

Risk score: 47

Runs every: 5m

Searches indices from: now-9m (https://www.elastic.co/guide/en/elasticsearch/reference/current/common-options.html#date-math[Date Math format], see also Additional look-back time)

Maximum alerts per execution: 100

References:

• https://gtfobins.github.io/gtfobins/c89/
• https://gtfobins.github.io/gtfobins/c99/

Tags:

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

Version: 1

Rule authors:

• Elastic

Rule license: Elastic License v2

process where event.type == "start" and process.name in ("sh", "dash", "bash") and
  process.parent.name in ("c89","c99") and process.parent.args == "-wrapper" and
  process.parent.args in ("sh,-s", "bash,-s", "dash,-s", "/bin/sh,-s", "/bin/bash,-s", "/bin/dash,-s")

Framework: MITRE ATT&CKTM

• 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/