sigma-rules/rules/linux/execution_shell_evasion_linux_binary.toml

[metadata]
creation_date = "2022/05/06"
integration = ["endpoint"]
maturity = "production"
min_stack_comments = "New fields added: required_fields, related_integrations, setup"
min_stack_version = "8.3.0"
updated_date = "2023/08/04"

[rule]
author = ["Elastic"]
description = """
Identifies the abuse of a Linux binary to break out of a restricted shell or environment by spawning an interactive 
system shell. The activity of spawning a shell from a binary is not common behavior for a user or system administrator, 
and may indicate an attempt to evade detection, increase capabilities or enhance the stability of an adversary.
"""
from = "now-9m"
index = ["logs-endpoint.events.*"]
language = "eql"
license = "Elastic License v2"
name = "Linux Restricted Shell Breakout via Linux Binary(s)"
note = """## Triage and analysis

### Investigating Shell Evasion via Linux Utilities
Detection alerts from this rule indicate that a Linux utility has been abused to breakout of restricted shells or
environments by spawning an interactive system shell.
Here are some possible avenues of investigation:
- Examine the entry point to the host and user in action via the Analyse View.
  - Identify the session entry leader and session user
- Examine the contents of session leading to the abuse via the Session View.
  - Examine the command execution pattern in the session, which may lead to suspricous activities
- Examine the execution of commands in the spawned shell.
  - Identify imment threat to the system from the executed commands
  - Take necessary incident response actions to contain any malicious behviour caused via this execution.

### Related rules

- A malicious spawned shell can execute any of the possible MITTRE ATT&CK vectors mainly to impair defences.
- Hence its adviced to enable defence evasion and privilige escalation rules accordingly in your environment

### Response and remediation

Initiate the incident response process based on the outcome of the triage.

- If the triage releaved suspicious netwrok activity from the malicious spawned shell,
  - Isolate the involved host to prevent further post-compromise behavior.
- If the triage identified malware execution via the maliciously spawned shell,
  - Search the environment for additional compromised hosts.
  - Implement temporary network rules, procedures, and segmentation to contain the malware.
  - Stop suspicious processes.
  - Immediately block the identified indicators of compromise (IoCs).
  - Inspect the affected systems for additional malware backdoors like reverse shells, reverse proxies, or droppers that attackers could use to reinfect the system.
- If the triage revelaed defence evasion for imparing defenses
  - Isolate the involved host to prevent further post-compromise behavior.
  - Identified the disabled security guard components on the host and take necessary steps in renebaling the same.
  - If any tools have been disbaled / uninstalled or config tampered work towards reenabling the same.
- If the triage revelaed addition of persistence mechanism exploit like auto start scripts
  - Isolate further login to the systems that can initae auto start scripts.
  - Identify the auto start scripts and disable and remove the same from the systems
- If the triage revealed data crawling or data export via remote copy
  - Investigate credential exposure on systems compromised / used / decoded by the attacker during the data crawling
  - Intiate compromised credential deactivation and credential rotation process for all exposed crednetials.
  - Investiagte if any IPR data was accessed during the data crawling and take appropriate actions.
- Determine the initial vector abused by the attacker and take action to prevent reinfection through the same vector.
- Using the incident response data, update logging and audit policies to improve the mean time to detect (MTTD) and the mean time to respond (MTTR).

## Setup

The session view analysis for the command alerted is avalible in versions 8.2 and above.
"""
references = [
    "https://gtfobins.github.io/gtfobins/apt/",
    "https://gtfobins.github.io/gtfobins/apt-get/",
    "https://gtfobins.github.io/gtfobins/nawk/",
    "https://gtfobins.github.io/gtfobins/mawk/",
    "https://gtfobins.github.io/gtfobins/awk/",
    "https://gtfobins.github.io/gtfobins/gawk/",
    "https://gtfobins.github.io/gtfobins/busybox/",
    "https://gtfobins.github.io/gtfobins/c89/",
    "https://gtfobins.github.io/gtfobins/c99/",
    "https://gtfobins.github.io/gtfobins/cpulimit/",
    "https://gtfobins.github.io/gtfobins/crash/",
    "https://gtfobins.github.io/gtfobins/env/",
    "https://gtfobins.github.io/gtfobins/expect/",
    "https://gtfobins.github.io/gtfobins/find/",
    "https://gtfobins.github.io/gtfobins/flock/",
    "https://gtfobins.github.io/gtfobins/gcc/",
    "https://gtfobins.github.io/gtfobins/mysql/",
    "https://gtfobins.github.io/gtfobins/nice/",
    "https://gtfobins.github.io/gtfobins/ssh/",
    "https://gtfobins.github.io/gtfobins/vi/",
    "https://gtfobins.github.io/gtfobins/vim/",
    "https://gtfobins.github.io/gtfobins/capsh/",
    "https://gtfobins.github.io/gtfobins/byebug/",
    "https://gtfobins.github.io/gtfobins/git/",
    "https://gtfobins.github.io/gtfobins/ftp/",
]
risk_score = 47
rule_id = "52376a86-ee86-4967-97ae-1a05f55816f0"
severity = "medium"
tags = ["Domain: Endpoint", "OS: Linux", "Use Case: Threat Detection", "Tactic: Execution", "Data Source: Elastic Endgame"]
timestamp_override = "event.ingested"
type = "eql"
query = '''
process where host.os.type == "linux" and event.type == "start" and
(
  /* launching shell from capsh */
  (process.name == "capsh" and process.args == "--") or
  
  /* launching shells from unusual parents or parent+arg combos */
  (process.name : "*sh" and (
    (process.parent.name : ("byebug", "ftp", "strace", "zip", "*awk", "git", "tar") and 
    (
      process.parent.args : "BEGIN {system(*)}" or
      (process.parent.args : ("*PAGER*", "!*sh", "exec *sh") or process.args : ("*PAGER*", "!*sh", "exec *sh")) or
      (
        (process.parent.args : "exec=*sh" or (process.parent.args : "-I" and process.parent.args : "*sh")) or
        (process.args : "exec=*sh" or (process.args : "-I" and process.args : "*sh"))
        )
      )
    ) or
    
    /* shells specified in parent args */
    /* nice rule is broken in 8.2 */
    (process.parent.args : "*sh" and
      (
        (process.parent.name == "nice") or
        (process.parent.name == "cpulimit" and process.parent.args == "-f") or
        (process.parent.name == "find" and process.parent.args == "-exec" and process.parent.args == ";" and process.parent.args == "-p") or
        (process.parent.name == "flock" and process.parent.args == "-u" and process.parent.args == "/")
      )
    )
  )) or

  /* shells specified in args */
  (process.args : "*sh" and (
    (process.parent.name == "crash" and process.parent.args == "-h") or
    (process.name == "sensible-pager" and process.parent.name in ("apt", "apt-get") and process.parent.args == "changelog")
    /* scope to include more sensible-pager invoked shells with different parent process to reduce noise and remove false positives */
    
  )) or
  (process.name == "busybox" and event.action == "exec" and process.args_count == 2 and process.args : "*sh" and not 
   process.executable : "/var/lib/docker/overlay2/*/merged/bin/busybox" and not (process.parent.args == "init" and
   process.parent.args == "runc") and not process.parent.args in ("ls-remote", "push", "fetch")) or
  (process.name == "env" and process.args_count == 2 and process.args : "*sh") or
  (process.parent.name in ("vi", "vim") and process.parent.args == "-c" and process.parent.args : ":!*sh") or
  (process.parent.name in ("c89", "c99", "gcc") and process.parent.args : "*sh,-s" and process.parent.args == "-wrapper") or
  (process.parent.name == "expect" and process.parent.args == "-c" and process.parent.args : "spawn *sh;interact") or
  (process.parent.name == "mysql" and process.parent.args == "-e" and process.parent.args : "\\!*sh") or
  (process.parent.name == "ssh" and process.parent.args == "-o" and process.parent.args : "ProxyCommand=;*sh 0<&2 1>&2")
)
'''

[[rule.threat]]
framework = "MITRE ATT&CK"

[[rule.threat.technique]]
id = "T1059"
name = "Command and Scripting Interpreter"
reference = "https://attack.mitre.org/techniques/T1059/"

[[rule.threat.technique.subtechnique]]
id = "T1059.004"
name = "Unix Shell"
reference = "https://attack.mitre.org/techniques/T1059/004/"

[rule.threat.tactic]
id = "TA0002"
name = "Execution"
reference = "https://attack.mitre.org/tactics/TA0002/"