sigma-rules/rules/linux/execution_shell_evasion_linux_binary.toml

[metadata]
creation_date = "2022/05/06"
maturity = "production"
updated_date = "2022/05/25"

[rule]
author = ["Elastic"]
description = """
Identifies Linux binary(s) abuse to breakout of restricted shells or environments by spawning an interactive system
shell. The linux utility(s) activity of spawning shell is not a standard use of the binary for a user or system
administrator. It may indicates an attempt to improve the capabilities or stability of an adversary access.
"""
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 = ["Elastic", "Host", "Linux", "Threat Detection", "Execution", "GTFOBins"]
timestamp_override = "event.ingested"
type = "eql"

query = '''
process where event.type == "start" and

  /* launch shells from unusual process */
  (process.name == "capsh" and process.args == "--") or

  /* launching shells from unusual parents or parent+arg combos */
  (process.name in ("bash", "sh", "dash","ash") and
    (process.parent.name in ("byebug","git","ftp")) or

    /* shells specified in parent args */
    /* nice rule is broken in 8.2 */
    (process.parent.args in ("/bin/sh", "/bin/bash", "/bin/dash", "/bin/ash", "sh", "bash", "dash", "ash") 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 == ";") or
        (process.parent.name == "flock" and process.parent.args == "-u" and process.parent.args == "/")
    ) or

    /* shells specified in args */
    (process.args in ("/bin/sh", "/bin/bash", "/bin/dash", "/bin/ash", "sh", "bash", "dash", "ash") 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 process.args_count == 2 and process.args in ("/bin/sh", "/bin/bash", "/bin/dash", "/bin/ash", "sh", "bash", "dash", "ash") )or
  (process.name == "env" and process.args_count == 2 and process.args in ("/bin/sh", "/bin/bash", "/bin/dash", "/bin/ash", "sh", "bash", "dash", "ash")) or
  (process.parent.name in ("vi", "vim") and process.parent.args == "-c" and process.parent.args in (":!/bin/bash", ":!/bin/sh", ":!bash", ":!sh")) or
  (process.parent.name in ("c89","c99", "gcc") and process.parent.args in ("sh,-s", "bash,-s", "dash,-s", "ash,-s", "/bin/sh,-s", "/bin/bash,-s", "/bin/dash,-s", "/bin/ash,-s") and process.parent.args == "-wrapper") or
  (process.parent.name == "expect" and process.parent.args == "-c" and process.parent.args in ("spawn /bin/sh;interact", "spawn /bin/bash;interact", "spawn /bin/dash;interact", "spawn sh;interact", "spawn bash;interact", "spawn dash;interact")) or
  (process.parent.name == "mysql" and process.parent.args == "-e" and process.parent.args in ("\\!*sh", "\\!*bash", "\\!*dash", "\\!*/bin/sh", "\\!*/bin/bash", "\\!*/bin/dash")) or
  (process.parent.name == "ssh" and process.parent.args == "-o" and process.parent.args in ("ProxyCommand=;sh 0<&2 1>&2", "ProxyCommand=;bash 0<&2 1>&2", "ProxyCommand=;dash 0<&2 1>&2", "ProxyCommand=;/bin/sh 0<&2 1>&2", "ProxyCommand=;/bin/bash 0<&2 1>&2", "ProxyCommand=;/bin/dash 0<&2 1>&2")) or
  (process.parent.name in ("nawk", "mawk", "awk", "gawk") and process.parent.args : "BEGIN {system(*)}")
'''


[[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/"