Linux Restricted Shell Breakout via Linux Binary(s)

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Linux Restricted Shell Breakout via Linux Binary(s)

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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.

Rule type: eql

Rule indices:

  • logs-endpoint.events.*

Severity: medium

Risk score: 47

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

  • Domain: Endpoint
  • OS: Linux
  • Use Case: Threat Detection
  • Tactic: Execution
  • Data Source: Elastic Endgame
  • Data Source: Elastic Defend

Version: 111

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guide

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## 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).

Rule query

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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 in ("bash", "dash", "ash", "sh", "tcsh", "csh", "zsh", "ksh", "fish") and (
    (process.parent.name : "*awk" and process.parent.args : "BEGIN {system(*)}") or
    (process.parent.name == "git" and process.parent.args : ("*PAGER*", "!*sh", "exec *sh") or
     process.args : ("*PAGER*", "!*sh", "exec *sh") and not process.name == "ssh" ) or
    (process.parent.name : ("byebug", "ftp", "strace", "zip", "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 == "." and process.parent.args == "-exec" and
         process.parent.args == ";" and process.parent.args : "/bin/*sh") 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") and not process.parent.name == "mkinitramfs") 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")
)

Framework: MITRE ATT&CKTM