Linux Restricted Shell Breakout via Linux Binary(s)edit

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.

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:

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

Version: 3

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guideedit

## 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 queryedit

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","strace")) 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(*)}")

Framework: MITRE ATT&CKTM