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 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: 5 minutes

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: 1

Added (Elastic Stack release): 8.3.0

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).

## Config

The session view analysis for the command alerted is avalible in versions 8.2 and above.

Rule query

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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(*)}")

Threat mapping

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Framework: MITRE ATT&CKTM