Shell Invocation Via Ssh - Linux

Status: test
Severity: high
Log source: category process_creation, product linux
Author: Li Ling, Andy Parkidomo, Robert Rakowski, Blake Hartstein (Bloomberg L.P.)
Source: github.com/SigmaHQ/sigma

Detects the use of the "ssh" utility to execute a shell. Such behavior may be associated with privilege escalation, unauthorized command execution, or to break out from restricted environments.

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1059` Command and Scripting Interpreter

Telemetry coverage

Platform	Record / event type
Linux	Event ID 1: Process Create

Rule body

title: Shell Invocation Via Ssh - Linux
id: 8737b7f6-8df3-4bb7-b1da-06019b99b687
status: test
description: |
    Detects the use of the "ssh" utility to execute a shell. Such behavior may be associated with privilege escalation, unauthorized command execution, or to break out from restricted environments.
references:
    - https://gtfobins.github.io/gtfobins/ssh/
    - https://www.elastic.co/guide/en/security/current/linux-restricted-shell-breakout-via-linux-binary-s.html
author: Li Ling, Andy Parkidomo, Robert Rakowski, Blake Hartstein (Bloomberg L.P.)
date: 2024-08-29
tags:
    - attack.execution
    - attack.t1059
logsource:
    category: process_creation
    product: linux
detection:
    selection_img:
        Image|endswith: '/ssh'
        CommandLine|contains:
            - 'ProxyCommand=;'
            - 'permitlocalcommand=yes'
            - 'localhost'
    selection_cli:
        CommandLine|contains:
            - '/bin/bash'
            - '/bin/dash'
            - '/bin/fish'
            - '/bin/sh'
            - '/bin/zsh'
            - 'sh 0<&2 1>&2'
            - 'sh 1>&2 0<&2'
    condition: all of selection_*
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_img`

selection_img:
    Image|endswith: '/ssh'
    CommandLine|contains:
        - 'ProxyCommand=;'
        - 'permitlocalcommand=yes'
        - 'localhost'

Stage 2: `selection_cli`

selection_cli:
    CommandLine|contains:
        - '/bin/bash'
        - '/bin/dash'
        - '/bin/fish'
        - '/bin/sh'
        - '/bin/zsh'
        - 'sh 0<&2 1>&2'
        - 'sh 1>&2 0<&2'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`CommandLine`	match	`/bin/bash` `/bin/dash` `/bin/fish` `/bin/sh` `/bin/zsh` `ProxyCommand=;` `localhost` `permitlocalcommand=yes` `sh 0<&2 1>&2` `sh 1>&2 0<&2`	`field:"CommandLine" kind:match`
`Image`	ends_with	`/ssh`	`field:"Image" kind:ends_with value:"/ssh"`