Shell Execution via Flock - 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 "flock" command 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
Discovery	`T1083` File and Directory Discovery

Telemetry coverage

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

Rule body

title: Shell Execution via Flock - Linux
id: 4b09c71e-4269-4111-9cdd-107d8867f0cc
status: test
description: |
    Detects the use of the "flock" command 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/flock/#shell
    - 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: 2024-09-02
tags:
    - attack.discovery
    - attack.t1083
logsource:
    category: process_creation
    product: linux
detection:
    selection_img:
        Image|endswith: '/flock'
        CommandLine|contains: ' -u '
    selection_cli:
        CommandLine|contains:
            - '/bin/bash'
            - '/bin/dash'
            - '/bin/fish'
            - '/bin/sh'
            - '/bin/zsh'
    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: '/flock'
    CommandLine|contains: ' -u '

Stage 2: `selection_cli`

selection_cli:
    CommandLine|contains:
        - '/bin/bash'
        - '/bin/dash'
        - '/bin/fish'
        - '/bin/sh'
        - '/bin/zsh'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`CommandLine`	match	`-u` `/bin/bash` `/bin/dash` `/bin/fish` `/bin/sh` `/bin/zsh`	`field:"CommandLine" kind:match`
`Image`	ends_with	`/flock`	`field:"Image" kind:ends_with value:"/flock"`