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Detection rules › Sigma

Indirect Command Execution From Script File Via Bash.EXE

Status
test
Severity
medium
Log source
product windows, category process_creation
Author
Nasreddine Bencherchali (Nextron Systems)
Source
github.com/SigmaHQ/sigma

Detects execution of Microsoft bash launcher without any flags to execute the content of a bash script directly. This can be used to potentially bypass defenses and execute Linux or Windows-based binaries directly via bash.

MITRE ATT&CK coverage

TacticTechniques
StealthT1202 Indirect Command Execution

Event coverage

ProviderEventTitle
SysmonEvent ID 1Process creation

Rule body yaml

title: Indirect Command Execution From Script File Via Bash.EXE
id: 2d22a514-e024-4428-9dba-41505bd63a5b
related:
    - id: 5edc2273-c26f-406c-83f3-f4d948e740dd
      type: similar
status: test
description: |
    Detects execution of Microsoft bash launcher without any flags to execute the content of a bash script directly.
    This can be used to potentially bypass defenses and execute Linux or Windows-based binaries directly via bash.
references:
    - https://lolbas-project.github.io/lolbas/Binaries/Bash/
    - https://linux.die.net/man/1/bash
    - Internal Research
author: Nasreddine Bencherchali (Nextron Systems)
date: 2023-08-15
tags:
    - attack.stealth
    - attack.t1202
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        - Image|endswith:
              - ':\Windows\System32\bash.exe'
              - ':\Windows\SysWOW64\bash.exe'
        - OriginalFileName: 'Bash.exe'
    filter_main_cli_flag:
        CommandLine|contains:
            # Note: we're not interested in flags being passed first
            - 'bash.exe -'
            - 'bash -'
    filter_main_no_cli:
        CommandLine: null
    filter_main_empty:
        CommandLine: ''
    filter_main_no_flag:
        CommandLine:
            - 'bash.exe'
            - 'bash'
    condition: selection and not 1 of filter_main_*
falsepositives:
    - Unknown
level: medium

Stages and Predicates

Stage 0: condition

selection and not 1 of filter_main_*

Stage 1: selection

selection:
    - Image|endswith:
          - ':\Windows\System32\bash.exe'
          - ':\Windows\SysWOW64\bash.exe'
    - OriginalFileName: 'Bash.exe'

Stage 2: not filter_main_*

filter_main_cli_flag:
    CommandLine|contains:
        - 'bash.exe -'
        - 'bash -'
filter_main_no_cli:
    CommandLine: null
filter_main_empty:
    CommandLine: ''
filter_main_no_flag:
    CommandLine:
        - 'bash.exe'
        - 'bash'

Exclusions

Top-level NOT(...) conjuncts: predicates this rule actively suppresses.

FieldKindExcluded values
CommandLineeqbash
CommandLineeqbash.exe
CommandLineis_null(no value, null check)
CommandLinematchbash -
CommandLinematchbash.exe -

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

FieldKindValues
Imageends_with
  • :\Windows\SysWOW64\bash.exe corpus 2 (sigma 2)
  • :\Windows\System32\bash.exe corpus 2 (sigma 2)
OriginalFileNameeq
  • Bash.exe corpus 4 (sigma 3, elastic 1)