Indirect Command Execution From Script File Via Bash.EXE

Status: test
Severity: medium
Log source: category process_creation, product windows
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

Tactic	Techniques
Stealth	`T1202` Indirect Command Execution

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 1: Process creation

Rule body

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

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`CommandLine`	eq	`bash`	`excludes:CommandLine` `field:"CommandLine" value:"bash"`
`CommandLine`	eq	`bash.exe`	`excludes:CommandLine` `field:"CommandLine" value:"bash.exe"`
`CommandLine`	is_null	(no value, null check)	`excludes:CommandLine`
`CommandLine`	match	`bash -`	`excludes:CommandLine` `field:"CommandLine" value:"bash -"`
`CommandLine`	match	`bash.exe -`	`excludes:CommandLine` `field:"CommandLine" value:"bash.exe -"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`Image`	ends_with	`:\Windows\SysWOW64\bash.exe` corpus 2 (sigma 2) `:\Windows\System32\bash.exe` corpus 2 (sigma 2)	`field:"Image" kind:ends_with`
`OriginalFileName`	eq	`Bash.exe` corpus 4 (sigma 3, elastic 1)	`field:"OriginalFileName" kind:eq value:"Bash.exe"`