Indirect Command Execution From Script File Via Bash.EXE

Severity: medium
Author: Nasreddine Bencherchali (Nextron Systems)
Source: upstream

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
Defense Evasion	`T1202` Indirect Command Execution

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `selection`

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

Stage 2: `not 1 of filter_main_*`

or:
CommandLine: ''
CommandLine: bash
CommandLine: bash.exe
CommandLine: null
CommandLine|contains: 'bash -'
CommandLine|contains: 'bash.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.

Field	Kind	Values
`CommandLine`	eq	`bash` `bash.exe`
`CommandLine`	match	`bash -` `bash.exe -`
`Image`	ends_with	`:\Windows\SysWOW64\bash.exe` corpus 2 (sigma 2) `:\Windows\System32\bash.exe` corpus 2 (sigma 2)
`OriginalFileName`	eq	`Bash.exe` corpus 3 (sigma 3)