PowerShell Script Dropped Via PowerShell.EXE

Severity: low
Author: frack113
Source: upstream

Detects PowerShell creating a PowerShell file (.ps1). While often times this behavior is benign, sometimes it can be a sign of a dropper script trying to achieve persistence.

Event coverage

Provider	Event ID	Title
Sysmon	11	FileCreate

Stages and Predicates

Stage 1: `selection`

or:
Image|endswith: '\powershell.exe'
Image|endswith: '\pwsh.exe'
TargetFilename|endswith: .ps1

Stage 2: `not 1 of filter_main_*`

or:
TargetFilename|contains: '\AppData\Local\Temp\'
TargetFilename|startswith: 'C:\Users\'
TargetFilename|contains: __PSScriptPolicyTest_
TargetFilename|startswith: 'C:\Windows\Temp\'

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
`Image`	ends_with	`\powershell.exe` corpus 143 (sigma 143) `\pwsh.exe` corpus 140 (sigma 140)
`TargetFilename`	ends_with	`.ps1` corpus 15 (sigma 15)
`TargetFilename`	match	`\AppData\Local\Temp\` corpus 11 (sigma 11) `__PSScriptPolicyTest_` corpus 2 (sigma 2)
`TargetFilename`	starts_with	`C:\Users\` corpus 12 (sigma 12) `C:\Windows\Temp\` corpus 4 (sigma 4)