Invoke-Obfuscation Obfuscated IEX Invocation - System

Status: test
Severity: high
Log source: product windows, service system
Author: Daniel Bohannon (@Mandiant/@FireEye), oscd.community
Source: github.com/SigmaHQ/sigma

Detects all variations of obfuscated powershell IEX invocation code generated by Invoke-Obfuscation framework from the code block linked in the references

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1027` Obfuscated Files or Information

Telemetry coverage

Provider	Record / event type
Service-Control-Manager	Event ID 7045: A service was installed in the system.

Rule body

title: Invoke-Obfuscation Obfuscated IEX Invocation - System
id: 51aa9387-1c53-4153-91cc-d73c59ae1ca9
status: test
description: Detects all variations of obfuscated powershell IEX invocation code generated by Invoke-Obfuscation framework from the code block linked in the references
references:
    - https://github.com/danielbohannon/Invoke-Obfuscation/blob/f20e7f843edd0a3a7716736e9eddfa423395dd26/Out-ObfuscatedStringCommand.ps1#L873-L888
author: Daniel Bohannon (@Mandiant/@FireEye), oscd.community
date: 2019-11-08
modified: 2022-11-27
tags:
    - attack.stealth
    - attack.t1027
logsource:
    product: windows
    service: system
detection:
    selection_eid:
        EventID: 7045
    selection_imagepath:
        - ImagePath|re: '\$PSHome\[\s*\d{1,3}\s*\]\s*\+\s*\$PSHome\['
        - ImagePath|re: '\$ShellId\[\s*\d{1,3}\s*\]\s*\+\s*\$ShellId\['
        - ImagePath|re: '\$env:Public\[\s*\d{1,3}\s*\]\s*\+\s*\$env:Public\['
        - ImagePath|re: '\$env:ComSpec\[(\s*\d{1,3}\s*,){2}'
        - ImagePath|re: '\\*mdr\*\W\s*\)\.Name'
        - ImagePath|re: '\$VerbosePreference\.ToString\('
        - ImagePath|re: '\String\]\s*\$VerbosePreference'
    condition: all of selection_*
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_eid`

selection_eid:
    EventID: 7045

Stage 2: `selection_imagepath`

selection_imagepath:
    - ImagePath|re: '\$PSHome\[\s*\d{1,3}\s*\]\s*\+\s*\$PSHome\['
    - ImagePath|re: '\$ShellId\[\s*\d{1,3}\s*\]\s*\+\s*\$ShellId\['
    - ImagePath|re: '\$env:Public\[\s*\d{1,3}\s*\]\s*\+\s*\$env:Public\['
    - ImagePath|re: '\$env:ComSpec\[(\s*\d{1,3}\s*,){2}'
    - ImagePath|re: '\\*mdr\*\W\s*\)\.Name'
    - ImagePath|re: '\$VerbosePreference\.ToString\('
    - ImagePath|re: '\String\]\s*\$VerbosePreference'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`ImagePath`	regex_match	`$PSHome[\s\d{1,3}\s]\s+\s$PSHome[` `$ShellId[\s\d{1,3}\s]\s+\s$ShellId[` `$VerbosePreference.ToString(` `$env:ComSpec[(\s\d{1,3}\s,){2}` `$env:Public[\s\d{1,3}\s]\s+\s$env:Public[` `\String]\s$VerbosePreference` `\mdr\W\s).Name`	`field:"ImagePath" kind:regex_match`