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Detection rules › Sigma

Powershell Token Obfuscation - Powershell

Status
test
Severity
medium
Log source
product windows, category ps_script
Author
frack113
Source
github.com/SigmaHQ/sigma

Detects TOKEN OBFUSCATION technique from Invoke-Obfuscation in Powershell scripts. Use this rule as a threat-hunting baseline to find obfuscated scripts in your environment. Once tested and tuned, consider deploying a production detection rule based on this hunting rule.

MITRE ATT&CK coverage

TacticTechniques
StealthT1027.009 Obfuscated Files or Information: Embedded Payloads

Event coverage

ProviderEventTitle
PowerShellEvent ID 4104Creating Scriptblock text (MessageNumber of MessageTotal).

Rule body yaml

title: Powershell Token Obfuscation - Powershell
id: f3a98ce4-6164-4dd4-867c-4d83de7eca51
related:
    - id: deb9b646-a508-44ee-b7c9-d8965921c6b6
      type: similar
status: test
description: |
    Detects TOKEN OBFUSCATION technique from Invoke-Obfuscation in Powershell scripts.
    Use this rule as a threat-hunting baseline to find obfuscated scripts in your environment.
    Once tested and tuned, consider deploying a production detection rule based on this hunting rule.
references:
    - https://github.com/danielbohannon/Invoke-Obfuscation
author: frack113
date: 2022-12-27
modified: 2025-10-19
tags:
    - attack.stealth
    - attack.t1027.009
    - detection.threat-hunting
logsource:
    product: windows
    category: ps_script
    definition: 'Requirements: Script Block Logging must be enabled'
detection:
    selection:
        # Examples:
        #   IN`V`o`Ke-eXp`ResSIOn (Ne`W-ob`ject Net.WebClient).DownloadString
        #   &('In'+'voke-Expressi'+'o'+'n') (.('New-Ob'+'jec'+'t') Net.WebClient).DownloadString
        #   &("{2}{3}{0}{4}{1}"-f 'e','Expression','I','nvok','-') (&("{0}{1}{2}"-f'N','ew-O','bject') Net.WebClient).DownloadString
        - ScriptBlockText|re: '\w+`(\w+|-|.)`[\w+|\s]'
        # - ScriptBlockText|re: '\((\'(\w|-|\.)+\'\+)+\'(\w|-|\.)+\'\)' TODO: fixme
        - ScriptBlockText|re: '"(\{\d\}){2,}"\s*-f'  # trigger on at least two placeholders. One might be used for legitimate string formatting
        #   ${e`Nv:pATh}
        - ScriptBlockText|re: '(?i)\$\{`?e`?n`?v`?:`?p`?a`?t`?h`?\}'
    filter_envpath:
        ScriptBlockText|contains: '${env:path}' # TODO: Fix this. See https://github.com/SigmaHQ/sigma/pull/4964
    filter_chocolatey:
        ScriptBlockText|contains:
            - 'it will return true or false instead'  # Chocolatey install script https://github.com/chocolatey/chocolatey
            - 'The function also prevents `Get-ItemProperty` from failing' # https://docs.chocolatey.org/en-us/create/functions/get-uninstallregistrykey
    filter_exchange:
        Path|startswith: 'C:\Program Files\Microsoft\Exchange Server\'
        Path|endswith: '\bin\servicecontrol.ps1'
        ScriptBlockText|contains: '`r`n'
    condition: selection and not 1 of filter_*
falsepositives:
    - Edge case might be possible with heavy use of string formatting or obfuscation in legitimate scripts.
level: medium

Stages and Predicates

Stage 0: condition

selection and not 1 of filter_*

Stage 1: selection

selection:
    - ScriptBlockText|re: '\w+`(\w+|-|.)`[\w+|\s]'
    - ScriptBlockText|re: '"(\{\d\}){2,}"\s*-f'
    - ScriptBlockText|re: '(?i)\$\{`?e`?n`?v`?:`?p`?a`?t`?h`?\}'

Stage 2: not filter_*

filter_envpath:
    ScriptBlockText|contains: '${env:path}'
filter_chocolatey:
    ScriptBlockText|contains:
        - 'it will return true or false instead'
        - 'The function also prevents `Get-ItemProperty` from failing'
filter_exchange:
    Path|startswith: 'C:\Program Files\Microsoft\Exchange Server\'
    Path|endswith: '\bin\servicecontrol.ps1'
    ScriptBlockText|contains: '`r`n'

Exclusions

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

FieldKindExcluded values
Pathends_with\bin\servicecontrol.ps1
Pathstarts_withC:\Program Files\Microsoft\Exchange Server\
ScriptBlockTextmatch`r`n
ScriptBlockTextmatch${env:path}
ScriptBlockTextmatchThe function also prevents `Get-ItemProperty` from failing
ScriptBlockTextmatchit will return true or false instead

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
ScriptBlockTextregex_match
  • "({\d}){2,}"\s*-f
  • (?i)${`?e`?n`?v`?:`?p`?a`?t`?h`?}
  • \w+`(\w+|-|.)`[\w+|\s]