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Detection rules › Sigma

Suspicious Encoded PowerShell Command Line

Status
test
Severity
high
Log source
product windows, category process_creation
Author
Florian Roth (Nextron Systems), Markus Neis, Jonhnathan Ribeiro, Daniil Yugoslavskiy, Anton Kutepov, oscd.community
Source
github.com/SigmaHQ/sigma

Detects suspicious powershell process starts with base64 encoded commands (e.g. Emotet)

MITRE ATT&CK coverage

TacticTechniques
ExecutionT1059.001 Command and Scripting Interpreter: PowerShell

Event coverage

ProviderEventTitle
SysmonEvent ID 1Process creation

Rule body yaml

title: Suspicious Encoded PowerShell Command Line
id: ca2092a1-c273-4878-9b4b-0d60115bf5ea
status: test
description: Detects suspicious powershell process starts with base64 encoded commands (e.g. Emotet)
references:
    - https://app.any.run/tasks/6217d77d-3189-4db2-a957-8ab239f3e01e
author: Florian Roth (Nextron Systems), Markus Neis, Jonhnathan Ribeiro, Daniil Yugoslavskiy, Anton Kutepov, oscd.community
date: 2018-09-03
modified: 2023-04-06
tags:
    - attack.execution
    - attack.t1059.001
logsource:
    category: process_creation
    product: windows
detection:
    selection_img:
        - Image|endswith:
              - '\powershell.exe'
              - '\pwsh.exe'
        - OriginalFileName:
              - 'PowerShell.EXE'
              - 'pwsh.dll'
    selection_cli_enc:
        CommandLine|contains: ' -e' # covers -en and -enc
    selection_cli_content:
        CommandLine|contains:
            - ' JAB'
            - ' SUVYI'
            - ' SQBFAFgA'
            - ' aQBlAHgA'
            - ' aWV4I'
            - ' IAA'
            - ' IAB'
            - ' UwB'
            - ' cwB'
    selection_standalone:
        CommandLine|contains:
            - '.exe -ENCOD '
            - ' BA^J e-' # Reversed
    filter_optional_remote_signed:
        CommandLine|contains: ' -ExecutionPolicy remotesigned '
    condition: selection_img and (all of selection_cli_* or selection_standalone) and not 1 of filter_optional_*
level: high

Stages and Predicates

Stage 0: condition

selection_img and (all of selection_cli_* or selection_standalone) and not 1 of filter_optional_*

Stage 1: selection_img

selection_img:
    - Image|endswith:
          - '\powershell.exe'
          - '\pwsh.exe'
    - OriginalFileName:
          - 'PowerShell.EXE'
          - 'pwsh.dll'

Stage 2: selection_cli_enc

selection_cli_enc:
    CommandLine|contains: ' -e'

Stage 3: selection_cli_content

selection_cli_content:
    CommandLine|contains:
        - ' JAB'
        - ' SUVYI'
        - ' SQBFAFgA'
        - ' aQBlAHgA'
        - ' aWV4I'
        - ' IAA'
        - ' IAB'
        - ' UwB'
        - ' cwB'

Stage 4: selection_standalone

selection_standalone:
    CommandLine|contains:
        - '.exe -ENCOD '
        - ' BA^J e-'

Stage 5: not filter_optional_remote_signed

filter_optional_remote_signed:
    CommandLine|contains: ' -ExecutionPolicy remotesigned '

Exclusions

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

FieldKindExcluded values
CommandLinematch -ExecutionPolicy remotesigned

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
CommandLinematch
  • -e corpus 6 (sigma 5, splunk 1)
  • BA^J e-
  • IAA
  • IAB corpus 2 (sigma 2)
  • JAB corpus 2 (sigma 2)
  • SQBFAFgA corpus 2 (sigma 2)
  • SUVYI corpus 2 (sigma 2)
  • UwB
  • aQBlAHgA corpus 2 (sigma 2)
  • aWV4I corpus 2 (sigma 2)
  • cwB
  • .exe -ENCOD
Imageends_with
  • \powershell.exe corpus 182 (sigma 182)
  • \pwsh.exe corpus 168 (sigma 168)
OriginalFileNameeq
  • PowerShell.EXE corpus 120 (sigma 84, splunk 30, elastic 6)
  • pwsh.dll corpus 112 (sigma 79, splunk 30, elastic 3)