Suspicious FromBase64String Usage On Gzip Archive - Ps Script

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

Detects attempts of decoding a base64 Gzip archive in a PowerShell script. This technique is often used as a method to load malicious content into memory afterward.

Known false positives

Legitimate administrative script

MITRE ATT&CK coverage

Tactic	Techniques
Command & Control	`T1132.001` Data Encoding: Standard Encoding

Telemetry coverage

Provider	Record / event type
PowerShell	Event ID 4104: Creating Scriptblock text (MessageNumber of MessageTotal).

Rule body

title: Suspicious FromBase64String Usage On Gzip Archive - Ps Script
id: df69cb1d-b891-4cd9-90c7-d617d90100ce
related:
    - id: d75d6b6b-adb9-48f7-824b-ac2e786efe1f
      type: similar
status: test
description: Detects attempts of decoding a base64 Gzip archive in a PowerShell script. This technique is often used as a method to load malicious content into memory afterward.
references:
    - https://speakerdeck.com/heirhabarov/hunting-for-powershell-abuse?slide=43
author: frack113
date: 2022-12-23
tags:
    - attack.command-and-control
    - attack.t1132.001
logsource:
    product: windows
    category: ps_script
    definition: 'Requirements: Script Block Logging must be enabled'
detection:
    selection:
        ScriptBlockText|contains|all:
            - 'FromBase64String'
            - 'MemoryStream'
            - 'H4sI'
    condition: selection
falsepositives:
    - Legitimate administrative script
level: medium

Stages and Predicates

Stage 0: `condition`

selection

Stage 1: `selection`

selection:
    ScriptBlockText|contains|all:
        - 'FromBase64String'
        - 'MemoryStream'
        - 'H4sI'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`ScriptBlockText`	match	`FromBase64String` corpus 4 (sigma 2, splunk 2) `H4sI` `MemoryStream`	`field:"ScriptBlockText" kind:match`