PowerShell CommandLine Uses FromBase64String to Decode Base64 Content (Windows)

Detects PowerShell process creation where the command line includes ::FromBase64String(, indicating Base64 decoding.

FreeReviewedSigma · High · v1

Product: windows
Category: process_creation
Author: Florian Roth (Nextron Systems) (SigmaHQ), DRL 1.1
Published: 2020-01-29
Updated: 2026-07-30

ATT&CK techniques

Execution → Defense Evasion

Recon
Resource Dev
Initial Access
Execution
- T1059.001PowerShell
Persistence
Priv Esc
Defense Evasion
- T1027Obfuscated Files or Information
- T1140Deobfuscate/Decode Files or Information
Cred Access
Discovery
Lateral Movement
Collection
C2
Exfiltration
Impact

What it detects

This rule flags Windows process creation events where the PowerShell command line contains the PowerShell method call "::FromBase64String(". Attackers often use Base64 encoding to hide or obfuscate script content passed to PowerShell, which can help evade casual inspection and some defenses. The detection relies on command-line text captured in process creation telemetry.

Reporting behind it

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Hidden PowerShell Archive Extraction via ExtractToDirectory

CastleLoader ClickFix PowerShell Hex Decode and Re-Execution

Malicious Non-Interactive Encoded PowerShell Stager (via process_creation)

PowerShell CommandLine Uses FromBase64String to Decode Base64 Content (Windows)

Pivot detection · T1059.001 · 9 related

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.

powershell-commandline-uses-frombase64string-to-decode-base64-content-windows-e32d4572

title: PowerShell CommandLine Uses FromBase64String to Decode Base64 Content (Windows)
id: ac7e8146-9e1a-4fef-8e2b-a726b1b5cef7
status: test
description: This rule flags Windows process creation events where the PowerShell command line contains the PowerShell method call "::FromBase64String(". Attackers often use Base64 encoding to hide or obfuscate script content passed to PowerShell, which can help evade casual inspection and some defenses. The detection relies on command-line text captured in process creation telemetry.
references:
  - https://gist.github.com/Neo23x0/6af876ee72b51676c82a2db8d2cd3639
  - https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_powershell_frombase64string.yml
author: Florian Roth (Nextron Systems), Huntrule Team
date: 2020-01-29
modified: 2023-01-26
tags:
  - attack.stealth
  - attack.t1027
  - attack.execution
  - attack.t1140
  - attack.t1059.001
logsource:
  category: process_creation
  product: windows
detection:
  selection:
    CommandLine|contains: ::FromBase64String(
  condition: selection
falsepositives:
  - Administrative script libraries
level: high
license: DRL-1.1
related:
  - id: e32d4572-9826-4738-b651-95fa63747e8a
    type: derived

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