Detect PowerShell COMPRESS OBFUSCATION using ASCII text encoding and stream/compression APIs

ATT&CK techniques

Execution → Defense Evasion

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

What it detects

This rule identifies PowerShell script block content consistent with obfuscation routines that use ASCII text encoding and .NET stream and compression objects. Attackers may use these patterns to hide payload logic and make scripts harder to inspect. The detection relies on ScriptBlockText inspection for specific .NET API usage and distinctive string fragments within the script block.

Reporting behind it

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Detect PowerShell COMPRESS OBFUSCATION using ASCII text encoding and stream/compression APIs

Pivot detection · T1027 · 9 related

Changelog

v2

v2
Candidate ingested via manual entry.2026-07-31
v1
No changelog recorded for this version.2026-07-30

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

powershell-obfuscation-using-compress-compression-streams-20e5497e

title: Detect PowerShell COMPRESS OBFUSCATION using ASCII text encoding and stream/compression APIs
id: bf220408-131c-487a-9d58-1b607223f845
status: test
description: This rule identifies PowerShell script block content consistent with obfuscation routines that use ASCII text encoding and .NET stream and compression objects. Attackers may use these patterns to hide payload logic and make scripts harder to inspect. The detection relies on ScriptBlockText inspection for specific .NET API usage and distinctive string fragments within the script block.
references:
  - https://github.com/SigmaHQ/sigma/issues/1009
  - https://github.com/SigmaHQ/sigma/blob/master/rules/windows/powershell/powershell_script/posh_ps_invoke_obfuscation_via_compress.yml
author: Timur Zinniatullin, oscd.community, Huntrule Team
date: 2020-10-18
modified: 2022-11-29
tags:
  - attack.stealth
  - attack.t1027
  - attack.execution
  - attack.t1059.001
logsource:
  product: windows
  category: ps_script
  definition: "Requirements: Script Block Logging must be enabled"
detection:
  selection_4104:
    ScriptBlockText|contains|all:
      - new-object
      - text.encoding]::ascii
    ScriptBlockText|contains:
      - system.io.compression.deflatestream
      - system.io.streamreader
    ScriptBlockText|endswith: readtoend
  condition: selection_4104
falsepositives:
  - Unknown
level: medium
license: DRL-1.1
related:
  - id: 20e5497e-331c-4cd5-8d36-935f6e2a9a07
    type: derived

Convert

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