Detection: Windows Cobalt Strike PowerShell Loader

Description

Detects the PowerShell pattern used at the end of a Cobalt Strike PowerShell loader to perform the decompression of the executable. This loader is used in attacks such as scripted web delivery. Cobalt Strike is a legitimate, commercial penetration testing tool that has been largely co-opted by ransomware gangs to launch attacks. Cobalt Strike's popularity is mainly due to its beacons or payload being stealthy, and easily customizable. Cobalt Strike Beacon provides encrypted communication with the C&C server to send information and receive commands.

Search

 1`powershell`
 2EventID="4104"
 3ScriptBlockText="*));IEX (New-Object IO.StreamReader(New-Object IO.Compression.GzipStream($s,[IO.Compression.CompressionMode]::Decompress))).ReadToEnd();"
 4
 5| fillnull
 6
 7| stats count min(_time) as firstTime
 8              max(_time) as lastTime
 9  by Computer EventID ScriptBlockText dest signature signature_id user_id vendor_product
10     Guid Opcode Name Path ProcessID ScriptBlockId
11
12
13| `security_content_ctime(firstTime)`
14
15| `security_content_ctime(lastTime)`
16
17| `windows_cobalt_strike_powershell_loader_filter`

Data Source

Name	Platform	Sourcetype	Source
Powershell Script Block Logging 4104	Windows	'XmlWinEventLog'	'XmlWinEventLog:Microsoft-Windows-PowerShell/Operational'

Macros Used

Annotations

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Implementation

The detection is based on data that originates from Endpoint Detection and Response (EDR) agents. These agents are designed to provide security-related telemetry from the endpoints where the agent is installed. To implement this search, you must ingest logs that contain the process GUID, process name, and parent process. Additionally, you must ingest complete command-line executions. These logs must be processed using the appropriate Splunk Technology Add-ons that are specific to the EDR product. The logs must also be mapped to the Processes node of the Endpoint data model. Use the Splunk Common Information Model (CIM) to normalize the field names and speed up the data modeling process.

Known False Positives

Some legitimate penetration testing activities and authorized red team exercises may use this PowerShell loader pattern. Verify and whitelist approved security testing tools to reduce false alerts.

Associated Analytic Story

• Cobalt Strike

Risk Based Analytics (RBA)

Risk Message:

Cobalt Strike PowerShell loader pattern observed on $dest$ via script block $ScriptBlockId$.

Detection Testing

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 1