Detection: Windows PowerShell Script TabExpansion Direct Call

Description

Detects specific indicators associated with the execution of the TabExpansion internal function in PowerShell. Calling this function directly is not normal and can be indicative of malicious activity such as TabShell. Due to the way PowerShell internals work it can be used in conjunction with directory transversal to load any PowerShell functions even in a sandboxed session. False positives could include legitimate usage of the TabExpansion function but calling it directly is very rare.

Search

 1`powershell`
 2EventID="4104"
 3ScriptBlockText="*$lastWord*"
 4ScriptBlockText="*$_val=' + $_expression*"
 5ScriptBlockText="*function Write-Members*"
 6
 7| fillnull
 8
 9| stats count min(_time) as firstTime
10              max(_time) as lastTime
11  by Computer EventID ScriptBlockText signature signature_id user_id vendor_product Guid
12     Opcode Name Path ProcessID ScriptBlockId
13
14
15| rename Computer as dest
16
17| `security_content_ctime(firstTime)`
18
19| `security_content_ctime(lastTime)`
20
21| `windows_powershell_script_tabexpansion_direct_call_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

Legitimate use of the TabExpansion function is rare but possible. Filter alerts if direct calls are from trusted administrative or development activities.

Associated Analytic Story

• Malicious PowerShell

Risk Based Analytics (RBA)

Risk Message:

Potential PowerShell TabExpansion activity observed on $dest$ via script block $ScriptBlockId$.

References

• https://blog.viettelcybersecurity.com/tabshell-owassrf/

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