Detection: Windows Command Obfuscation with Environment Variable Substrings

Description

Detects command obfuscation by using a technique to build a target command using character indexes from environment variables. This hides the true intent of the command by building it on the fly. In Windows command prompt, you can use the :~ format to extract substrings from environment variable values. This behavior has been observed in various malware families, including Cobalt Strike and Meterpreter.

Search

 1
 2| tstats `security_content_summariesonly`
 3  count min(_time) as firstTime
 4        max(_time) as lastTime
 5
 6from datamodel=Endpoint.Processes where
 7
 8Processes.process="*%%*:~*,*"
 9
10by Processes.process Processes.vendor_product Processes.user_id Processes.process_hash
11   Processes.parent_process_name Processes.parent_process_exec Processes.action
12   Processes.dest Processes.process_current_directory Processes.process_path
13   Processes.process_integrity_level Processes.original_file_name
14   Processes.parent_process Processes.parent_process_path
15   Processes.parent_process_guid Processes.parent_process_id
16   Processes.process_guid Processes.process_id
17   Processes.user Processes.process_name
18
19
20| `drop_dm_object_name(Processes)`
21
22| `security_content_ctime(firstTime)`
23
24| `security_content_ctime(lastTime)`
25
26| `windows_command_obfuscation_with_environment_variable_substrings_filter`

Data Source

Name	Platform	Sourcetype	Source
CrowdStrike ProcessRollup2	Other	'crowdstrike:events:sensor'	'crowdstrike'
Sysmon EventID 1	Windows	'XmlWinEventLog'	'XmlWinEventLog:Microsoft-Windows-Sysmon/Operational'
Windows Event Log Security 4688	Windows	'XmlWinEventLog'	'XmlWinEventLog:Security'

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 scripts and system maintenance tools may use substring extraction from environment variables for dynamic command construction. Review and whitelist authorized scripts to reduce false positives.

Associated Analytic Story

• Malicious PowerShell

Risk Based Analytics (RBA)

Risk Message:

Potential Command Obfuscation with Environment Variable Substrings activity from $process$ observed on $dest$.

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