Detection: Windows PowerShell Process Implementing Manual Base64 Decoder

Description

The following analytic identifies Windows PowerShell processes that implement a manual Base64 decoder. Threat actors often use Base64 encoding to obfuscate malicious payloads or commands within PowerShell scripts. By manually decoding Base64 strings, attackers can evade detection mechanisms that look for standard decoding functions like using the "-enc" flag or the "FromBase64String" function. This detection focuses on PowerShell processes that exhibit characteristics of manual Base64 decoding, such as the presence of specific string manipulation methods and bitwise operations. Security teams should investigate any instances of such activity, especially if found in conjunction with other suspicious behaviors or on systems that should not be using PowerShell for such tasks.

Search

 1
 2| tstats `security_content_summariesonly`
 3  count min(_time) as firstTime
 4        max(_time) as lastTime
 5
 6  from datamodel=Endpoint.Processes where
 7
 8  `process_powershell`
 9  Processes.process = "*ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/*"
10  Processes.process = "*.Substring(*"
11  Processes.process = "*.GetString(*"
12  Processes.process = "*.IndexOf(*"
13  Processes.process IN ("*-shl*", *-shr*, "*-bxor*", "*-bor*", "*-band*")
14
15by Processes.action Processes.dest Processes.original_file_name Processes.parent_process Processes.parent_process_exec
16   Processes.parent_process_guid Processes.parent_process_id Processes.parent_process_name Processes.parent_process_path
17   Processes.process Processes.process_exec Processes.process_guid Processes.process_hash Processes.process_id
18   Processes.process_integrity_level Processes.process_name Processes.process_path Processes.user Processes.user_id
19   Processes.vendor_product
20
21
22| `drop_dm_object_name(Processes)`
23
24| `security_content_ctime(firstTime)`
25
26| `security_content_ctime(lastTime)`
27
28| `windows_powershell_process_implementing_manual_base64_decoder_filter`

Data Source

Name	Platform	Sourcetype	Source
CrowdStrike ProcessRollup2	N/A	'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

Legitimate use of PowerShell for decoding Base64 strings by administrators or power users may trigger this detection. It is essential to review the context of the PowerShell execution to determine if it is malicious, suspicious or even authorized activity.

Associated Analytic Story

• Compromised Windows Host

• Deobfuscate-Decode Files or Information

Risk Based Analytics (RBA)

Risk Message:

An instance of $process_name$ executing a manual Base64 decoding routine $process$ was identified on endpoint $dest$ by user $user$.

References

• https://www.virustotal.com/gui/file/4b3ab4d9f2332da6b6cd8d9d0f4910a5eb85ac8c969108acb3ad49631812f998/behavior

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