Detection: Windows Exfiltration Over C2 Via Powershell UploadString

Updated Date: 2024-05-27 ID: 59e8bf41-7472-412a-90d3-00f3afa452e9 Author: Teoderick Contreras, Splunk Type: TTP Product: Splunk Enterprise Security

Description

The following analytic identifies potential data exfiltration using the PowerShell net.webclient command with the UploadString method. It leverages PowerShell Script Block Logging to detect instances where this command is executed. This activity is significant as it may indicate an attempt to upload sensitive data, such as desktop screenshots or files, to an external or internal URI, often associated with malware like Winter-Vivern. If confirmed malicious, this could lead to unauthorized data transfer, compromising sensitive information and potentially leading to further exploitation of the compromised host.

Search

1`powershell` EventCode=4104 ScriptBlockText = "*Net.webclient*" AND ScriptBlockText = "*.UploadString*" 
2| stats count min(_time) as firstTime max(_time) as lastTime by EventCode ScriptBlockText Computer UserID 
3| `security_content_ctime(firstTime)` 
4| `security_content_ctime(lastTime)` 
5| `windows_exfiltration_over_c2_via_powershell_uploadstring_filter`

Data Source

Name	Platform	Sourcetype	Source	Supported App
Powershell Script Block Logging 4104	Windows	`'xmlwineventlog'`	`'XmlWinEventLog:Microsoft-Windows-PowerShell/Operational'`	N/A

Macros Used

Name	Value
powershell	`(source=WinEventLog:Microsoft-Windows-PowerShell/Operational OR source="XmlWinEventLog:Microsoft-Windows-PowerShell/Operational")`
windows_exfiltration_over_c2_via_powershell_uploadstring_filter	`search *`

windows_exfiltration_over_c2_via_powershell_uploadstring_filter is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Annotations

- MITRE ATT&CK

+ Kill Chain Phases

+ NIST

+ CIS

- Threat Actors

ID	Technique	Tactic
T1041	Exfiltration Over C2 Channel	Exfiltration

KillChainPhase.ACTIONS_ON_OBJECTIVES

NistCategory.DE_CM

Cis18Value.CIS_10

APT3

APT32

APT39

Chimera

Confucius

GALLIUM

Gamaredon Group

Higaisa

Ke3chang

Kimsuky

Lazarus Group

Leviathan

LuminousMoth

MuddyWater

Sandworm Team

Stealth Falcon

Wizard Spider

ZIRCONIUM

Default Configuration

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

Setting	Value
Disabled	true
Cron Schedule	`0 * * * *`
Earliest Time	`-70m@m`
Latest Time	`-10m@m`
Schedule Window	`auto`
Creates Notable	Yes
Rule Title	`%name%`
Rule Description	`%description%`
Notable Event Fields	user, dest
Creates Risk Event	True

This configuration file applies to all detections of type TTP. These detections will use Risk Based Alerting and generate Notable Events.

Implementation

To successfully implement this analytic, you will need to enable PowerShell Script Block Logging on some or all endpoints. Additional setup here https://docs.splunk.com/Documentation/UBA/5.0.4.1/GetDataIn/AddPowerShell#Configure_module_logging_for_PowerShell.

Known False Positives

False positives should be limited. Filter as needed.

Associated Analytic Story

Winter Vivern

Risk Based Analytics (RBA)

Risk Message	Risk Score	Impact	Confidence
A PowerShell script on $Computer$ is attempting to transfer files to a remote URL.	49	70	70

The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

References

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`XmlWinEventLog:Microsoft-Windows-PowerShell/Operational`	`xmlwineventlog`
Integration	✅ Passing	Dataset	`XmlWinEventLog:Microsoft-Windows-PowerShell/Operational`	`xmlwineventlog`

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: 2