Detection: Windows Alternate DataStream - Base64 Content

Updated Date: 2024-05-28 ID: 683f48de-982f-4a7e-9aac-9cec550da498 Author: Steven Dick, Teoderick Contreras, Michael Haag, Splunk Type: TTP Product: Splunk Enterprise Security

Description

The following analytic detects the creation of Alternate Data Streams (ADS) with Base64 content on Windows systems. It leverages Sysmon EventID 15, which captures file creation events, including the content of named streams. ADS can conceal malicious payloads, making them significant for SOC monitoring. This detection identifies hidden streams that may contain executables, scripts, or configuration data, often used by malware to evade detection. If confirmed malicious, this activity could allow attackers to hide and execute payloads, persist in the environment, or access sensitive information without being easily detected.

Search

 1`sysmon` EventCode=15 NOT Contents IN ("-","[ZoneTransfer]*") 
 2| regex TargetFilename="(?<!\/)\b\w+(\.\w+)?:\w+(\.\w+)?$" 
 3| regex Contents="(?:[A-Za-z0-9+/]{128,})(?:[A-Za-z0-9+/]{2}==
 4|[A-Za-z0-9+/]{3}=)?$" 
 5| eval file_name = replace(TargetFilename,"(.*\\\)",""), process = Image , file_path = TargetFilename , base64 = Contents, file_hash = coalesce(SHA256,SHA1,MD5,Hash) 
 6| stats count min(_time) as firstTime max(_time) as lastTime values(user) as user by dest, process, process_guid, file_name, file_path, file_hash, base64 
 7| `base64decode(base64)` 
 8| fields - base64 
 9| rename base64_decode as command 
10| `security_content_ctime(firstTime)` 
11| `security_content_ctime(lastTime)` 
12| `windows_alternate_datastream___base64_content_filter`

Data Source

Name	Platform	Sourcetype	Source	Supported App
Sysmon EventID 15	Windows	`'xmlwineventlog'`	`'XmlWinEventLog:Microsoft-Windows-Sysmon/Operational'`	N/A

Macros Used

Name	Value
base64decode	`eval b64x_split=split($b64in$,"")
windows_alternate_datastream___base64_content_filter	`search *`

windows_alternate_datastream___base64_content_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
T1564	Hide Artifacts	Defense Evasion
T1564.004	NTFS File Attributes	Defense Evasion

KillChainPhase.EXPLOITAITON

NistCategory.DE_CM

Cis18Value.CIS_10

APT32

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

Target environment must ingest sysmon data, specifically Event ID 15.

Known False Positives

Unknown

Associated Analytic Story

Windows Defense Evasion Tactics

Risk Based Analytics (RBA)

Risk Message	Risk Score	Impact	Confidence
Base64 content written to an NTFS alternate data stream by $user$, see command field for details.	80	100	80

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-Sysmon/Operational`	`XmlWinEventLog`
Integration	✅ Passing	Dataset	`XmlWinEventLog:Microsoft-Windows-Sysmon/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: 3