Detection: Windows Symlink Evaluation Change via Fsutil

Updated Date: 2025-10-07 ID: 9777e7e3-2499-4a16-a519-ebe33630c1e8 Author: Nasreddine Bencherchali, Splunk Type: Anomaly Product: Splunk Enterprise Security

Description

This analytic detects the execution of the Windows built-in tool Fsutil.exe with the "behavior", "set" and "SymlinkEvaluation" parameters. Attackers can abuse this to alter symlink evaluation behavior on Windows, potentially enabling remote traversal over SMB shares or evading defenses. Such changes should be uncommon or even rare in enterprise environments and should be investigated.

Search

 1
 2| tstats `security_content_summariesonly`
 3    count min(_time) as firstTime max(_time) as lastTime
 4  from datamodel=Endpoint.Processes where 
 5  
 6  (
 7    Processes.process_name="fsutil.exe"
 8    OR Processes.original_file_name="fsutil.exe"
 9  )
10
11  Processes.process="*behavior*"
12  Processes.process="*set*"
13  Processes.process="*SymlinkEvaluation*"
14  
15  by Processes.action Processes.dest Processes.original_file_name Processes.parent_process
16  Processes.parent_process_exec Processes.parent_process_guid Processes.parent_process_id
17  Processes.parent_process_name Processes.parent_process_path Processes.process Processes.process_exec
18  Processes.process_guid Processes.process_hash Processes.process_id Processes.process_integrity_level
19  Processes.process_name Processes.process_path Processes.user Processes.user_id 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_symlink_evaluation_change_via_fsutil_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

Name	Value
security_content_ctime	`convert timeformat="%Y-%m-%dT%H:%M:%S" ctime($field$)`
windows_symlink_evaluation_change_via_fsutil_filter	`search *`

windows_symlink_evaluation_change_via_fsutil_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
T1222.001	Windows File and Directory Permissions Modification	Defense Evasion

Exploitation

DE.AE

CIS 10

Wizard Spider

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 Risk Event	True

This configuration file applies to all detections of type anomaly. These detections will use Risk Based Alerting.

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

Edge cases may exist in environments where this command is used for legitimate purposes. However, such usage is expected to be uncommon. It is recommended to investigate any occurrences of this command, and apply filters as necessary.

Associated Analytic Story

Windows Post-Exploitation

Risk Based Analytics (RBA)

Risk Message:

process $process_name$ with command line "$process$" modified SymlinkEvaluation on $dest$

Risk Object	Risk Object Type	Risk Score	Threat Objects
dest	system	35	parent_process_name

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: 1