GitHub Repo

Detection: Windows Defender ASR Block Events

Updated Date: 2026-09-22 ID: 026f5f4e-e99f-4155-9e63-911ba587300b Author: Michael Haag, Splunk Type: Anomaly Product: Splunk Enterprise Security

Description

This detection searches for ASR block events that are generated when a process or application attempts to perform an action that is blocked by an ASR rule. Typically, these will be enabled in block most after auditing and tuning the ASR rules themselves.

 1`ms_defender`
 2EventCode IN (
 3    1121,
 4    1126,
 5    1129,
 6    1131,
 7    1133
 8)
 9
10| fillnull value=NULL
11
12| stats count min(_time) as firstTime
13              max(_time) as lastTime
14  by host EventCode
15     Parent_Commandline Process_Name Target_Commandline
16     Path ID RuleType
17
18
19| lookup local=t asr_rules ID OUTPUT ASR_Rule
20
21
22| rename host as dest
23
24
25| `security_content_ctime(firstTime)`
26
27| `security_content_ctime(lastTime)`
28
29| `windows_defender_asr_block_events_filter`

Data Source

Name Platform Sourcetype Source
Windows Event Log Defender 1131 Windows icon Windows 'XmlWinEventLog' 'XmlWinEventLog:Security'
Windows Event Log Defender 1133 Windows icon Windows 'XmlWinEventLog' 'XmlWinEventLog:Security'
Windows Event Log Defender 1126 Windows icon Windows 'XmlWinEventLog' 'XmlWinEventLog:Security'
Windows Event Log Defender 1121 Windows icon Windows 'XmlWinEventLog' 'WinEventLog:Microsoft-Windows-Windows Defender/Operational'
Windows Event Log Defender 1129 Windows icon Windows 'XmlWinEventLog' 'WinEventLog:Microsoft-Windows-Windows Defender/Operational'

Macros Used

Name Value
security_content_ctime convert timeformat="%Y-%m-%dT%H:%M:%S" ctime($field$)
windows_defender_asr_block_events_filter search *
windows_defender_asr_block_events_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
T1059 Command and Scripting Interpreter Execution
T1566.001 Spearphishing Attachment Initial Access
T1566.002 Spearphishing Link Initial Access
Delivery
Installation
DE.AE
CIS 10

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 Finding (Notable) No
Creates Intermediate Finding (Risk Event) Yes
Anomaly detections generate Intermediate Findings (Risk Events). They do not generate a Finding (Notable) directly.

Implementation

The following analytic requires collection of Windows Defender Operational logs in either XML or multi-line. To collect, setup a new input for the Windows Defender Operational logs. In addition, it does require a lookup that maps the ID to ASR Rule name. Note that Audit and block Event IDs have different fields, therefore the analytic will need to be modified for each type of event.

Known False Positives

Some false positives are expected initially as the ASR rules are tuned and configured.

Associated Analytic Story

Intermediate Findings

Message Entity Field Entity Type Risk Score
The ASR block event [$ASR_Rule$] was triggered on [$dest$]. dest system 20

Threat Objects

Field Type
Target_Commandline process
Parent_Commandline parent_process
Process_Name file_path

References

Detection Testing

Test Type Status Dataset Source Sourcetype
Validation ✅ Passing N/A N/A N/A
Unit ✅ Passing Dataset XmlWinEventLog:Microsoft-Windows-Windows Defender/Operational XmlWinEventLog
Integration ✅ Passing Dataset XmlWinEventLog:Microsoft-Windows-Windows Defender/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: 11