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| Create custom detection rules in Microsoft Defender ATP | Learn how to create custom detection rules based on advanced hunting queries | custom detections, create, manage, alerts, edit, run on demand, frequency, interval, detection rules, advanced hunting, hunt, query, response actions, mdatp, microsoft defender atp | eADQiWindows 10XVcnh | met150 | w10 | deploy | library | security | lomayor | lomayor | medium | dansimp | ITPro | M365-security-compliance | article |
Create custom detection rules
Applies to:
Custom detection rules built from advanced hunting queries let you proactively monitor various events and system states, including suspected breach activity and misconfigured devices. You can set them to run at regular intervals, generating alerts and taking response actions whenever there are matches.
Read this article to learn how to create new custom detection rules. Or see viewing and managing existing rules.
1. Check required permissions
To create or manage custom detections, your role needs to have the manage security settings permission.
2. Prepare the query
In Microsoft Defender Security Center, go to Advanced hunting and select an existing query or create a new query. When using a new query, run the query to identify errors and understand possible results.
Important
To prevent the service from returning too many alerts, each rule is limited to generating only 100 alerts whenever it runs. Before creating a rule, tweak your query to avoid alerting for normal, day-to-day activity.
Required columns in the query results
To use a query for a custom detection rule, the query must return the Timestamp, DeviceId, and ReportId columns in the results. Simple queries, such as those that don't use the project or summarize operator to customize or aggregate results, typically return these common columns.
There are various ways to ensure more complex queries return these columns. For example, if you prefer to aggregate and count by DeviceId, you can still return Timestamp and ReportId by getting them from the most recent event involving each device.
The sample query below counts the number of unique devices (DeviceId) with antivirus detections and uses this count to find only the devices with more than five detections. To return the latest Timestamp and the corresponding ReportId, it uses the summarize operator with the arg_max function.
DeviceEvents
| where Timestamp > ago(7d)
| where ActionType == "AntivirusDetection"
| summarize (Timestamp, ReportId)=arg_max(Timestamp, ReportId), count() by DeviceId
| where count_ > 5
3. Create new rule and provide alert details
With the query in the query editor, select Create detection rule and specify the following alert details:
- Detection name—name of the detection rule
- Frequency—interval for running the query and taking action. See additional guidance below
- Alert title—title displayed with alerts triggered by the rule
- Severity—potential risk of the component or activity identified by the rule. Read about alert severities
- Category—type of threat component or activity, if any. Read about alert categories
- MITRE ATT&CK techniques—one or more attack techniques identified by the rule as documented in the MITRE ATT&CK framework. This section is not available with certain alert categories, such as malware, ransomware, suspicious activity, and unwanted software
- Description—more information about the component or activity identified by the rule
- Recommended actions—additional actions that responders might take in response to an alert
For more information about how alert details are displayed, read about the alert queue.
Rule frequency
When saved, a new or edited custom detection rule immediately runs and checks for matches from the past 30 days of data. The rule then runs again at fixed intervals and lookback durations based on the frequency you choose:
- Every 24 hours—runs every 24 hours, checking data from the past 30 days
- Every 12 hours—runs every 12 hours, checking data from the past 24 hours
- Every 3 hours—runs every 3 hours, checking data from the past 6 hours
- Every hour—runs hourly, checking data from the past 2 hours
Select the frequency that matches how closely you want to monitor detections, and consider your organization's capacity to respond to the alerts.
4. Specify actions on files or devices
Your custom detection rule can automatically take actions on files or devices that are returned by the query.
Actions on devices
These actions are applied to devices in the DeviceId column of the query results:
- Isolate device—applies full network isolation, preventing the device from connecting to any application or service, except for the Microsoft Defender ATP service. Learn more about device isolation
- Collect investigation package—collects device information in a ZIP file. Learn more about the investigation package
- Run antivirus scan—performs a full Microsoft Defender Antivirus scan on the device
- Initiate investigation—starts an automated investigation on the device
Actions on files
These actions are applied to files in the SHA1 or the InitiatingProcessSHA1 column of the query results:
- Allow/Block—automatically adds the file to your custom indicator list so that it is always allowed to run or blocked from running. You can set the scope of this action so that it is taken only on selected device groups. This scope is independent of the scope of the rule.
- Quarantine file—deletes the file from its current location and places a copy in quarantine
5. Set the rule scope
Set the scope to specify which devices are covered by the rule:
- All devices
- Specific device groups
Only data from devices in scope will be queried. Also, actions will be taken only on those devices.
6. Review and turn on the rule
After reviewing the rule, select Create to save it. The custom detection rule immediately runs. It runs again based on configured frequency to check for matches, generate alerts, and take response actions.