Custom detection rules in Microsoft 365 Defender allow security analysts to proactively monitor and respond to specific threats tailored to their organization's unique environment. These rules leverage Advanced Hunting queries written in Kusto Query Language (KQL) to identify suspicious activities …Custom detection rules in Microsoft 365 Defender allow security analysts to proactively monitor and respond to specific threats tailored to their organization's unique environment. These rules leverage Advanced Hunting queries written in Kusto Query Language (KQL) to identify suspicious activities and automatically generate alerts or take response actions.
To configure custom detection rules, navigate to the Microsoft 365 Defender portal and access the Hunting section. First, create or refine an Advanced Hunting query that identifies the specific behavior or threat pattern you want to detect. The query must return results that include essential columns such as Timestamp, ReportId, and relevant entity identifiers like DeviceId or AccountObjectId.
Once your query is validated and returns meaningful results, you can convert it into a detection rule by selecting 'Create detection rule.' You must specify several parameters including the rule name, frequency of execution (ranging from every 24 hours to continuous), alert severity level, and the MITRE ATT&CK category that best describes the threat.
The rule configuration also requires mapping entities from your query results. This enables correlation with other alerts and proper entity page population. You can configure automated response actions such as isolating devices, collecting investigation packages, running antivirus scans, or disabling user accounts.
Managing custom detection rules involves regular review and optimization. Monitor rule performance through the detection rule management interface, checking for false positives and adjusting query logic as needed. Rules can be enabled, disabled, modified, or deleted based on evolving security requirements.
Best practices include testing queries thoroughly before creating rules, setting appropriate alert thresholds to minimize noise, documenting rule purposes and logic, and periodically reviewing rule effectiveness. Custom detection rules complement built-in detections by addressing organization-specific threats and filling coverage gaps in your security monitoring strategy.
Configure and Manage Custom Detection Rules
Why It Is Important
Custom detection rules are essential for Security Operations Analysts because they allow organizations to detect threats specific to their environment that built-in detections may not cover. Every organization has unique applications, processes, and potential attack vectors that require tailored detection capabilities. Mastering custom detection rules enables analysts to proactively identify suspicious activities, reduce alert fatigue by fine-tuning detections, and respond to emerging threats faster.
What Are Custom Detection Rules?
Custom detection rules in Microsoft 365 Defender are user-defined rules that run advanced hunting queries on a scheduled basis. When the query returns results, alerts are automatically generated, and response actions can be triggered. These rules extend the detection capabilities beyond Microsoft's built-in analytics by allowing security teams to create detections based on their specific threat intelligence, compliance requirements, and organizational context.
How Custom Detection Rules Work
Custom detection rules operate through the following process:
1. Query Creation: Analysts write Kusto Query Language (KQL) queries in Advanced Hunting that identify specific suspicious patterns or behaviors.
2. Rule Configuration: The query is converted into a detection rule with parameters including: - Detection frequency: How often the rule runs (every 1, 3, 12, or 24 hours) - Alert title and severity: How the resulting alerts appear - Impacted entities: Devices, users, or mailboxes affected - Automated actions: Response actions to take when triggered
3. Continuous Monitoring: The rule runs automatically at the specified interval, scanning data for matches.
4. Alert Generation: When results are found, alerts are created in the incidents queue for investigation.
Key Components to Remember
- Custom detection rules can look back up to 30 days of data - Rules can trigger automated response actions such as isolating devices, quarantining files, or disabling users - Queries must include timestamp and ReportId columns for proper alert grouping - You need appropriate permissions to create and manage custom detections - Rules can be enabled or disabled as needed
Exam Tips: Answering Questions on Configure and Manage Custom Detection Rules
Focus on Frequency Options: Remember the available detection frequencies: 1 hour, 3 hours, 12 hours, and 24 hours. Questions often test whether you know these specific intervals.
Understand Entity Mapping: Know that you must map impacted entities (DeviceId, AccountObjectId, etc.) for proper alert association. Exam scenarios may present queries and ask which entities should be mapped.
Know the Prerequisites: Questions may test whether you understand that custom detections require a valid Advanced Hunting query with specific columns.
Response Actions: Be familiar with available automated actions for different entity types. Device actions differ from user account actions.
Lookback Period: Remember the 30-day maximum lookback period for custom detection queries.
Permissions: Understand that creating custom detections requires Security Administrator or equivalent permissions in Microsoft 365 Defender.
Scenario-Based Questions: When presented with a scenario requiring detection of specific behaviors, identify whether a custom detection rule is the appropriate solution versus built-in analytics or other features.
Query Requirements: Pay attention to questions about required columns and query structure. The Timestamp column is essential for proper rule execution.