Elastic Security Detection Rules

M365 Exchange Mailbox Items Accessed Excessively

Last updated 22 days ago on 2025-12-10
Created 6 months ago on 2025-06-17

About

Identifies an excessive number of Microsoft 365 mailbox items accessed by a user either via aggregated counts or throttling. Microsoft audits mailbox access via the MailItemsAccessed event, which is triggered when a user accesses mailbox items. If more than 1000 mailbox items are accessed within a 24-hour period, it is then throttled. Excessive mailbox access may indicate an adversary attempting to exfiltrate sensitive information or perform reconnaissance on a target's mailbox. This rule detects both the throttled and unthrottled events with a high threshold.
Tags
Domain: CloudDomain: EmailData Source: Microsoft 365Data Source: Microsoft 365 Audit LogsUse Case: Threat DetectionTactic: CollectionLanguage: kuery
Severity
medium
Risk Score
47
MITRE ATT&CK™

Collection (TA0009)(external, opens in a new tab or window)

↳ Email Collection (T1114)(external, opens in a new tab or window)

False Positive Examples
Legitimate users may access a large number of mailbox items in a short period, especially in environments with high email volume or during data migrations. If this is expected behavior, consider adjusting the rule or adding exceptions for specific users or groups.
License
Elastic License v2(external, opens in a new tab or window)

Definition

Rule Type
Query (Kibana Query Language)
Integration Pack
Prebuilt Security Detection Rules
Index Patterns
filebeat-*logs-o365.audit-*
Related Integrations

o365(external, opens in a new tab or window)

Query
text code block:
event.dataset: "o365.audit" and
    event.provider: "Exchange" and
    event.action: "MailItemsAccessed" and
    event.code: "ExchangeItemAggregated" and
    (
        (
            o365.audit.OperationProperties.Name: "IsThrottled" and
            o365.audit.OperationProperties.Value: "True"
        ) or o365.audit.OperationCount >= 100
    )

Install detection rules in Elastic Security

Detect M365 Exchange Mailbox Items Accessed Excessively in the Elastic Security detection engine by installing this rule into your Elastic Stack.

To setup this rule, check out the installation guide for Prebuilt Security Detection Rules(external, opens in a new tab or window).