Exchange mailbox item operations (aggregated) M365-ExchangeItemAggregated

2 operations, identified by Operation in the audit log.

Operation	Description
_catch_all	Catch-all for M365-ExchangeItemAggregated rules matching the RecordType but no specific Operation.
MailItemsAccessed	A mail client or application accessed mailbox items via MAPI, EWS, or REST; logged for OAuth-authenticated access and admin access.

_catch_all: Exchange mailbox item operations (aggregated) (catch-all)

RecordType: M365-ExchangeItemAggregated

Description

Catch-all for M365-ExchangeItemAggregated rules matching the RecordType but no specific Operation.

Fields #

Name	Description
`RecordType`	AuditLogRecordType enum value (named string in Sentinel OfficeActivity; integer in the raw Management Activity API JSON).
`Operation`	The name of the user or admin activity. For Exchange admin activity this is the cmdlet name; for DLP it is DlpRuleMatch/DlpRuleUndo/DlpInfo. The per-event discriminator (event.action in Elastic ECS).
`Workload`	The Microsoft 365 service where the activity occurred (Exchange, SharePoint, AzureActiveDirectory, MicrosoftTeams, ...). Surfaces as event.provider in Elastic ECS.
`ResultStatus`	Whether the action succeeded: Succeeded, PartiallySucceeded, or Failed (True/False for Exchange admin). For STS-logon events this reflects only the HTTP operation, not logon success: check LogonError.
`UserId`	UPN of the user who performed the action; system-account and app@sharepoint values also appear.
`UserType`	Type of actor: Regular, Admin, System, DcAdmin, Application, ServicePrincipal.
`UserKey`	Alternative ID for the user (the PUID for SharePoint / OneDrive / Exchange events).
`ClientIP`	IPv4/IPv6 address of the device used; may be a trusted-app IP, and is null for Microsoft Entra ID events.
`ObjectId`	For SharePoint/OneDrive, the full path of the file/folder; for Exchange admin, the object modified by the cmdlet; shape is operation-specific.
`OrganizationId`	GUID of the tenant's Office 365 organization (constant per tenant).
`Id`	Unique identifier (GUID) for the audit record.
`CreationTime`	UTC date/time when the audit record was generated.
`Scope`	Source of the event: online (a Microsoft 365 service) or onprem (an on-premises service; currently SharePoint only).
`AppAccessContext`	Application context for the user or service principal that performed the action.
`LogonType`	Type of user who accessed the mailbox and performed the operation (Owner, Admin, Delegate, ...).
`MailboxOwnerUPN`	Email address of the person who owns the accessed mailbox.
`MailboxGuid`	Exchange GUID of the accessed mailbox.
`LogonUserSid`	SID of the user who performed the operation.
`ExternalAccess`	True if the logon user's domain differs from the mailbox owner's domain.
`ClientInfoString`	Information about the email client used (browser/Outlook version, mobile device info).
`ClientIPAddress`	IP address of the device used when the operation was logged.
`ClientProcessName`	The email client used to access the mailbox.
`SessionId`	Unique session identifier; helps separate attacker actions from day-to-day activity on a compromised account.

References #

Office 365 Management Activity API schema (AuditLogRecordType enum) https://learn.microsoft.com/en-us/office/office-365-management-api/office-365-management-activity-api-schema
Microsoft Purview audit log activities reference https://learn.microsoft.com/en-us/purview/audit-log-activities

MailItemsAccessed

RecordType: M365-ExchangeItemAggregated

Description

A mail client or application accessed mailbox items via MAPI, EWS, or REST; logged for OAuth-authenticated access and admin access.

Fields #

Name	Description
`RecordType`	AuditLogRecordType enum value (named string in Sentinel OfficeActivity; integer in the raw Management Activity API JSON).
`Operation`	The name of the user or admin activity. For Exchange admin activity this is the cmdlet name; for DLP it is DlpRuleMatch/DlpRuleUndo/DlpInfo. The per-event discriminator (event.action in Elastic ECS).
`Workload`	The Microsoft 365 service where the activity occurred (Exchange, SharePoint, AzureActiveDirectory, MicrosoftTeams, ...). Surfaces as event.provider in Elastic ECS.
`ResultStatus`	Whether the action succeeded: Succeeded, PartiallySucceeded, or Failed (True/False for Exchange admin). For STS-logon events this reflects only the HTTP operation, not logon success: check LogonError.
`UserId`	UPN of the user who performed the action; system-account and app@sharepoint values also appear.
`UserType`	Type of actor: Regular, Admin, System, DcAdmin, Application, ServicePrincipal.
`UserKey`	Alternative ID for the user (the PUID for SharePoint / OneDrive / Exchange events).
`ClientIP`	IPv4/IPv6 address of the device used; may be a trusted-app IP, and is null for Microsoft Entra ID events.
`ObjectId`	For SharePoint/OneDrive, the full path of the file/folder; for Exchange admin, the object modified by the cmdlet; shape is operation-specific.
`OrganizationId`	GUID of the tenant's Office 365 organization (constant per tenant).
`Id`	Unique identifier (GUID) for the audit record.
`CreationTime`	UTC date/time when the audit record was generated.
`Scope`	Source of the event: online (a Microsoft 365 service) or onprem (an on-premises service; currently SharePoint only).
`AppAccessContext`	Application context for the user or service principal that performed the action.
`LogonType`	Type of user who accessed the mailbox and performed the operation (Owner, Admin, Delegate, ...).
`MailboxOwnerUPN`	Email address of the person who owns the accessed mailbox.
`MailboxGuid`	Exchange GUID of the accessed mailbox.
`LogonUserSid`	SID of the user who performed the operation.
`ExternalAccess`	True if the logon user's domain differs from the mailbox owner's domain.
`ClientInfoString`	Information about the email client used (browser/Outlook version, mobile device info).
`ClientIPAddress`	IP address of the device used when the operation was logged.
`ClientProcessName`	The email client used to access the mailbox.
`SessionId`	Unique session identifier; helps separate attacker actions from day-to-day activity on a compromised account.

Common Indicators #

Field/value combinations most frequently checked by detection rules targeting this event, derived from cross-vendor predicate analysis.

Field	Kind	Value	Rules	Vendors
`EventType`	eq	`MailItemsAccessed`	2 rules	elastic
`Provider_Name`	eq	`Exchange`	2 rules	elastic
`Operation`	eq	`MailItemsAccessed`	1 rule	kusto, splunk
`ResultStatus`	eq	`Succeeded`	1 rule	kusto
`anomalies`	gt	`0`	1 rule	kusto
`m365::UserType`	in	`0`	1 rule	elastic
`m365::UserType`	in	`10`	1 rule	elastic
`m365::UserType`	in	`2`	1 rule	elastic
`m365::UserType`	in	`3`	1 rule	elastic
`m365::Workload`	eq	`Exchange`	1 rule	kusto

Detection Rules #

View all rules referencing this event →

Elastic #

M365 Exchange Mailbox Items Accessed Excessively source medium: 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.
M365 Exchange Mailbox Accessed by Unusual Client source medium: Identifies suspicious Microsoft 365 mail access by ClientAppId. This rule detects when a user accesses their mailbox using a client application that is not typically used by the user, which may indicate potential compromise or unauthorized access attempts. Adversaries may use custom or third-party applications to access mailboxes, bypassing standard security controls. First-party Microsoft applications are also abused after OAuth tokens are compromised, allowing adversaries to access mailboxes without raising suspicion.

Kusto #

Exchange workflow MailItemsAccessed operation anomaly source medium: Identifies anomalous increases in Exchange mail items accessed operations. The query leverages KQL built-in anomaly detection algorithms to find large deviations from baseline patterns. Sudden increases in execution frequency of sensitive actions should be further investigated for malicious activity. Manually change scorethreshold from 1.5 to 3 or higher to reduce the noise based on outliers flagged from the query criteria. Read more about MailItemsAccessed- https://learn.microsoft.com/en-us/purview/audit-log-investigate-accounts

YARA-L #

Office 365 mail accessed via unexpected application source: Detect mail items accessed via API through an unapproved Entra ID application.

References #

Office 365 Management Activity API schema (AuditLogRecordType enum) https://learn.microsoft.com/en-us/office/office-365-management-api/office-365-management-activity-api-schema
Microsoft Purview audit log activities reference https://learn.microsoft.com/en-us/purview/audit-log-activities

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`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)

Exchange mailbox item operations (aggregated) M365-ExchangeItemAggregated

_catch_all: Exchange mailbox item operations (aggregated) (catch-all)

Description

Fields #

References #

MailItemsAccessed

Description

Fields #

Common Indicators #

Detection Rules #

Elastic #

Kusto #

YARA-L #

References #

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