IMPORTANT: No additional bug fixes or documentation updates will be released for this version. For the latest information, see the current release documentation.

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O365 Exchange Suspicious Mailbox Right Delegation

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O365 Exchange Suspicious Mailbox Right Delegation

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Identifies the assignment of rights to access content from another mailbox. An adversary may use the compromised account to send messages to other accounts in the network of the target organization while creating inbox rules, so messages can evade spam/phishing detection mechanisms.

Rule type: query

Rule indices:

filebeat-*
logs-o365*

Severity: low

Risk score: 21

Runs every: 5 minutes

Searches indices from: now-6m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

Tags:

Elastic
Cloud
Microsoft 365
Continuous Monitoring
SecOps
Configuration Audit

Version: 4 (version history)

Added (Elastic Stack release): 7.16.0

Last modified (Elastic Stack release): 8.4.0

Rule authors: Elastic, Austin Songer

Rule license: Elastic License v2

Potential false positives

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Assignment of rights to a service account.

Investigation guide

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Rule query

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event.dataset:o365.audit and event.provider:Exchange and
event.action:Add-MailboxPermission and
o365.audit.Parameters.AccessRights:(FullAccess or SendAs or
SendOnBehalf) and event.outcome:success and not user.id : "NT
AUTHORITY\SYSTEM (Microsoft.Exchange.Servicehost)"

Threat mapping

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Framework: MITRE ATT&CK^TM

Tactic:
- Name: Persistence
- ID: TA0003
- Reference URL: https://attack.mitre.org/tactics/TA0003/
Technique:
- Name: Account Manipulation
- ID: T1098
- Reference URL: https://attack.mitre.org/techniques/T1098/

Rule version history

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Version 4 (8.4.0 release)

Updated query, changed from:

event.dataset:o365.audit and event.provider:Exchange and
event.action:Add-MailboxPermission and
o365.audit.Parameters.AccessRights:(FullAccess or SendAs or
SendOnBehalf) and event.outcome:success and not user.id : "NT
AUTHORITY\SYSTEM (Microsoft.Exchange.Servicehost)"

Version 2 (8.1.0 release)

Updated query, changed from:

event.dataset:o365.audit and event.provider:Exchange and
event.action:Add-MailboxPermission and
o365.audit.Parameters.AccessRights:(FullAccess or SendAs or
SendOnBehalf) and event.outcome:success

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