Elastic Security Detection Rules

Suspicious Mailbox Permission Delegation in Exchange Online

Last updated 2 days ago on 2025-05-07
Created 4 years ago on 2021-05-17

About

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.
Tags
Domain: CloudDomain: SaaSData Source: Microsoft 365Data Source: Microsoft ExchangeData Source: Microsoft 365 Audit LogsUse Case: Configuration AuditTactic: PersistenceLanguage: kuery
Severity
low
Risk Score
21
MITRE ATT&CK™

Persistence (TA0003)(opens in a new tab or window)

↳ Account Manipulation (T1098)(opens in a new tab or window)

False Positive Examples
Assignment of rights to a service account.Delegation by first-party applications that require mailbox access.
License
Elastic License v2(opens in a new tab or window)

Definition

Rule Type
New Terms Rule
Integration Pack
Prebuilt Security Detection Rules
Index Patterns
filebeat-*logs-o365.audit-*
Related Integrations

o365(opens in a new tab or window)

Query
event.dataset: "o365.audit" and
event.provider: "Exchange" and
event.outcome: "success" and
not o365.audit.UserType : (3 or 4) and
(
    (event.action: "Add-MailboxPermission" and o365.audit.Parameters.AccessRights: "FullAccess") or
    (event.action: "Add-RecipientPermission" and o365.audit.Parameters.AccessRights: "SendAs") or
    (event.action: "Set-Mailbox" and o365.audit.Parameters.GrantSendOnBehalfTo: *)
) and
not user.id:(
    "NT AUTHORITY\SYSTEM (Microsoft.Exchange.ServiceHost)" or
    "NT AUTHORITY\SYSTEM (Microsoft.Exchange.AdminApi.NetCore)" or
    "NT AUTHORITY\SYSTEM (w3wp)"
    )

Install detection rules in Elastic Security

Detect Suspicious Mailbox Permission Delegation in Exchange Online 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(opens in a new tab or window).