Office365: Mailbox SendAs or SendOnBehalf has occurred

Rule ID

PH_RULE_O365_MailSendAsSendOnBehalfAction

Default Status

Enabled

Description

Detects when a Office365 user sends mail as or on behalf of another mailbox. This can be used to spoof the true sender of the email. Many uses are legitimate, but should be monitored for malicious activity or misconfigurations.

Severity

MITRE ATT&CK® Tactics

Collection

Collection consists of techniques adversaries may use to gather information and the sources information is collected from that are relevant to following through on the adversary's objectives. Frequently, the next goal after collecting data is to steal (exfiltrate) the data. Common target sources include various drive types, browsers, audio, video, and email. Common collection methods include capturing screenshots and keyboard input.

https://attack.mitre.org/tactics/TA0009

MITRE ATT&CK® Techniques

T1114.003

Email Collection: Email Forwarding Rule

Adversaries may setup email forwarding rules to collect sensitive information. Adversaries may abuse email-forwarding rules to monitor the activities of a victim, steal information, and further gain intelligence on the victim or the victim's organization to use as part of further exploits or operations. Any user or administrator within the organization (or adversary with valid credentials) can create rules to automatically forward all received messages to another recipient, forward emails to different locations based on the sender, and more.

https://attack.mitre.org/techniques/T1114/003

Impacts

Application

Data Source

Office 365 via Management Activity API

Detection

Correlation

Remediation Guidance

Ensure that the SendAs or SendOnBehalf operation was legitimate based on business requirements.

Time Window

If the following pattern or patterns match an ingested event within the given time window in seconds, trigger an incident.

300 seconds

Trigger Conditions

If the following defined pattern/s occur within a 300 second time window.

mail_impersonate

SubPattern Definitions

SubPattern Name: mail_impersonate

This is the named definition of the event query, this is important if multiple subpatterns are defined to distinguish them.

SubPattern Query

This is the query logic that matches incoming events

eventType IN ("MS_OFFICE365_Exchange_SendAs","MS_OFFICE365_Exchange_SendOnBehalf")

Group by Attributes

This defines how matching events are aggregated, only events with the same matching attribute values are grouped into one unique incident ID

user,targetUser,srcIpAddr

Aggregate Constraint

This is most typically a numerical constraint that defines when the rule should trigger an incident

COUNT(*) >= 1

Incident Attribute Mapping

This section defines which fields in matching raw events should be mapped to the incident attributes in the resulting incident.

The available raw event attributes to map are limited to the group by attributes and the aggregate event constraint fields for each subpattern

srcIpAddr=mail_impersonate.srcIpAddr,
user=mail_impersonate.user,
targetUser=mail_impersonate.targetUser,
incidentCount=mail_impersonate.COUNT(*)

FortiSIEM Rules