Detection: O365 Suspicious User Email Forwarding

Description

DEPRECATION NOTE - This search has been deprecated and replaced with O365 Mailbox Email Forwarding Enabled. The following analytic detects when multiple users have configured a forwarding rule to the same destination to proactively identify and investigate potential security risks related to email forwarding and take appropriate actions to protect the organizations data and prevent unauthorized access or data breaches. This detection is made by a Splunk query to O365 management activity logs with the operation Set-Mailbox to gather information about mailbox configurations. Then, the query uses the spath function to extract the parameters and rename the "Identity" field as "src_user" and searches for entries where the "ForwardingSmtpAddress" field is not empty, which indicates the presence of a forwarding rule. Next, the analytic uses the stats command to group the results by the forwarding email address and count the number of unique source users (src_user). Finally, it filters the results and only retains entries where the count of source users (count_src_user) is greater than 1, which indicates that multiple users have set up forwarding rules to the same destination. This detection is important because it suggests that multiple users are forwarding emails to the same destination without proper authorization, which can lead to the exposure of sensitive information, loss of data control, or unauthorized access to confidential emails. Investigating and addressing this issue promptly can help prevent data breaches and mitigate potential damage.indicates a potential security risk since multiple users forwarding emails to the same destination can be a sign of unauthorized access, data exfiltration, or a compromised account. Additionally, it also helps to determine if the forwarding rules are legitimate or if they indicate a security incident. False positives can occur if there are legitimate reasons for multiple users to forward emails to the same destination, such as a shared mailbox or a team collaboration scenario. Next steps include further investigation and context analysis to determine the legitimacy of the forwarding rules.

Search

1`o365_management_activity` Operation=Set-Mailbox 
2| spath input=Parameters 
3| rename Identity AS src_user 
4| search ForwardingSmtpAddress=* 
5| stats dc(src_user) AS count_src_user earliest(_time) as firstTime latest(_time) as lastTime values(src_user) AS src_user values(user) AS user by ForwardingSmtpAddress 
6| where count_src_user > 1 
7|`security_content_ctime(firstTime)` 
8|`security_content_ctime(lastTime)` 
9|`o365_suspicious_user_email_forwarding_filter`

Data Source

Macros Used

Annotations

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Implementation

You must install splunk Microsoft Office 365 add-on. This search works with o365:management:activity

Known False Positives

unknown

Associated Analytic Story

• Data Exfiltration

• Office 365 Collection Techniques

Risk Based Analytics (RBA)

Detection Testing

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 2