This detection has been marked deprecated by the Splunk Threat Research team. This means that it will no longer be maintained or supported.

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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.

  • Type: Anomaly
  • Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud

  • Last Updated: 2020-12-16
  • Author: Patrick Bareiss, Splunk
  • ID: f8dfe015-dbb3-4569-ba75-b13787e06aa4




ID Technique Tactic
T1114.003 Email Forwarding Rule Collection
T1114 Email Collection Collection
Kill Chain Phase
  • Exploitation
  • DE.AE
  • CIS 10
`o365_management_activity` Operation=Set-Mailbox 
| spath input=Parameters 
| rename Identity AS src_user 
| search ForwardingSmtpAddress=* 
| 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 
| where count_src_user > 1 


The SPL above uses the following Macros:

:information_source: o365_suspicious_user_email_forwarding_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Required fields

List of fields required to use this analytic.

  • _time
  • Operation
  • Parameters

How To Implement

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

Known False Positives


Associated Analytic Story


Risk Score Impact Confidence Message
48.0 80 60 User $user$ configured multiple users $src_user$ with a count of $count_src_user$, a forwarding rule to same destination $ForwardingSmtpAddress$

:information_source: The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.


Test Dataset

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

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