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Description

The following analytic identifies instances where the inbox folder of a mailbox in Office 365 is shared with all users within the tenant. Sharing the inbox folder with all users is an unusual and risky configuration. Attackers have been known to exploit this setting to surreptitiously read a target user's emails from another account. Such unauthorized access can lead to data breaches, leakage of confidential information, or further compromise based on the information gathered from the emails. Monitoring for this configuration change ensures that inadvertent or malicious sharing is promptly identified and addressed. If an attacker successfully configures the inbox to be shared with all users, they can access and read all emails in the affected mailbox from any account within the tenant. This can lead to data exfiltration, spear-phishing attacks based on the information in the emails, or further malicious activities using sensitive information gathered from the mailbox.

  • Type: TTP

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

  • Last Updated: 2023-09-07
  • Author: Mauricio Velazco, Splunk
  • ID: 21421896-a692-4594-9888-5faeb8a53106

Annotations

ATT&CK

ATT&CK

ID Technique Tactic
T1114 Email Collection Collection
T1114.002 Remote Email Collection Collection
Kill Chain Phase
  • Exploitation
NIST
  • DE.CM
CIS20
  • CIS 10
CVE 


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`o365_management_activity` Operation=ModifyFolderPermissions Workload=Exchange object=Inbox Item.ParentFolder.MemberUpn=Everyone 
| eval isReadRole=if(match('Item.ParentFolder.MemberRights', "(ReadAny)"), "true", "false")  
| search isReadRole = "true" 
| stats count earliest(_time) as firstTime latest(_time) as lastTime by Operation, UserId, object, MailboxOwnerUPN, Item.ParentFolder.MemberUpn, Item.ParentFolder.MemberRights 
| `security_content_ctime(firstTime)` 
| `security_content_ctime(lastTime)` 
| `o365_mailbox_inbox_folder_shared_with_all_users_filter`

Macros

The SPL above uses the following Macros:

:information_source: o365_mailbox_inbox_folder_shared_with_all_users_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
  • Workload
  • object
  • Item.ParentFolder.MemberUpn
  • Item.ParentFolder.MemberRights
  • UserId
  • MailboxOwnerUPN

How To Implement

You must install the Splunk Microsoft Office 365 Add-on and ingest Office 365 management activity events.

Known False Positives

Administrators might temporarily share a mailbox with all users for legitimate reasons, such as troubleshooting, migrations, or other administrative tasks. Some organizations use shared mailboxes for teams or departments where multiple users need access to the same mailbox. Filter as needed.

Associated Analytic Story

RBA

Risk Score Impact Confidence Message
56.0 80 70 Inbox folder for the $MailboxOwnerUPN$ mailbox was shared with all users.

: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.

Reference

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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