LoFP / 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.

Techniques

• T1114
• T1114.002

Sample rules

O365 Mailbox Inbox Folder Shared with All Users

• source: splunk
• technicques:
  • T1114
  • T1114.002

Description

The following analytic detects instances where the inbox folder of an Office 365 mailbox is shared with all users within the tenant. It leverages Office 365 management activity events to identify when the ‘Inbox’ folder permissions are modified to include ‘Everyone’ with read rights. This activity is significant as it represents a potential security risk, allowing unauthorized access to sensitive emails. If confirmed malicious, this could lead to data breaches, exfiltration of confidential information, and further compromise through spear-phishing or other malicious activities based on the accessed email content.

Detection logic

`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`