ID | Technique | Tactic |
---|---|---|
T1114 | Email Collection | Collection |
T1114.002 | Remote Email Collection | Collection |
Detection: O365 Mailbox Inbox Folder Shared with All Users
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.
Search
1`o365_management_activity` Operation=ModifyFolderPermissions Workload=Exchange object=Inbox Item.ParentFolder.MemberUpn=Everyone
2| eval isReadRole=if(match('Item.ParentFolder.MemberRights', "(ReadAny)"), "true", "false")
3| search isReadRole = "true"
4| stats count earliest(_time) as firstTime latest(_time) as lastTime by Operation, UserId, object, MailboxOwnerUPN, Item.ParentFolder.MemberUpn, Item.ParentFolder.MemberRights
5| `security_content_ctime(firstTime)`
6| `security_content_ctime(lastTime)`
7| `o365_mailbox_inbox_folder_shared_with_all_users_filter`
Data Source
Name | Platform | Sourcetype | Source | Supported App |
---|---|---|---|---|
O365 ModifyFolderPermissions | N/A | 'o365:management:activity' |
'o365' |
N/A |
Macros Used
Name | Value |
---|---|
o365_management_activity | sourcetype=o365:management:activity |
o365_mailbox_inbox_folder_shared_with_all_users_filter | search * |
o365_mailbox_inbox_folder_shared_with_all_users_filter
is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.
Annotations
Default Configuration
This detection is configured by default in Splunk Enterprise Security to run with the following settings:
Setting | Value |
---|---|
Disabled | true |
Cron Schedule | 0 * * * * |
Earliest Time | -70m@m |
Latest Time | -10m@m |
Schedule Window | auto |
Creates Notable | Yes |
Rule Title | %name% |
Rule Description | %description% |
Notable Event Fields | user, dest |
Creates Risk Event | True |
Implementation
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
Risk Based Analytics (RBA)
Risk Message | Risk Score | Impact | Confidence |
---|---|---|---|
Inbox folder for the $MailboxOwnerUPN$ mailbox was shared with all users. | 56 | 80 | 70 |
References
Detection Testing
Test Type | Status | Dataset | Source | Sourcetype |
---|---|---|---|---|
Validation | ✅ Passing | N/A | N/A | N/A |
Unit | ✅ Passing | Dataset | o365 |
o365:management:activity |
Integration | ✅ Passing | Dataset | o365 |
o365:management:activity |
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