Detection: O365 Mailbox Inbox Folder Shared with All Users

Updated Date: 2024-09-30 ID: 21421896-a692-4594-9888-5faeb8a53106 Author: Mauricio Velazco, Splunk Type: TTP Product: Splunk Enterprise Security

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.

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
O365 ModifyFolderPermissions N/A 'o365:management:activity' 'o365'

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

- MITRE ATT&CK
+ Kill Chain Phases
+ NIST
+ CIS
- Threat Actors
ID Technique Tactic
T1114 Email Collection Collection
T1114.002 Remote Email Collection Collection
KillChainPhase.EXPLOITAITON
NistCategory.DE_CM
Cis18Value.CIS_10
Ember Bear
Magic Hound
Scattered Spider
Silent Librarian
APT1
APT28
APT29
Chimera
Dragonfly
FIN4
HAFNIUM
Ke3chang
Kimsuky
Leafminer
Magic Hound
Star Blizzard

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
This configuration file applies to all detections of type TTP. These detections will use Risk Based Alerting and generate Notable Events.

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

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: 3