O365 Mailbox Inbox Folder Shared with All Users

Status: production
Severity: medium
Group by: "Item.ParentFolder.MemberRights", "Item.ParentFolder.MemberUpn", MailboxOwnerUPN, aws::recipientAccountId, dest, object, signature, src, user, vendor_product
Author: Mauricio Velazco, Splunk
Source: github.com/splunk/security_content

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.

MITRE ATT&CK coverage

Tactic	Techniques
Collection	`T1114.002` Email Collection: Remote Email Collection

Rules detecting the same action

Other rules on this platform that filter on the same API call or operation.

O365 Mailbox Folder Read Permission Assigned (Splunk)

Rule body splunk

name: O365 Mailbox Inbox Folder Shared with All Users
id: 21421896-a692-4594-9888-5faeb8a53106
version: 10
creation_date: '2023-12-06'
modification_date: '2026-05-13'
author: Mauricio Velazco, Splunk
status: production
type: TTP
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.
data_source:
    - O365 ModifyFolderPermissions
search: "`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\" | rename UserId as user | fillnull | stats count earliest(_time) as firstTime latest(_time) as lastTime by signature, user, dest, vendor_account, vendor_product, object, MailboxOwnerUPN, Item.ParentFolder.MemberUpn, Item.ParentFolder.MemberRights, src | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `o365_mailbox_inbox_folder_shared_with_all_users_filter`"
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.
references:
    - https://attack.mitre.org/techniques/T1114/002/
    - https://www.mandiant.com/sites/default/files/2022-08/remediation-hardening-strategies-for-m365-defend-against-apt29-white-paper.pdf
    - https://www.blackhillsinfosec.com/abusing-exchange-mailbox-permissions-mailsniper/
    - https://learn.microsoft.com/en-us/purview/audit-mailboxes
    - https://learn.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxodlgt/5610c6e6-3268-44e3-adff-8804f5315946
drilldown_searches:
    - name: View the detection results for - "$MailboxOwnerUPN$"
      search: '%original_detection_search% | search  MailboxOwnerUPN = "$MailboxOwnerUPN$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$MailboxOwnerUPN$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$MailboxOwnerUPN$") | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: 7d
      latest_offset: "0"
finding:
    title: Inbox folder for the $MailboxOwnerUPN$ mailbox was shared with all users.
    entity:
        field: MailboxOwnerUPN
        type: user
        score: 50
analytic_story:
    - Office 365 Persistence Mechanisms
asset_type: O365 Tenant
mitre_attack_id:
    - T1114.002
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: cloud
security_domain: access
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1114.002/o365_inbox_shared_with_all_users/o365_inbox_shared_with_all_users.log
          source: o365
          sourcetype: o365:management:activity
      test_type: unit

Stages and Predicates

Stage 1: `search`

`o365_management_activity` Operation=ModifyFolderPermissions Workload=Exchange object=Inbox Item.ParentFolder.MemberUpn=Everyone

Stage 2: `eval`

| eval isReadRole=if(match('Item.ParentFolder.MemberRights',"(ReadAny)"), "true", "false")

isReadRole =

ifmatch('Item.ParentFolder.MemberRights', "(ReadAny)")"true"

else"false"

Stage 3: `search`

| search isReadRole = "true"

Stage 4: `rename`

| rename UserId as user

Stage 5: `fillnull`

| fillnull

Stage 6: `stats`

| stats count earliest(_time) as firstTime latest(_time) as lastTime by signature, user, dest, vendor_account, vendor_product, object, MailboxOwnerUPN, Item.ParentFolder.MemberUpn, Item.ParentFolder.MemberRights, src

Stage 7: `search`

| `security_content_ctime(firstTime)`

Stage 8: `search`

| `security_content_ctime(lastTime)`

Stage 9: `search`

| `o365_mailbox_inbox_folder_shared_with_all_users_filter`

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

Field	Kind	Values
`Item.ParentFolder.MemberUpn`	eq	`Everyone`
`Operation`	eq	`ModifyFolderPermissions`
`Workload`	eq	`Exchange`
`isReadRole`	eq	`"true"`
`object`	eq	`Inbox`
`sourcetype`	eq	`o365:management:activity`

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)

O365 Mailbox Inbox Folder Shared with All Users

MITRE ATT&CK coverage

Rules detecting the same action

Rule body splunk

Stages and Predicates

Stage 1: `search`

Stage 2: `eval`

Stage 3: `search`

Stage 4: `rename`

Stage 5: `fillnull`

Stage 6: `stats`

Stage 7: `search`

Stage 8: `search`

Stage 9: `search`

Indicators

Keyboard shortcuts

Search operators

O365 Mailbox Inbox Folder Shared with All Users

MITRE ATT&CK coverage

Rules detecting the same action

Rule body splunk

Stages and Predicates

Stage 1: search

Stage 2: eval

Stage 3: search

Stage 4: rename

Stage 5: fillnull

Stage 6: stats

Stage 7: search

Stage 8: search

Stage 9: search

Indicators

Stage 1: `search`

Stage 2: `eval`

Stage 3: `search`

Stage 4: `rename`

Stage 5: `fillnull`

Stage 6: `stats`

Stage 7: `search`

Stage 8: `search`

Stage 9: `search`