Accessed files shared by temporary external user

Status: available
Severity: low
Time window: 1h
Group by: OfficeObjectId
Source: github.com/Azure/Azure-Sentinel

This detection identifies when an external user is added to a Team or Teams chat and shares a file which is accessed by many users (>10) and the users is removed within short period of time. This might be an indicator of suspicious activity.

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1566` Phishing

Event coverage

Provider	Event
M365-SharePointFileOperation	FileAccessed
M365-SharePointFileOperation	FileUploaded

Rule body kusto

id: bff058b2-500e-4ae5-bb49-a5b1423cbd5b
name: Accessed files shared by temporary external user
description: |
  'This detection identifies when an external user is added to a Team or Teams chat and shares a file which is accessed by many users (>10) and the users is removed within short period of time. This might be an indicator of suspicious activity.'
severity: Low
status: Available 
requiredDataConnectors:
  - connectorId: Office365
    dataTypes:
      - OfficeActivity (Teams)
  - connectorId: Office365
    dataTypes:
      - OfficeActivity (SharePoint)
queryFrequency: 1h
queryPeriod: 1h
triggerOperator: gt
triggerThreshold: 0
tactics:
  - InitialAccess
relevantTechniques:
  - T1566
query: |
  let fileAccessThrehold = 10;
  OfficeActivity
  | where OfficeWorkload =~ "MicrosoftTeams"
  | where Operation =~ "MemberAdded"
  | extend MemberAdded = tostring(parse_json(Members)[0].UPN)
  | where MemberAdded contains ("#EXT#")
  | project TimeAdded=TimeGenerated, Operation, MemberAdded, UserWhoAdded = UserId, TeamName
  | join kind = inner (
    OfficeActivity
    | where OfficeWorkload =~ "MicrosoftTeams"
    | where Operation =~ "MemberRemoved"
    | extend MemberAdded = tostring(parse_json(Members)[0].UPN)
    | where MemberAdded contains ("#EXT#")
    | project TimeDeleted=TimeGenerated, Operation, MemberAdded, UserWhoDeleted = UserId, TeamName
    ) on MemberAdded
  | where TimeDeleted > TimeAdded
  | join kind=inner (
    OfficeActivity
    | where RecordType == "SharePointFileOperation"
    | where SourceRelativeUrl has "Microsoft Teams Chat Files"
    | where Operation == "FileUploaded"
    | extend MemberAdded = UserId
    | join kind = inner (
        OfficeActivity
        | where RecordType == "SharePointFileOperation"
        | where Operation  == "FileAccessed"
        | where SourceRelativeUrl has "Microsoft Teams Chat Files"
        | summarize FileAccessCount = count() by OfficeObjectId
        | where FileAccessCount > fileAccessThrehold
        ) on $left.OfficeObjectId == $right.OfficeObjectId
    )on MemberAdded
  | project-away MemberAdded1, MemberAdded2, OfficeObjectId1, Operation1, Operation2, TeamName1, TeamName2
  | extend MemberAddedAccountName = tostring(split(MemberAdded, "@")[0]), MemberAddedAccountUPNSuffix = tostring(split(MemberAdded, "@")[1])
  | extend UserWhoAddedAccountName = tostring(split(UserWhoAdded, "@")[0]), UserWhoAddedAccountUPNSuffix = tostring(split(UserWhoAdded, "@")[1])
  | extend UserWhoDeletedAccountName = tostring(split(UserWhoDeleted, "@")[0]), UserWhoDeletedAccountUPNSuffix = tostring(split(UserWhoDeleted, "@")[1])
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: MemberAdded
      - identifier: Name
        columnName: MemberAddedAccountName
      - identifier: UPNSuffix
        columnName: MemberAddedAccountUPNSuffix
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: UserWhoAdded
      - identifier: Name
        columnName: UserWhoAddedAccountName
      - identifier: UPNSuffix
        columnName: UserWhoAddedAccountUPNSuffix
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: UserWhoDeleted
      - identifier: Name
        columnName: UserWhoDeletedAccountName
      - identifier: UPNSuffix
        columnName: UserWhoDeletedAccountUPNSuffix
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: ClientIP
version: 2.1.1
kind: Scheduled

Stages and Predicates

Parameters

let fileAccessThrehold = 10;

Stage 1: `source`

OfficeActivity

Stage 2: `where`

| where OfficeWorkload =~ "MicrosoftTeams"

Stage 3: `where`

| where Operation =~ "MemberAdded"

Stage 4: `extend`

| extend MemberAdded = tostring(parse_json(Members)[0].UPN)

Stage 5: `where`

| where MemberAdded contains ("#EXT#")

Stage 6: `project`

| project TimeAdded=TimeGenerated, Operation, MemberAdded, UserWhoAdded = UserId, TeamName

Stage 7: `join`

| join kind = inner (
  OfficeActivity
  | where OfficeWorkload =~ "MicrosoftTeams"
  | where Operation =~ "MemberRemoved"
  | extend MemberAdded = tostring(parse_json(Members)[0].UPN)
  | where MemberAdded contains ("#EXT#")
  | project TimeDeleted=TimeGenerated, Operation, MemberAdded, UserWhoDeleted = UserId, TeamName
  ) on MemberAdded

Stage 8: `where`

| where TimeDeleted > TimeAdded

Stage 9: `join`

| join kind=inner (
  OfficeActivity
  | where RecordType == "SharePointFileOperation"
  | where SourceRelativeUrl has "Microsoft Teams Chat Files"
  | where Operation == "FileUploaded"
  | extend MemberAdded = UserId
  | join kind = inner (
      OfficeActivity
      | where RecordType == "SharePointFileOperation"
      | where Operation  == "FileAccessed"
      | where SourceRelativeUrl has "Microsoft Teams Chat Files"
      | summarize FileAccessCount = count() by OfficeObjectId
      | where FileAccessCount > fileAccessThrehold
      ) on $left.OfficeObjectId == $right.OfficeObjectId
  )on MemberAdded

Stage 10: `project-away`

| project-away MemberAdded1, MemberAdded2, OfficeObjectId1, Operation1, Operation2, TeamName1, TeamName2

Stage 11: `extend` (3 consecutive steps)

| extend MemberAddedAccountName = tostring(split(MemberAdded, "@")[0]), MemberAddedAccountUPNSuffix = tostring(split(MemberAdded, "@")[1])
| extend UserWhoAddedAccountName = tostring(split(UserWhoAdded, "@")[0]), UserWhoAddedAccountUPNSuffix = tostring(split(UserWhoAdded, "@")[1])
| extend UserWhoDeletedAccountName = tostring(split(UserWhoDeleted, "@")[0]), UserWhoDeletedAccountUPNSuffix = tostring(split(UserWhoDeleted, "@")[1])

Stage 12: `summarize`

summarize by OfficeObjectId

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
`FileAccessCount`	gt	`10` transforms: `cased`
`MemberAdded`	contains	`#EXT#`
`OfficeWorkload`	eq	`MicrosoftTeams`
`Operation`	eq	`FileAccessed` transforms: `cased` `FileUploaded` transforms: `cased` `MemberAdded` `MemberRemoved`
`RecordType`	eq	`SharePointFileOperation` transforms: `cased`
`SourceRelativeUrl`	match	`Microsoft Teams Chat Files` transforms: `term`
`TimeDeleted`	gt	`TimeAdded` transforms: `cased`

Output fields

Fields the rule emits when it matches. Chronicle authors list these in the outcome block; they appear on the detection and $risk_score drives alerting. Sentinel / Defender XDR rules build them up through project / summarize / extend stages. Sentinel maps these into alert fields via entityMappings and customDetails; Defender XDR custom detections surface them as alert fields directly.

Field	Source
`OfficeObjectId`	`summarize`

`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)

Accessed files shared by temporary external user

MITRE ATT&CK coverage

Event coverage

Rule body kusto

Stages and Predicates

Parameters

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `extend`

Stage 5: `where`

Stage 6: `project`

Stage 7: `join`

Stage 8: `where`

Stage 9: `join`

Stage 10: `project-away`

Stage 11: `extend` (3 consecutive steps)

Stage 12: `summarize`

Indicators

Output fields

Keyboard shortcuts

Search operators

Accessed files shared by temporary external user

MITRE ATT&CK coverage

Event coverage

Rule body kusto

Stages and Predicates

Parameters

Stage 1: source

Stage 2: where

Stage 3: where

Stage 4: extend

Stage 5: where

Stage 6: project

Stage 7: join

Stage 8: where

Stage 9: join

Stage 10: project-away

Stage 11: extend (3 consecutive steps)

Stage 12: summarize

Indicators

Output fields

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `extend`

Stage 5: `where`

Stage 6: `project`

Stage 7: `join`

Stage 8: `where`

Stage 9: `join`

Stage 10: `project-away`

Stage 11: `extend` (3 consecutive steps)

Stage 12: `summarize`