Accessed files shared by temporary external user

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Id	bff058b2-500e-4ae5-bb49-a5b1423cbd5b
Rulename	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
Tactics	InitialAccess
Techniques	T1566
Required data connectors	Office365
Kind	Scheduled
Query frequency	1h
Query period	1h
Trigger threshold	0
Trigger operator	gt
Source Uri	https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Microsoft 365/Analytic Rules/External User added to Team and immediately uploads file.yaml
Version	2.1.1
Arm template	bff058b2-500e-4ae5-bb49-a5b1423cbd5b.json

KQL

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])

YAML

relevantTechniques:
- T1566
name: Accessed files shared by temporary external user
requiredDataConnectors:
- dataTypes:
  - OfficeActivity (Teams)
  connectorId: Office365
- dataTypes:
  - OfficeActivity (SharePoint)
  connectorId: Office365
entityMappings:
- 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: Account
- fieldMappings:
  - identifier: Address
    columnName: ClientIP
  entityType: IP
triggerThreshold: 0
id: bff058b2-500e-4ae5-bb49-a5b1423cbd5b
tactics:
- InitialAccess
version: 2.1.1
OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Microsoft 365/Analytic Rules/External User added to Team and immediately uploads file.yaml
queryPeriod: 1h
kind: Scheduled
queryFrequency: 1h
severity: Low
status: Available
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.'
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])  
triggerOperator: gt

ARM

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "workspace": {
      "type": "String"
    }
  },
  "resources": [
    {
      "apiVersion": "2024-01-01-preview",
      "id": "[concat(resourceId('Microsoft.OperationalInsights/workspaces/providers', parameters('workspace'), 'Microsoft.SecurityInsights'),'/alertRules/bff058b2-500e-4ae5-bb49-a5b1423cbd5b')]",
      "kind": "Scheduled",
      "name": "[concat(parameters('workspace'),'/Microsoft.SecurityInsights/bff058b2-500e-4ae5-bb49-a5b1423cbd5b')]",
      "properties": {
        "alertRuleTemplateName": "bff058b2-500e-4ae5-bb49-a5b1423cbd5b",
        "customDetails": null,
        "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.'\n",
        "displayName": "Accessed files shared by temporary external user",
        "enabled": true,
        "entityMappings": [
          {
            "entityType": "Account",
            "fieldMappings": [
              {
                "columnName": "MemberAdded",
                "identifier": "FullName"
              },
              {
                "columnName": "MemberAddedAccountName",
                "identifier": "Name"
              },
              {
                "columnName": "MemberAddedAccountUPNSuffix",
                "identifier": "UPNSuffix"
              }
            ]
          },
          {
            "entityType": "Account",
            "fieldMappings": [
              {
                "columnName": "UserWhoAdded",
                "identifier": "FullName"
              },
              {
                "columnName": "UserWhoAddedAccountName",
                "identifier": "Name"
              },
              {
                "columnName": "UserWhoAddedAccountUPNSuffix",
                "identifier": "UPNSuffix"
              }
            ]
          },
          {
            "entityType": "Account",
            "fieldMappings": [
              {
                "columnName": "UserWhoDeleted",
                "identifier": "FullName"
              },
              {
                "columnName": "UserWhoDeletedAccountName",
                "identifier": "Name"
              },
              {
                "columnName": "UserWhoDeletedAccountUPNSuffix",
                "identifier": "UPNSuffix"
              }
            ]
          },
          {
            "entityType": "IP",
            "fieldMappings": [
              {
                "columnName": "ClientIP",
                "identifier": "Address"
              }
            ]
          }
        ],
        "OriginalUri": "https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Microsoft 365/Analytic Rules/External User added to Team and immediately uploads file.yaml",
        "query": "let fileAccessThrehold = 10;\nOfficeActivity\n| where OfficeWorkload =~ \"MicrosoftTeams\"\n| where Operation =~ \"MemberAdded\"\n| extend MemberAdded = tostring(parse_json(Members)[0].UPN)\n| where MemberAdded contains (\"#EXT#\")\n| project TimeAdded=TimeGenerated, Operation, MemberAdded, UserWhoAdded = UserId, TeamName\n| join kind = inner (\n  OfficeActivity\n  | where OfficeWorkload =~ \"MicrosoftTeams\"\n  | where Operation =~ \"MemberRemoved\"\n  | extend MemberAdded = tostring(parse_json(Members)[0].UPN)\n  | where MemberAdded contains (\"#EXT#\")\n  | project TimeDeleted=TimeGenerated, Operation, MemberAdded, UserWhoDeleted = UserId, TeamName\n  ) on MemberAdded\n| where TimeDeleted > TimeAdded\n| join kind=inner (\n  OfficeActivity\n  | where RecordType == \"SharePointFileOperation\"\n  | where SourceRelativeUrl has \"Microsoft Teams Chat Files\"\n  | where Operation == \"FileUploaded\"\n  | extend MemberAdded = UserId\n  | join kind = inner (\n      OfficeActivity\n      | where RecordType == \"SharePointFileOperation\"\n      | where Operation  == \"FileAccessed\"\n      | where SourceRelativeUrl has \"Microsoft Teams Chat Files\"\n      | summarize FileAccessCount = count() by OfficeObjectId\n      | where FileAccessCount > fileAccessThrehold\n      ) on $left.OfficeObjectId == $right.OfficeObjectId\n  )on MemberAdded\n| project-away MemberAdded1, MemberAdded2, OfficeObjectId1, Operation1, Operation2, TeamName1, TeamName2\n| extend MemberAddedAccountName = tostring(split(MemberAdded, \"@\")[0]), MemberAddedAccountUPNSuffix = tostring(split(MemberAdded, \"@\")[1])\n| extend UserWhoAddedAccountName = tostring(split(UserWhoAdded, \"@\")[0]), UserWhoAddedAccountUPNSuffix = tostring(split(UserWhoAdded, \"@\")[1])\n| extend UserWhoDeletedAccountName = tostring(split(UserWhoDeleted, \"@\")[0]), UserWhoDeletedAccountUPNSuffix = tostring(split(UserWhoDeleted, \"@\")[1])\n",
        "queryFrequency": "PT1H",
        "queryPeriod": "PT1H",
        "severity": "Low",
        "status": "Available",
        "subTechniques": [],
        "suppressionDuration": "PT1H",
        "suppressionEnabled": false,
        "tactics": [
          "InitialAccess"
        ],
        "techniques": [
          "T1566"
        ],
        "templateVersion": "2.1.1",
        "triggerOperator": "GreaterThan",
        "triggerThreshold": 0
      },
      "type": "Microsoft.OperationalInsights/workspaces/providers/alertRules"
    }
  ]
}