Tailscale External shared-in device added

let recent =
    Tailscale_Devices_CL
    | where TimeGenerated > ago(1h)
    | summarize arg_max(TimeGenerated, *) by DeviceId
    | where IsExternal == true
    | project DeviceId, DeviceName, Hostname, User, Os, ClientVersion, Created, LastSeen, Tags;
let prior =
    Tailscale_Devices_CL
    | where TimeGenerated between (ago(2d) .. ago(1h))
    | summarize arg_max(TimeGenerated, *) by DeviceId
    | where IsExternal == true
    | distinct DeviceId;
recent
| join kind=leftanti prior on DeviceId
| project TimeGenerated = now(), DeviceName, Hostname, User, Os, ClientVersion, Created, LastSeen, Tags

entityMappings:
- entityType: Host
  fieldMappings:
  - identifier: HostName
    columnName: Hostname
- entityType: Account
  fieldMappings:
  - identifier: FullName
    columnName: User
name: 'Tailscale: External (shared-in) device added'
kind: Scheduled
triggerOperator: gt
status: Available
queryPeriod: 2d
requiredDataConnectors:
- connectorId: TailscaleCCF
  dataTypes:
  - Tailscale_Devices_CL
tactics:
- InitialAccess
query: |
  let recent =
      Tailscale_Devices_CL
      | where TimeGenerated > ago(1h)
      | summarize arg_max(TimeGenerated, *) by DeviceId
      | where IsExternal == true
      | project DeviceId, DeviceName, Hostname, User, Os, ClientVersion, Created, LastSeen, Tags;
  let prior =
      Tailscale_Devices_CL
      | where TimeGenerated between (ago(2d) .. ago(1h))
      | summarize arg_max(TimeGenerated, *) by DeviceId
      | where IsExternal == true
      | distinct DeviceId;
  recent
  | join kind=leftanti prior on DeviceId
  | project TimeGenerated = now(), DeviceName, Hostname, User, Os, ClientVersion, Created, LastSeen, Tags
description: |
  Identifies new external (shared-in) devices joining the tailnet that were not present in the prior 24-hour baseline. Each shared-in device expands the trust boundary - confirm the share matches a documented agreement and ACL scope.
severity: Medium
incidentConfiguration:
  groupingConfiguration:
    lookbackDuration: P1D
    enabled: true
    reopenClosedIncident: false
    matchingMethod: AllEntities
  createIncident: true
id: b2c3d4e5-6789-0123-4567-890123450043
triggerThreshold: 0
queryFrequency: 1h
OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Tailscale%20%28CCF%29/Analytic%20Rules/TailscaleExternalDeviceAdded.yaml
version: 1.0.0
relevantTechniques:
- T1078

{
  "$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/b2c3d4e5-6789-0123-4567-890123450043')]",
      "kind": "Scheduled",
      "name": "[concat(parameters('workspace'),'/Microsoft.SecurityInsights/b2c3d4e5-6789-0123-4567-890123450043')]",
      "properties": {
        "alertRuleTemplateName": "b2c3d4e5-6789-0123-4567-890123450043",
        "customDetails": null,
        "description": "Identifies new external (shared-in) devices joining the tailnet that were not present in the prior 24-hour baseline. Each shared-in device expands the trust boundary - confirm the share matches a documented agreement and ACL scope.\n",
        "displayName": "Tailscale: External (shared-in) device added",
        "enabled": true,
        "entityMappings": [
          {
            "entityType": "Host",
            "fieldMappings": [
              {
                "columnName": "Hostname",
                "identifier": "HostName"
              }
            ]
          },
          {
            "entityType": "Account",
            "fieldMappings": [
              {
                "columnName": "User",
                "identifier": "FullName"
              }
            ]
          }
        ],
        "incidentConfiguration": {
          "createIncident": true,
          "groupingConfiguration": {
            "enabled": true,
            "lookbackDuration": "P1D",
            "matchingMethod": "AllEntities",
            "reopenClosedIncident": false
          }
        },
        "OriginalUri": "https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Tailscale%20%28CCF%29/Analytic%20Rules/TailscaleExternalDeviceAdded.yaml",
        "query": "let recent =\n    Tailscale_Devices_CL\n    | where TimeGenerated > ago(1h)\n    | summarize arg_max(TimeGenerated, *) by DeviceId\n    | where IsExternal == true\n    | project DeviceId, DeviceName, Hostname, User, Os, ClientVersion, Created, LastSeen, Tags;\nlet prior =\n    Tailscale_Devices_CL\n    | where TimeGenerated between (ago(2d) .. ago(1h))\n    | summarize arg_max(TimeGenerated, *) by DeviceId\n    | where IsExternal == true\n    | distinct DeviceId;\nrecent\n| join kind=leftanti prior on DeviceId\n| project TimeGenerated = now(), DeviceName, Hostname, User, Os, ClientVersion, Created, LastSeen, Tags\n",
        "queryFrequency": "PT1H",
        "queryPeriod": "P2D",
        "severity": "Medium",
        "status": "Available",
        "subTechniques": [],
        "suppressionDuration": "PT1H",
        "suppressionEnabled": false,
        "tactics": [
          "InitialAccess"
        ],
        "techniques": [
          "T1078"
        ],
        "templateVersion": "1.0.0",
        "triggerOperator": "GreaterThan",
        "triggerThreshold": 0
      },
      "type": "Microsoft.OperationalInsights/workspaces/providers/alertRules"
    }
  ]
}