Tailscale: Policy file (ACL) modified

Status: available
Severity: medium
Time window: 15m
Source: github.com/Azure/Azure-Sentinel

Identifies when the tailnet ACL/policy file is modified. Review the diff - incorrect ACLs can silently expand blast radius across the tailnet.

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1556` Modify Authentication Process

Rule body

id: 1e7249c2-1a9d-05fd-45cb-c859eef5b8ae
name: "Tailscale: Policy file (ACL) modified"
description: |
  Identifies when the tailnet ACL/policy file is modified. Review the diff - incorrect ACLs can silently expand blast radius across the tailnet.
severity: Medium
status: Available
requiredDataConnectors:
  - connectorId: TailscaleCCF
    dataTypes:
      - Tailscale_Audit_CL
queryFrequency: 15m
queryPeriod: 15m
triggerOperator: gt
triggerThreshold: 0
tactics:
  - DefenseEvasion
  - Persistence
relevantTechniques:
  - T1556
query: |
  Tailscale_Audit_CL
  | where Action == "UPDATE"
  | where tostring(Target.type) == "TAILNET"
  | where tostring(Target.property) == "ACL"
  | extend ActorLogin = tostring(Actor.loginName)
  | project TimeGenerated, ActorLogin, Action, Target, Origin, Old, New
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: ActorLogin
incidentConfiguration:
  createIncident: true
  groupingConfiguration:
    enabled: true
    reopenClosedIncident: false
    lookbackDuration: PT5H
    matchingMethod: AllEntities
kind: Scheduled
version: 1.0.0

Stages and Predicates

Stage 1: `source`

Tailscale_Audit_CL

Stage 2: `where`

| where Action == "UPDATE"

Stage 3: `where`

| where tostring(Target.type) == "TAILNET"

Stage 4: `where`

| where tostring(Target.property) == "ACL"

Stage 5: `extend`

| extend ActorLogin = tostring(Actor.loginName)

Stage 6: `project`

| project TimeGenerated, ActorLogin, Action, Target, Origin, Old, New

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`Action`	eq	`UPDATE`	`field:"Action" kind:eq value:"UPDATE"`
`property`	eq	`ACL` transforms: `tostring`	`field:"property" kind:eq value:"ACL"`
`type`	eq	`TAILNET` transforms: `tostring`	`field:"type" kind:eq value:"TAILNET"`

Output fields

These fields are emitted when the rule matches.

Field	Source
`Action`	`project`
`ActorLogin`	`project`
`New`	`project`
`Old`	`project`
`Origin`	`project`
`Target`	`project`
`TimeGenerated`	`project`

Tailscale: Policy file (ACL) modified

MITRE ATT&CK coverage

Rule body

Stages and Predicates

Stage 1: source

Stage 2: where

Stage 3: where

Stage 4: where

Stage 5: extend

Stage 6: project