Tailscale: Split-DNS configuration modified

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

Identifies when the tailnet split-DNS configuration is modified. Split-DNS overrides per-domain resolution within the tailnet - an attacker adding a new domain mapping or changing the resolver IP can hijack DNS for that domain.

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1556` Modify Authentication Process
Defense Impairment	`T1556` Modify Authentication Process
Credential Access	`T1556` Modify Authentication Process
Command & Control	`T1568` Dynamic Resolution

Rule body

id: b4c5d6e7-1234-5678-90ab-cdef12345010
name: "Tailscale: Split-DNS configuration modified"
description: |
  Identifies when the tailnet split-DNS configuration is modified. Split-DNS overrides per-domain resolution within the tailnet - an attacker adding a new domain mapping or changing the resolver IP can hijack DNS for that domain.
severity: High
status: Available
requiredDataConnectors:
  - connectorId: TailscaleCCF
    dataTypes:
      - Tailscale_Audit_CL
queryFrequency: 15m
queryPeriod: 15m
triggerOperator: gt
triggerThreshold: 0
tactics:
  - DefenseEvasion
  - CommandAndControl
relevantTechniques:
  - T1556
  - T1568
query: |
  Tailscale_Audit_CL
  | where Action == "UPDATE"
  | where tostring(Target.type) == "TAILNET"
  | where tostring(Target.property) == "DNS_SPLIT_DNS"
  | extend ActorLogin = tostring(Actor.loginName)
  | extend OldDomains = bag_keys(Old)
  | extend NewDomains = bag_keys(New)
  | extend AddedDomains = set_difference(NewDomains, OldDomains)
  | extend RemovedDomains = set_difference(OldDomains, NewDomains)
  | project TimeGenerated, ActorLogin, AddedDomains, RemovedDomains, Old, New, Origin
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: ActorLogin
incidentConfiguration:
  createIncident: true
  groupingConfiguration:
    enabled: true
    reopenClosedIncident: false
    lookbackDuration: P1D
    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) == "DNS_SPLIT_DNS"

Stage 5: `extend` (5 consecutive steps)

| extend ActorLogin = tostring(Actor.loginName)
| extend OldDomains = bag_keys(Old)
| extend NewDomains = bag_keys(New)
| extend AddedDomains = set_difference(NewDomains, OldDomains)
| extend RemovedDomains = set_difference(OldDomains, NewDomains)

Stage 6: `project`

| project TimeGenerated, ActorLogin, AddedDomains, RemovedDomains, Old, New, Origin

Indicators

These rows show field, operator, and value matches.

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

Output fields

These fields are emitted when the rule matches.

Field	Source
`ActorLogin`	`project`
`AddedDomains`	`project`
`New`	`project`
`Old`	`project`
`Origin`	`project`
`RemovedDomains`	`project`
`TimeGenerated`	`project`

Tailscale: Split-DNS configuration 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 (5 consecutive steps)

Stage 6: project