Tailscale: DNS nameservers modified

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

Identifies when the tailnet's global DNS nameserver list is modified. Adding an attacker-controlled resolver as a tailnet-wide nameserver enables broad DNS hijacking for every device using MagicDNS resolution.

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: c5d6e7f8-2345-6789-01ab-cdef12345011
name: "Tailscale: DNS nameservers modified"
description: |
  Identifies when the tailnet's global DNS nameserver list is modified. Adding an attacker-controlled resolver as a tailnet-wide nameserver enables broad DNS hijacking for every device using MagicDNS resolution.
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_NAMESERVERS"
  | extend ActorLogin = tostring(Actor.loginName)
  | extend OldServers = Old.dns
  | extend NewServers = New.dns
  | extend Added = set_difference(NewServers, OldServers)
  | extend Removed = set_difference(OldServers, NewServers)
  | project TimeGenerated, ActorLogin, OldServers, NewServers, Added, Removed, 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_NAMESERVERS"

Stage 5: `extend` (5 consecutive steps)

| extend ActorLogin = tostring(Actor.loginName)
| extend OldServers = Old.dns
| extend NewServers = New.dns
| extend Added = set_difference(NewServers, OldServers)
| extend Removed = set_difference(OldServers, NewServers)

Stage 6: `project`

| project TimeGenerated, ActorLogin, OldServers, NewServers, Added, Removed, 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_NAMESERVERS` transforms: `tostring`	`field:"property" kind:eq value:"DNS_NAMESERVERS"`
`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`
`Added`	`project`
`NewServers`	`project`
`OldServers`	`project`
`Origin`	`project`
`Removed`	`project`
`TimeGenerated`	`project`

Tailscale: DNS nameservers 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