Possible SignIn from Azure Backdoor

Severity: medium
Time window: 1h
Group by: DomainName, InitiatedBy, UserDomain
Source: github.com/Azure/Azure-Sentinel

Identifies when a user adds an unverified domain as an authentication method, followed by a sign-in from a user the newly added domain. Threat actors may add custom domains to create a backdoor to your tenant. It's important to monitor whenever custom domains are added to the tenant.

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1098` Account Manipulation

Event coverage

Provider	Event
Entra-AuditLogs	Add unverified domain

Rules detecting the same action

Other rules on this platform that filter on the same API call or operation.

Anomalous sign-in location by user account and authenticating application (Kusto)
Anomalous Single Factor Signin (Kusto)
Authentications of Privileged Accounts Outside of Expected Controls (Kusto)
Azure Many Failed SignIns (Panther)
Azure Portal sign in from another Azure Tenant (Kusto)
Azure Service Principal Sign-In Followed by Arc Cluster Credential Access (Elastic)
Azure SignIn via Legacy Authentication Protocol (Panther)
Detect non-admin requesting token for admin applications (Kusto)

Rule body kusto

id: fa00014c-c5f4-4715-8f5b-ba567e19e41e
name: Possible SignIn from Azure Backdoor
description: |
  'Identifies when a user adds an unverified domain as an authentication method, followed by a sign-in from a user the newly added domain. Threat actors may add custom domains to create a backdoor to your tenant. It's important to monitor whenever custom domains are added to the tenant.'
severity: Medium
requiredDataConnectors:
  - connectorId: AzureActiveDirectory
    dataTypes:
      - SigninLogs
      - AuditLogs 
queryFrequency: 1h
queryPeriod: 1h
triggerOperator: gt
triggerThreshold: 0
tactics:
  - Persistence
relevantTechniques:
  - T1098
query: |
  // Microsoft Entra ID  Backdoors: Identity Federation
  //Ref: https://www.inversecos.com/2021/11/how-to-detect-azure-active-directory.html
  AuditLogs
  | where OperationName == "Add unverified domain"
  | where Result == "success"
  | extend InitiatedBy = tostring(parse_json(tostring(InitiatedBy.user)).userPrincipalName)
  | extend DomainName = tostring(TargetResources[0].displayName)
  | summarize DomainAddedTime = min(TimeGenerated), ModifiedProperties = make_set(parse_json(TargetResources[0].modifiedProperties),1048576) by InitiatedBy, DomainName
  | join kind=inner (
  SigninLogs
  | where ResultType == "0"
  | extend UserDomain = tostring(parse_json(split(UserPrincipalName,"@",1)[0]))
  | summarize SignInTime = min(TimeGenerated)  by UserPrincipalName, IPAddress, tostring(LocationDetails),AppDisplayName,ResourceDisplayName,UserDomain
  ) on $left.DomainName == $right.UserDomain
  // Getting UserName and Domain
  | extend Name = split(UserPrincipalName,"@",0), Domain = split(UserPrincipalName,"@",1)
  | mv-expand Name,Domain
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: NTDomain
        columnName: UserDomain
      - identifier: FullName
        columnName: UserPrincipalName
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: IPAddress
customDetails:
  AppDisplayName: AppDisplayName
  ResourceDisplayName: ResourceDisplayName
  DomainAdded: DomainName
  InitiatedBy: InitiatedBy
  ModifiedProperties: ModifiedProperties
  DomainAddedTime: DomainAddedTime
  SignInTime: SignInTime
version: 1.0.0
kind: Scheduled

Stages and Predicates

Stage 1: `source`

AuditLogs

Stage 2: `where`

| where OperationName == "Add unverified domain"

Stage 3: `where`

| where Result == "success"

Stage 4: `extend`

| extend InitiatedBy = tostring(parse_json(tostring(InitiatedBy.user)).userPrincipalName)

Stage 5: `extend`

| extend DomainName = tostring(TargetResources[0].displayName)

Stage 6: `summarize`

| summarize DomainAddedTime = min(TimeGenerated), ModifiedProperties = make_set(parse_json(TargetResources[0].modifiedProperties),1048576) by InitiatedBy, DomainName

Stage 7: `join`

| join kind=inner (
SigninLogs
| where ResultType == "0"
| extend UserDomain = tostring(parse_json(split(UserPrincipalName,"@",1)[0]))
| summarize SignInTime = min(TimeGenerated)  by UserPrincipalName, IPAddress, tostring(LocationDetails),AppDisplayName,ResourceDisplayName,UserDomain
) on $left.DomainName == $right.UserDomain

Stage 8: `extend`

| extend Name = split(UserPrincipalName,"@",0), Domain = split(UserPrincipalName,"@",1)

Stage 9: `mv-expand`

| mv-expand Name,Domain

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

Field	Kind	Values
`OperationName`	eq	`Add unverified domain` transforms: `cased`
`Result`	eq	`success` transforms: `cased`
`ResultType`	eq	`0` transforms: `cased`

Output fields

Fields the rule emits when it matches. Chronicle authors list these in the outcome block; they appear on the detection and $risk_score drives alerting. Sentinel / Defender XDR rules build them up through project / summarize / extend stages. Sentinel maps these into alert fields via entityMappings and customDetails; Defender XDR custom detections surface them as alert fields directly.

Field	Source
`DomainAddedTime`	`summarize`
`DomainName`	`summarize`
`InitiatedBy`	`summarize`
`ModifiedProperties`	`summarize`
`Domain`	`extend`
`Name`	`extend`

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)

Possible SignIn from Azure Backdoor

MITRE ATT&CK coverage

Event coverage

Rules detecting the same action

Rule body kusto

Stages and Predicates

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `extend`

Stage 5: `extend`

Stage 6: `summarize`

Stage 7: `join`

Stage 8: `extend`

Stage 9: `mv-expand`

Indicators

Output fields

Keyboard shortcuts

Search operators

Possible SignIn from Azure Backdoor

MITRE ATT&CK coverage

Event coverage

Rules detecting the same action

Rule body kusto

Stages and Predicates

Stage 1: source

Stage 2: where

Stage 3: where

Stage 4: extend

Stage 5: extend

Stage 6: summarize

Stage 7: join

Stage 8: extend

Stage 9: mv-expand

Indicators

Output fields

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `extend`

Stage 5: `extend`

Stage 6: `summarize`

Stage 7: `join`

Stage 8: `extend`

Stage 9: `mv-expand`