Mail redirect via ExO transport rule

Status: available
Severity: medium
Time window: 1h
Source: github.com/Azure/Azure-Sentinel

Identifies when Exchange Online transport rule configured to forward emails. This could be an adversary mailbox configured to collect mail from multiple user accounts.

MITRE ATT&CK coverage

Tactic	Techniques
Collection	`T1114` Email Collection
Exfiltration	`T1020` Automated Exfiltration

Event coverage

Provider	Event
M365-ExchangeAdmin	New-TransportRule
M365-ExchangeAdmin	Set-TransportRule

Rules detecting the same action

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

M365 Exchange Inbox Forwarding Rule Created (Elastic)
M365 Exchange Mail Flow Transport Rule Created (Elastic)
Threat Essentials - Mail redirect via ExO transport rule (Kusto)

Rule body kusto

id: 500415fb-bba7-4227-a08a-9857fb61b6a7
name: Mail redirect via ExO transport rule
description: |
  'Identifies when Exchange Online transport rule configured to forward emails.
  This could be an adversary mailbox configured to collect mail from multiple user accounts.'
severity: Medium
status: Available 
requiredDataConnectors:
  - connectorId: Office365
    dataTypes:
      - OfficeActivity (Exchange)
queryFrequency: 1h
queryPeriod: 1h
triggerOperator: gt
triggerThreshold: 0
tactics:
  - Collection
  - Exfiltration
relevantTechniques:
  - T1114
  - T1020
query: |
  OfficeActivity
  | where OfficeWorkload == "Exchange"
  | where Operation in~ ("New-TransportRule", "Set-TransportRule")
  | mv-apply DynamicParameters = todynamic(Parameters) on (summarize ParsedParameters = make_bag(pack(tostring(DynamicParameters.Name), DynamicParameters.Value)))
  | extend RuleName = case(
      Operation =~ "Set-TransportRule", OfficeObjectId,
      Operation =~ "New-TransportRule", ParsedParameters.Name,
      "Unknown")
  | mv-expand ExpandedParameters = todynamic(Parameters)
  | where ExpandedParameters.Name in~ ("BlindCopyTo", "RedirectMessageTo") and isnotempty(ExpandedParameters.Value)
  | extend RedirectTo = ExpandedParameters.Value
  | extend ClientIPValues = extract_all(@'\[?(::ffff:)?(?P<IPAddress>(\d+\.\d+\.\d+\.\d+)|[^\]]+)\]?([-:](?P<Port>\d+))?', dynamic(["IPAddress", "Port"]), ClientIP)[0]
  | extend From = ParsedParameters.From
  | project TimeGenerated, RedirectTo, IPAddress = tostring(ClientIPValues[0]), Port = tostring(ClientIPValues[1]), UserId, From, Operation, RuleName, Parameters
  | extend AccountName = tostring(split(UserId, "@")[0]), AccountUPNSuffix = tostring(split(UserId, "@")[1])
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: UserId
      - identifier: Name
        columnName: AccountName
      - identifier: UPNSuffix
        columnName: AccountUPNSuffix
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: IPAddress
version: 2.0.4
kind: Scheduled

Stages and Predicates

Stage 1: `source`

OfficeActivity

Stage 2: `where`

| where OfficeWorkload == "Exchange"

Stage 3: `where`

| where Operation in~ ("New-TransportRule", "Set-TransportRule")

Stage 4: `kusto:mv-apply`

| mv-apply DynamicParameters = todynamic(Parameters) on (summarize ParsedParameters = make_bag(pack(tostring(DynamicParameters.Name), DynamicParameters.Value)))

Stage 5: `extend`

| extend RuleName = case(
    Operation =~ "Set-TransportRule", OfficeObjectId,
    Operation =~ "New-TransportRule", ParsedParameters.Name,
    "Unknown")

RuleName =

ifOperation =~ "Set-TransportRule"OfficeObjectId

elifOperation =~ "New-TransportRule"ParsedParameters.Name

else"Unknown"

Stage 6: `mv-expand`

| mv-expand ExpandedParameters = todynamic(Parameters)

Stage 7: `where`

| where ExpandedParameters.Name in~ ("BlindCopyTo", "RedirectMessageTo") and isnotempty(ExpandedParameters.Value)

Stage 8: `extend` (3 consecutive steps)

| extend RedirectTo = ExpandedParameters.Value
| extend ClientIPValues = extract_all(@'\[?(::ffff:)?(?P<IPAddress>(\d+\.\d+\.\d+\.\d+)|[^\]]+)\]?([-:](?P<Port>\d+))?', dynamic(["IPAddress", "Port"]), ClientIP)[0]
| extend From = ParsedParameters.From

Stage 9: `project`

| project TimeGenerated, RedirectTo, IPAddress = tostring(ClientIPValues[0]), Port = tostring(ClientIPValues[1]), UserId, From, Operation, RuleName, Parameters

Stage 10: `extend`

| extend AccountName = tostring(split(UserId, "@")[0]), AccountUPNSuffix = tostring(split(UserId, "@")[1])

Stage 11: `summarize`

summarize

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
`Name`	in	`BlindCopyTo` `RedirectMessageTo`
`OfficeWorkload`	eq	`Exchange` transforms: `cased`
`Operation`	in	`New-TransportRule` `Set-TransportRule`
`Value`	is_not_null	(no value, null check)

`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)

Mail redirect via ExO transport rule

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: `kusto:mv-apply`

Stage 5: `extend`

Stage 6: `mv-expand`

Stage 7: `where`

Stage 8: `extend` (3 consecutive steps)

Stage 9: `project`

Stage 10: `extend`

Stage 11: `summarize`

Indicators

Keyboard shortcuts

Search operators

Mail redirect via ExO transport rule

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: kusto:mv-apply

Stage 5: extend

Stage 6: mv-expand

Stage 7: where

Stage 8: extend (3 consecutive steps)

Stage 9: project

Stage 10: extend

Stage 11: summarize

Indicators

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `kusto:mv-apply`

Stage 5: `extend`

Stage 6: `mv-expand`

Stage 7: `where`

Stage 8: `extend` (3 consecutive steps)

Stage 9: `project`

Stage 10: `extend`

Stage 11: `summarize`