NRT Multiple users email forwarded to same destination

Severity: medium
Group by: ClientIP
Author: Pete Bryan
Source: github.com/Azure/Azure-Sentinel

'Identifies when multiple (more than one) users mailboxes are configured to forward to the same destination. This could be an attacker-controlled destination mailbox configured to collect mail from multiple compromised user accounts.'

MITRE ATT&CK coverage

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

Rules detecting the same action

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

Multiple users email forwarded to same destination (Kusto)
NRT Malicious Inbox Rule (Kusto)

Rule body kusto

id: 3b05727d-a8d1-477d-bbdd-d957da96ac7b
name: NRT Multiple users email forwarded to same destination
description: |
  'Identifies when multiple (more than one) users mailboxes are configured to forward to the same destination.
  This could be an attacker-controlled destination mailbox configured to collect mail from multiple compromised user accounts.'
severity: Medium
requiredDataConnectors:
  - connectorId: Office365
    dataTypes:
      - OfficeActivity
tactics:
  - Collection
  - Exfiltration
relevantTechniques:
  - T1114
  - T1020
query: |
  OfficeActivity
  | where OfficeWorkload =~ "Exchange"
  | where Parameters has_any ("ForwardTo", "RedirectTo", "ForwardingSmtpAddress")
  | mv-apply DynamicParameters = todynamic(Parameters) on (summarize ParsedParameters = make_bag(pack(tostring(DynamicParameters.Name), DynamicParameters.Value)))
  | evaluate bag_unpack(ParsedParameters, columnsConflict='replace_source')
  | extend DestinationMailAddress = tolower(case(
      isnotempty(column_ifexists("ForwardTo", "")), column_ifexists("ForwardTo", ""),
      isnotempty(column_ifexists("RedirectTo", "")), column_ifexists("RedirectTo", ""),
      isnotempty(column_ifexists("ForwardingSmtpAddress", "")), trim_start(@"smtp:", column_ifexists("ForwardingSmtpAddress", "")),
      ""))
  | where isnotempty(DestinationMailAddress)
  | mv-expand split(DestinationMailAddress, ";")
  | extend ClientIPValues = extract_all(@'\[?(::ffff:)?(?P<IPAddress>(\d+\.\d+\.\d+\.\d+)|[^\]]+)\]?([-:](?P<Port>\d+))?', dynamic(["IPAddress", "Port"]), ClientIP)[0]
  | extend ClientIP = tostring(ClientIPValues[0]), Port = tostring(ClientIPValues[1])
  | summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated), DistinctUserCount = dcount(UserId), UserId = make_set(UserId, 250), Ports = make_set(Port, 250), EventCount = count() by tostring(DestinationMailAddress), ClientIP
  | where DistinctUserCount > 1
  | mv-expand UserId to typeof(string)
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: UserId
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: ClientIP
version: 1.0.3
kind: NRT
metadata:
    source:
        kind: Community
    author:
        name: Pete Bryan
    support:
        tier: Community
    categories:
        domains: [ "Security - Threat Protection" ]

Stages and Predicates

Stage 1: `source`

OfficeActivity

Stage 2: `where`

| where OfficeWorkload =~ "Exchange"

Stage 3: `where`

| where Parameters has_any ("ForwardTo", "RedirectTo", "ForwardingSmtpAddress")

Stage 4: `kusto:mv-apply`

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

Stage 5: `evaluate`

| evaluate bag_unpack(ParsedParameters, columnsConflict='replace_source')

Stage 6: `extend`

| extend DestinationMailAddress = tolower(case(
    isnotempty(column_ifexists("ForwardTo", "")), column_ifexists("ForwardTo", ""),
    isnotempty(column_ifexists("RedirectTo", "")), column_ifexists("RedirectTo", ""),
    isnotempty(column_ifexists("ForwardingSmtpAddress", "")), trim_start(@"smtp:", column_ifexists("ForwardingSmtpAddress", "")),
    ""))

Stage 7: `where`

| where isnotempty(DestinationMailAddress)

Stage 8: `mv-expand`

| mv-expand split(DestinationMailAddress, ";")

Stage 9: `extend`

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

Stage 10: `extend`

| extend ClientIP = tostring(ClientIPValues[0]), Port = tostring(ClientIPValues[1])

Stage 11: `summarize`

| summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated), DistinctUserCount = dcount(UserId), UserId = make_set(UserId, 250), Ports = make_set(Port, 250), EventCount = count() by tostring(DestinationMailAddress), ClientIP

Threshold: gt 1

Stage 12: `where`

| where DistinctUserCount > 1

Stage 13: `mv-expand`

| mv-expand UserId to typeof(string)

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
`DestinationMailAddress`	is_not_null	(no value, null check)
`DistinctUserCount`	gt	`1` transforms: `cased`
`OfficeWorkload`	eq	`Exchange`
`Parameters`	match	`ForwardTo` `ForwardingSmtpAddress` `RedirectTo`

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
`ClientIP`	`summarize`
`DistinctUserCount`	`summarize`
`EndTime`	`summarize`
`EventCount`	`summarize`
`Ports`	`summarize`
`StartTime`	`summarize`
`UserId`	`summarize`

`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)

NRT Multiple users email forwarded to same destination

MITRE ATT&CK 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: `evaluate`

Stage 6: `extend`

Stage 7: `where`

Stage 8: `mv-expand`

Stage 9: `extend`

Stage 10: `extend`

Stage 11: `summarize`

Stage 12: `where`

Stage 13: `mv-expand`

Indicators

Output fields

Keyboard shortcuts

Search operators

NRT Multiple users email forwarded to same destination

MITRE ATT&CK 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: evaluate

Stage 6: extend

Stage 7: where

Stage 8: mv-expand

Stage 9: extend

Stage 10: extend

Stage 11: summarize

Stage 12: where

Stage 13: mv-expand

Indicators

Output fields

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `kusto:mv-apply`

Stage 5: `evaluate`

Stage 6: `extend`

Stage 7: `where`

Stage 8: `mv-expand`

Stage 9: `extend`

Stage 10: `extend`

Stage 11: `summarize`

Stage 12: `where`

Stage 13: `mv-expand`