Malicious BEC Inbox Rule

Severity: medium
Time window: 1d
Group by: ClientIPAddress, Keyword, OfficeObjectId, Operation, OriginatingServer, ResultStatus, RuleDetail, UserId
Source: github.com/Azure/Azure-Sentinel

'Often times after the initial compromise in a BEC attack the attackers create inbox rules to delete emails that contain certain keywords related to their BEC attack. This is done so as to limit ability to warn compromised users that they've been compromised.

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1078` Valid Accounts, `T1098` Account Manipulation
Stealth	`T1078` Valid Accounts

Event coverage

Provider	Event
M365-ExchangeItem	New-InboxRule

Rules detecting the same action

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

Inbox Rules Creation Or Update Activity in O365 (Sigma)
M365 Exchange Inbox Phishing Evasion Rule Created (Elastic)
Mail Forwarding/Redirecting Activity In O365 (Sigma)
Rare and potentially high-risk Office operations (Kusto)

Rule body kusto

id: 8ac77493-3cae-4840-8634-15fb23f8fb68
name: Malicious BEC Inbox Rule
description: |
  'Often times after the initial compromise in a BEC attack the attackers create inbox rules to delete emails that contain certain keywords related to their BEC attack.
   This is done so as to limit ability to warn compromised users that they've been compromised. 
severity: Medium
requiredDataConnectors:
  - connectorId: Office365
    dataTypes:
      - OfficeActivity
queryFrequency: 1d
queryPeriod: 1d
triggerOperator: gt
triggerThreshold: 0
tactics:
  - Persistence
  - DefenseEvasion
relevantTechniques:
  - T1098
  - T1078
tags:
  - BEC
query: |
 let BEC_Keywords = dynamic([ 'invoice','payment','paycheck','transfer','bank statement','bank details','closing','funds','bank account','account details','remittance','purchase','deposit',"PO#","Zahlung","Rechnung","Paiement", "virement bancaire","Bankuberweisung",'hacked','phishing']);
 OfficeActivity
 | where Operation =~ "New-InboxRule"
 | where Parameters has "Deleted Items" or Parameters has "Junk Email"  or Parameters has "DeleteMessage"
 | extend Events=todynamic(Parameters)
 | parse Events  with * "SubjectContainsWords" SubjectContainsWords '}'*
 | parse Events  with * "BodyContainsWords" BodyContainsWords '}'*
 | parse Events  with * "SubjectOrBodyContainsWords" SubjectOrBodyContainsWords '}'*
 | where SubjectContainsWords has_any (BEC_Keywords)
  or BodyContainsWords has_any (BEC_Keywords)
  or SubjectOrBodyContainsWords has_any (BEC_Keywords)
 | extend ClientIPAddress = case( ClientIP has ".", tostring(split(ClientIP,":")[0]), ClientIP has "[", tostring(trim_start(@'[[]',tostring(split(ClientIP,"]")[0]))), ClientIP )
 | extend Keyword = iff(isnotempty(SubjectContainsWords), SubjectContainsWords, (iff(isnotempty(BodyContainsWords),BodyContainsWords,SubjectOrBodyContainsWords )))
 | extend RuleDetail = case(OfficeObjectId contains '/' , tostring(split(OfficeObjectId, '/')[-1]) , tostring(split(OfficeObjectId, '\\')[-1]))
 | summarize count(), StartTimeUtc = min(TimeGenerated), EndTimeUtc = max(TimeGenerated) by  Operation, UserId, ClientIPAddress, ResultStatus, Keyword, OriginatingServer, OfficeObjectId, RuleDetail
 | extend UserName = split(UserId, '@')[0], DomainName = split(UserId, '@')[1]
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: UserId
      - identifier: Name
        columnName: UserName
      - identifier: UPNSuffix
        columnName: DomainName
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: ClientIPAddress
version: 1.0.2
kind: Scheduled

Stages and Predicates

Let binding: `BEC_Keywords`

let BEC_Keywords = dynamic([ 'invoice','payment','paycheck','transfer','bank statement','bank details','closing','funds','bank account','account details','remittance','purchase','deposit',"PO#","Zahlung","Rechnung","Paiement", "virement bancaire","Bankuberweisung",'hacked','phishing']);

Stage 1: `source`

OfficeActivity

Stage 2: `where`

| where Operation =~ "New-InboxRule"

Stage 3: `where`

| where Parameters has "Deleted Items" or Parameters has "Junk Email"  or Parameters has "DeleteMessage"

Stage 4: `extend`

| extend Events=todynamic(Parameters)

Stage 5: `parse`

| parse Events  with * "SubjectContainsWords" SubjectContainsWords '}'*

Stage 6: `parse`

| parse Events  with * "BodyContainsWords" BodyContainsWords '}'*

Stage 7: `parse`

| parse Events  with * "SubjectOrBodyContainsWords" SubjectOrBodyContainsWords '}'*

Stage 8: `where`

| where SubjectContainsWords has_any (BEC_Keywords)
 or BodyContainsWords has_any (BEC_Keywords)
 or SubjectOrBodyContainsWords has_any (BEC_Keywords)

References BEC_Keywords (defined above).

Stage 9: `extend` (3 consecutive steps)

| extend ClientIPAddress = case( ClientIP has ".", tostring(split(ClientIP,":")[0]), ClientIP has "[", tostring(trim_start(@'[[]',tostring(split(ClientIP,"]")[0]))), ClientIP )
| extend Keyword = iff(isnotempty(SubjectContainsWords), SubjectContainsWords, (iff(isnotempty(BodyContainsWords),BodyContainsWords,SubjectOrBodyContainsWords )))
| extend RuleDetail = case(OfficeObjectId contains '/' , tostring(split(OfficeObjectId, '/')[-1]) , tostring(split(OfficeObjectId, '\\')[-1]))

ClientIPAddress =

ifClientIP has "."tostring(split(ClientIP, ":")[0])

elifClientIP has "["tostring(trim_start(@'[[]', tostring(split(ClientIP, "]")[0])))

elseClientIP

Stage 10: `summarize`

| summarize count(), StartTimeUtc = min(TimeGenerated), EndTimeUtc = max(TimeGenerated) by  Operation, UserId, ClientIPAddress, ResultStatus, Keyword, OriginatingServer, OfficeObjectId, RuleDetail

Stage 11: `extend`

| extend UserName = split(UserId, '@')[0], DomainName = split(UserId, '@')[1]

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
`BodyContainsWords`	match	`Bankuberweisung` `PO#` `Paiement` `Rechnung` `Zahlung` `account details` `bank account` `bank details` `bank statement` `closing` `deposit` `funds` `hacked` `invoice` `paycheck` `payment` `phishing` `purchase` `remittance` `transfer` `virement bancaire`
`Operation`	eq	`New-InboxRule`
`Parameters`	match	`DeleteMessage` transforms: `term` `Deleted Items` transforms: `term` `Junk Email` transforms: `term`
`SubjectContainsWords`	match	`Bankuberweisung` `PO#` `Paiement` `Rechnung` `Zahlung` `account details` `bank account` `bank details` `bank statement` `closing` `deposit` `funds` `hacked` `invoice` `paycheck` `payment` `phishing` `purchase` `remittance` `transfer` `virement bancaire`
`SubjectOrBodyContainsWords`	match	`Bankuberweisung` `PO#` `Paiement` `Rechnung` `Zahlung` `account details` `bank account` `bank details` `bank statement` `closing` `deposit` `funds` `hacked` `invoice` `paycheck` `payment` `phishing` `purchase` `remittance` `transfer` `virement bancaire`

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
`ClientIPAddress`	`summarize`
`EndTimeUtc`	`summarize`
`Keyword`	`summarize`
`OfficeObjectId`	`summarize`
`Operation`	`summarize`
`OriginatingServer`	`summarize`
`ResultStatus`	`summarize`
`RuleDetail`	`summarize`
`StartTimeUtc`	`summarize`
`UserId`	`summarize`
`DomainName`	`extend`
`UserName`	`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)

Malicious BEC Inbox Rule

MITRE ATT&CK coverage

Event coverage

Rules detecting the same action

Rule body kusto

Stages and Predicates

Let binding: `BEC_Keywords`

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `extend`

Stage 5: `parse`

Stage 6: `parse`

Stage 7: `parse`

Stage 8: `where`

Stage 9: `extend` (3 consecutive steps)

Stage 10: `summarize`

Stage 11: `extend`

Indicators

Output fields

Keyboard shortcuts

Search operators

Malicious BEC Inbox Rule

MITRE ATT&CK coverage

Event coverage

Rules detecting the same action

Rule body kusto

Stages and Predicates

Let binding: BEC_Keywords

Stage 1: source

Stage 2: where

Stage 3: where

Stage 4: extend

Stage 5: parse

Stage 6: parse

Stage 7: parse

Stage 8: where

Stage 9: extend (3 consecutive steps)

Stage 10: summarize

Stage 11: extend

Indicators

Output fields

Let binding: `BEC_Keywords`

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `extend`

Stage 5: `parse`

Stage 6: `parse`

Stage 7: `parse`

Stage 8: `where`

Stage 9: `extend` (3 consecutive steps)

Stage 10: `summarize`

Stage 11: `extend`