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

Telemetry coverage

Platform	Record / event type
Microsoft 365	Exchange audit operation New-InboxRule: Federated domain added

Rules detecting the same action

These rules filter on the same operation.

Rule body

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` used in Stage 8

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)

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

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`BodyContainsWords`	match	`Bankuberweisung` transforms: `term` `PO#` transforms: `term` `Paiement` transforms: `term` `Rechnung` transforms: `term` `Zahlung` transforms: `term` `account details` transforms: `term` `bank account` transforms: `term` `bank details` transforms: `term` `bank statement` transforms: `term` `closing` transforms: `term` `deposit` transforms: `term` `funds` transforms: `term` `hacked` transforms: `term` `invoice` transforms: `term` `paycheck` transforms: `term` `payment` transforms: `term` `phishing` transforms: `term` `purchase` transforms: `term` `remittance` transforms: `term` `transfer` transforms: `term` `virement bancaire` transforms: `term`	`field:"BodyContainsWords" kind:match`
`Operation`	eq	`New-InboxRule`	`field:"Operation" kind:eq value:"New-InboxRule"`
`Parameters`	match	`DeleteMessage` transforms: `term` `Deleted Items` transforms: `term` `Junk Email` transforms: `term`	`field:"m365::Parameters" kind:match`
`SubjectContainsWords`	match	`Bankuberweisung` transforms: `term` `PO#` transforms: `term` `Paiement` transforms: `term` `Rechnung` transforms: `term` `Zahlung` transforms: `term` `account details` transforms: `term` `bank account` transforms: `term` `bank details` transforms: `term` `bank statement` transforms: `term` `closing` transforms: `term` `deposit` transforms: `term` `funds` transforms: `term` `hacked` transforms: `term` `invoice` transforms: `term` `paycheck` transforms: `term` `payment` transforms: `term` `phishing` transforms: `term` `purchase` transforms: `term` `remittance` transforms: `term` `transfer` transforms: `term` `virement bancaire` transforms: `term`	`field:"SubjectContainsWords" kind:match`
`SubjectOrBodyContainsWords`	match	`Bankuberweisung` transforms: `term` `PO#` transforms: `term` `Paiement` transforms: `term` `Rechnung` transforms: `term` `Zahlung` transforms: `term` `account details` transforms: `term` `bank account` transforms: `term` `bank details` transforms: `term` `bank statement` transforms: `term` `closing` transforms: `term` `deposit` transforms: `term` `funds` transforms: `term` `hacked` transforms: `term` `invoice` transforms: `term` `paycheck` transforms: `term` `payment` transforms: `term` `phishing` transforms: `term` `purchase` transforms: `term` `remittance` transforms: `term` `transfer` transforms: `term` `virement bancaire` transforms: `term`	`field:"SubjectOrBodyContainsWords" kind:match`

Output fields

These fields are emitted when the rule matches.

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`

Malicious BEC Inbox Rule

MITRE ATT&CK coverage

Telemetry coverage

Rules detecting the same action

Rule body

Stages and Predicates

Let binding: BEC_Keywords used in Stage 8

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