Malicious Inbox Rule

Status: available
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 the attackers create inbox rules to delete emails that contain certain keywords. This is done so as to limit ability to warn compromised users that they've been compromised. Below is a sample query that tries to detect this. Reference: https://www.reddit.com/r/sysadmin/comments/7kyp0a/recent_phishing_attempts_my_experience_and_what/

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.

M365 Exchange Inbox Forwarding Rule Created (Elastic)
M365 Exchange Inbox Rule with Obfuscated Name (Elastic)
O365 BEC Email Hiding Rule Created (Splunk)
O365 Email New Inbox Rule Created (Splunk)

Rule body

id: 7b907bf7-77d4-41d0-a208-5643ff75bf9a
name: Malicious Inbox Rule
description: |
  'Often times after the initial compromise the attackers create inbox rules to delete emails that contain certain keywords.
   This is done so as to limit ability to warn compromised users that they've been compromised. Below is a sample query that tries to detect this.
  Reference: https://www.reddit.com/r/sysadmin/comments/7kyp0a/recent_phishing_attempts_my_experience_and_what/'
severity: Medium
status: Available
requiredDataConnectors:
  - connectorId: Office365
    dataTypes:
      - OfficeActivity (Exchange)
queryFrequency: 1d
queryPeriod: 1d
triggerOperator: gt
triggerThreshold: 0
tactics:
  - Persistence
  - DefenseEvasion
relevantTechniques:
  - T1098
  - T1078
query: |
 let Keywords = dynamic(["helpdesk", " alert", " suspicious", "fake", "malicious", "phishing", "spam", "do not click", "do not open", "hijacked", "Fatal"]);
 OfficeActivity
 | where OfficeWorkload =~ "Exchange" 
 | where Operation =~ "New-InboxRule" and (ResultStatus =~ "True" or ResultStatus =~ "Succeeded")
 | 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 (Keywords)
  or BodyContainsWords has_any (Keywords)
  or SubjectOrBodyContainsWords has_any (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 AccountName = tostring(split(UserId, "@")[0]), AccountUPNSuffix = tostring(split(UserId, "@")[1])
 | extend OriginatingServerName = tostring(split(OriginatingServer, " ")[0])
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: UserId
      - identifier: Name
        columnName: AccountName
      - identifier: UPNSuffix
        columnName: AccountUPNSuffix
  - entityType: Host
    fieldMappings:
      - identifier: FullName
        columnName: OriginatingServerName
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: ClientIPAddress
version: 2.0.4
kind: Scheduled

Stages and Predicates

Parameters

let Keywords = dynamic(["helpdesk", " alert", " suspicious", "fake", "malicious", "phishing", "spam", "do not click", "do not open", "hijacked", "Fatal"]);

Stage 1: `source`

OfficeActivity

Stage 2: `where`

| where OfficeWorkload =~ "Exchange"

Stage 3: `where`

| where Operation =~ "New-InboxRule" and (ResultStatus =~ "True" or ResultStatus =~ "Succeeded")

Stage 4: `where`

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

Stage 5: `extend`

| extend Events=todynamic(Parameters)

Stage 6: `parse`

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

Stage 7: `parse`

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

Stage 8: `parse`

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

Stage 9: `where`

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

Stage 10: `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 11: `summarize`

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

Stage 12: `extend`

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

Stage 13: `extend`

| extend OriginatingServerName = tostring(split(OriginatingServer, " ")[0])

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`BodyContainsWords`	match	`alert` transforms: `term` `suspicious` transforms: `term` `Fatal` transforms: `term` `do not click` transforms: `term` `do not open` transforms: `term` `fake` transforms: `term` `helpdesk` transforms: `term` `hijacked` transforms: `term` `malicious` transforms: `term` `phishing` transforms: `term` `spam` transforms: `term`	`field:"BodyContainsWords" kind:match`
`OfficeWorkload`	eq	`Exchange`	`field:"m365::Workload" kind:eq value:"Exchange"`
`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`
`ResultStatus`	eq	`Succeeded` `True`	`field:"ResultStatus" kind:eq`
`SubjectContainsWords`	match	`alert` transforms: `term` `suspicious` transforms: `term` `Fatal` transforms: `term` `do not click` transforms: `term` `do not open` transforms: `term` `fake` transforms: `term` `helpdesk` transforms: `term` `hijacked` transforms: `term` `malicious` transforms: `term` `phishing` transforms: `term` `spam` transforms: `term`	`field:"SubjectContainsWords" kind:match`
`SubjectOrBodyContainsWords`	match	`alert` transforms: `term` `suspicious` transforms: `term` `Fatal` transforms: `term` `do not click` transforms: `term` `do not open` transforms: `term` `fake` transforms: `term` `helpdesk` transforms: `term` `hijacked` transforms: `term` `malicious` transforms: `term` `phishing` transforms: `term` `spam` 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`
`AccountName`	`extend`
`AccountUPNSuffix`	`extend`
`OriginatingServerName`	`extend`

Malicious Inbox Rule

MITRE ATT&CK coverage

Telemetry coverage

Rules detecting the same action

Rule body

Stages and Predicates

Parameters

Stage 1: source

Stage 2: where

Stage 3: where

Stage 4: where

Stage 5: extend

Stage 6: parse

Stage 7: parse

Stage 8: parse

Stage 9: where

Stage 10: extend (3 consecutive steps)

Stage 11: summarize

Stage 12: extend

Stage 13: extend