Microsoft Exchange External Forwarding

Severity: high
Log types: Microsoft365.Audit.Exchange
Tags: Microsoft365, Exchange, Data Exfiltration, Email Security
Reference: https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/outbound-spam-policies-external-email-forwarding?view=o365-worldwide
Source: github.com/panther-labs/panther-analysis

Detects when a user creates email forwarding rules to external organizations in Microsoft Exchange Online. This can indicate data exfiltration attempts, where an attacker sets up forwarding to collect emails outside the organization. The rule detects both mailbox forwarding (Set-Mailbox) and inbox rules (New-InboxRule). The detection includes: 1. External organization forwarding based on domain comparison 2. Suspicious forwarding patterns like: - Forwarding without keeping a copy - Deleting messages after forwarding - Stopping rule processing after forwarding 3. Multiple forwarding destinations 4. Various forwarding methods (SMTP, redirect, forward as attachment)

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1137.005` Office Application Startup: Outlook Rules
Collection	`T1114.003` Email Collection: Email Forwarding Rule
Exfiltration	`T1020` Automated Exfiltration

Rule body yaml

AnalysisType: rule
Description: >
  Detects when a user creates email forwarding rules to external organizations in Microsoft Exchange Online.
  This can indicate data exfiltration attempts, where an attacker sets up forwarding to collect emails outside
  the organization. The rule detects both mailbox forwarding (Set-Mailbox) and inbox rules (New-InboxRule).
  
  The detection includes:
  1. External organization forwarding based on domain comparison
  2. Suspicious forwarding patterns like:
     - Forwarding without keeping a copy
     - Deleting messages after forwarding
     - Stopping rule processing after forwarding
  3. Multiple forwarding destinations
  4. Various forwarding methods (SMTP, redirect, forward as attachment)
DisplayName: "Microsoft Exchange External Forwarding"
Enabled: true
Filename: microsoft_exchange_external_forwarding.py
Reports:
  MITRE ATT&CK:
    - TA0003:T1137.005 # Persistence - Office Application Startup: Outlook Rules
    - TA0009:T1114.003 # Collection - Email Collection: Email Forwarding Rule
    - TA0010:T1020 # Exfiltration - Automated Exfiltration
Reference: https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/outbound-spam-policies-external-email-forwarding?view=o365-worldwide
Severity: High
Tags:
  - Microsoft365
  - Exchange
  - Data Exfiltration
  - Email Security
DedupPeriodMinutes: 60
LogTypes:
  - Microsoft365.Audit.Exchange
RuleID: "Microsoft365.Exchange.External.Forwarding"
Threshold: 1
SummaryAttributes:
  - userid
  - parameters
  - organizationname
Runbook: |
  1. Investigate the forwarding configuration:
     - Check if the forwarding is legitimate and approved
     - Verify the destination addresses
     - Review any suspicious patterns (deletion, no copy kept)
  2. If unauthorized:
     - Remove the forwarding rule
     - Check for any data that may have been forwarded
     - Review the user's recent activity
  3. If authorized:
     - Document the business justification
     - Ensure it complies with security policies
     - Monitor for any changes to the forwarding configuration
Tests:
  - Name: External Organization Forwarding
    ExpectedResult: true
    Log:
      clientip: 1.2.3.4
      creationtime: "2022-12-12 22:19:00"
      externalaccess: false
      id: 111-22-33
      objectid: homer.simpson
      operation: Set-Mailbox
      organizationid: 11-aa-bb
      organizationname: simpsons.onmicrosoft.com
      originatingserver: QWERTY (1.2.3.4)
      parameters:
        - Name: Identity
          Value: homer.simpson@simpsons.onmicrosoft.com
        - Name: ForwardingSmtpAddress
          Value: smtp:peter.griffin@familyguy.com
        - Name: DeliverToMailboxAndForward
          Value: "False"
      recordtype: 1
      resultstatus: "True"
      userid: homer.simpson@simpsons.onmicrosoft.com
      userkey: "12345"
      usertype: 2
      workload: Exchange

  - Name: Internal Organization Forwarding
    ExpectedResult: false
    Log:
      clientip: 1.2.3.4
      creationtime: "2022-12-12 22:19:00"
      externalaccess: false
      id: 111-22-33
      objectid: homer.simpson
      operation: Set-Mailbox
      organizationid: 11-aa-bb
      organizationname: simpsons.onmicrosoft.com
      originatingserver: QWERTY (1.2.3.4)
      parameters:
        - Name: Identity
          Value: marge.simpson@simpsons.onmicrosoft.com
        - Name: ForwardingSmtpAddress
          Value: smtp:marge.simpson@simpsons.com
        - Name: DeliverToMailboxAndForward
          Value: "True"
      recordtype: 1
      resultstatus: "True"
      userid: homer.simpson@simpsons.com
      userkey: "12345"
      usertype: 2
      workload: Exchange

  - Name: Suspicious Forwarding Pattern
    ExpectedResult: true
    Log:
      clientip: 1.2.3.4
      creationtime: "2022-12-12 22:19:00"
      externalaccess: false
      id: 111-22-33
      objectid: homer.simpson
      operation: New-InboxRule
      organizationid: 11-aa-bb
      organizationname: simpsons.onmicrosoft.com
      originatingserver: QWERTY (1.2.3.4)
      parameters:
        - Name: Identity
          Value: "Delete and Forward Rule"
        - Name: Mailbox
          Value: homer.simpson@simpsons.onmicrosoft.com
        - Name: ForwardTo
          Value: external@example.com
        - Name: DeleteMessage
          Value: "True"
        - Name: StopProcessingRules
          Value: "True"
      recordtype: 1
      resultstatus: "True"
      userid: homer.simpson@simpsons.onmicrosoft.com
      userkey: "12345"
      usertype: 2
      workload: Exchange

  - Name: Multiple Forwarding Addresses
    ExpectedResult: true
    Log:
      clientip: 1.2.3.4
      creationtime: "2022-12-12 22:19:00"
      externalaccess: false
      id: 111-22-33
      objectid: homer.simpson
      operation: New-InboxRule
      organizationid: 11-aa-bb
      organizationname: simpsons.onmicrosoft.com
      originatingserver: QWERTY (1.2.3.4)
      parameters:
        - Name: Identity
          Value: "Multiple Forward Rule"
        - Name: Mailbox
          Value: homer.simpson@simpsons.onmicrosoft.com
        - Name: ForwardTo
          Value: "external1@example.com;external2@example.com;external3@example.com"
        - Name: DeliverToMailboxAndForward
          Value: "False"
      recordtype: 1
      resultstatus: "True"
      userid: homer.simpson@simpsons.onmicrosoft.com
      userkey: "12345"
      usertype: 2
      workload: Exchange

  - Name: Invalid Identity Format
    ExpectedResult: true
    Log:
      clientip: 1.2.3.4
      creationtime: "2022-12-12 22:19:00"
      externalaccess: false
      id: 111-22-33
      objectid: homer.simpson
      operation: Set-Mailbox
      organizationid: 11-aa-bb
      organizationname: simpsons.onmicrosoft.com
      originatingserver: QWERTY (1.2.3.4)
      parameters:
        - Name: Identity
          Value: Invalid/Path/Format
        - Name: ForwardingSmtpAddress
          Value: smtp:hello@familyguy.com
        - Name: DeliverToMailboxAndForward
          Value: "False"
      recordtype: 1
      resultstatus: "True"
      userid: homer.simpson@simpsons.onmicrosoft.com
      userkey: "12345"
      usertype: 2
      workload: Exchange

  - Name: Missing Organization Name
    ExpectedResult: true
    Log:
      clientip: 1.2.3.4
      creationtime: "2022-12-12 22:19:00"
      externalaccess: false
      id: 111-22-33
      objectid: homer.simpson
      operation: Set-Mailbox
      organizationid: 11-aa-bb
      organizationname: ""
      originatingserver: QWERTY (1.2.3.4)
      parameters:
        - Name: Identity
          Value: ABC1.prod.outlook.com/Microsoft Exchange Hosted Organizations/simpsons.onmicrosoft.com/homer.simpson
        - Name: ForwardingSmtpAddress
          Value: smtp:hello@familyguy.com
        - Name: DeliverToMailboxAndForward
          Value: "False"
      recordtype: 1
      resultstatus: "True"
      userid: homer.simpson@simpsons.onmicrosoft.com
      userkey: "12345"
      usertype: 2
      workload: Exchange

  - Name: Subdomain Forwarding (Internal)
    ExpectedResult: false
    Log:
      clientip: 1.2.3.4
      creationtime: "2022-12-12 22:19:00"
      externalaccess: false
      id: 111-22-33
      objectid: homer.simpson
      operation: Set-Mailbox
      organizationid: 11-aa-bb
      organizationname: simpsons.onmicrosoft.com
      originatingserver: QWERTY (1.2.3.4)
      parameters:
        - Name: Identity
          Value: homer.simpson@simpsons.onmicrosoft.com
        - Name: ForwardingSmtpAddress
          Value: smtp:bart.simpson@springfield.simpsons.com
        - Name: DeliverToMailboxAndForward
          Value: "True"
      recordtype: 1
      resultstatus: "True"
      userid: homer.simpson@simpsons.com
      userkey: "12345"
      usertype: 2
      workload: Exchange

  - Name: Similar Domain Forwarding (External)
    ExpectedResult: true
    Log:
      clientip: 1.2.3.4
      creationtime: "2022-12-12 22:19:00"
      externalaccess: false
      id: 111-22-33
      objectid: homer.simpson
      operation: Set-Mailbox
      organizationid: 11-aa-bb
      organizationname: simpsons.onmicrosoft.com
      originatingserver: QWERTY (1.2.3.4)
      parameters:
        - Name: Identity
          Value: homer.simpson@simpsons.onmicrosoft.com
        - Name: ForwardingSmtpAddress
          Value: smtp:evil@simpsons2.com
        - Name: DeliverToMailboxAndForward
          Value: "True"
      recordtype: 1
      resultstatus: "True"
      userid: homer.simpson@simpsons.com
      userkey: "12345"
      usertype: 2
      workload: Exchange

  - Name: Non-Com TLD Organization
    ExpectedResult: false
    Log:
      clientip: 1.2.3.4
      creationtime: "2022-12-12 22:19:00"
      externalaccess: false
      id: 111-22-33
      objectid: homer.simpson
      operation: Set-Mailbox
      organizationid: 11-aa-bb
      organizationname: simpsons.onmicrosoft.com
      originatingserver: QWERTY (1.2.3.4)
      parameters:
        - Name: Identity
          Value: john@justice.org
        - Name: ForwardingSmtpAddress
          Value: smtp:bruce@justice.org
        - Name: DeliverToMailboxAndForward
          Value: "True"
      recordtype: 1
      resultstatus: "True"
      userid: john@justice.org
      userkey: "12345"
      usertype: 2
      workload: Exchange
  - Name: Internal Forwarding with Suspicious Pattern
    ExpectedResult: false
    Log:
      clientip: "1.2.3.4:10736"
      creationtime: "2025-07-07 09:11:11.000000000"
      externalaccess: false
      id: 111-22-33
      objectid: "ABC001.prod.outlook.com/Microsoft Exchange Hosted Organizations/simpsons.onmicrosoft.com/444-55-66\\Move GitHub emails"
      operation: New-InboxRule
      organizationid: 11-aa-bb
      organizationname: simpsons.onmicrosoft.com
      originatingserver: QWERTY (1.2.3.4)
      parameters:
        - Name: AlwaysDeleteOutlookRulesBlob
          Value: "False"
        - Name: Force
          Value: "False"
        - Name: MoveToFolder
          Value: "MYFolder"
        - Name: Name
          Value: "Move emails to another folder"
        - Name: FromAddressContainsWords
          Value: "specialsender"
        - Name: StopProcessingRules
          Value: "True"
      recordtype: 1
      resultstatus: "True"
      userid: homer.simpson@simpsons.onmicrosoft
      userkey: homer.simpson@simpsons.onmicrosoft
      usertype: 2
      workload: Exchange

Detection logic

Condition

operation in ["Set-Mailbox", "New-InboxRule"]

This rule also runs imperative logic the parser cannot express as a filter; the conditions above are the structured part it could extract.

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
`operation`	in	`New-InboxRule` `Set-Mailbox`

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
`operation`	`Operation`
`organization_id`	`OrganizationId`
`client_ip`	`ClientIp`
`extended_properties`	`ExtendedProperties`
`modified_properties`	`ModifiedProperties`
`application`	`Application`
`actor`	`Actor`
`userid`

`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)