Mail Forwarding/Redirecting Activity Via ExchangePowerShell Cmdlet

Status: experimental
Severity: medium
Log source: product windows, category ps_script
Author: Nasreddine Bencherchali (Nextron Systems), Marco Pedrinazzi (@pedrinazziM) (InTheCyber)
Source: github.com/SigmaHQ/sigma

Detects email forwarding or redirecting activity via ExchangePowerShell Cmdlet

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1564.008` Hide Artifacts: Email Hiding Rules
Collection	`T1114.003` Email Collection: Email Forwarding Rule
Exfiltration	`T1020` Automated Exfiltration

Event coverage

Provider	Event	Title
PowerShell	Event ID 4104	Creating Scriptblock text (MessageNumber of MessageTotal).

Rule body yaml

title: Mail Forwarding/Redirecting Activity Via ExchangePowerShell Cmdlet
id: 0c7686d5-c74e-4292-b224-2a08e956ebc4
related:
    - id: c726e007-2cd0-4a55-abfb-79730fbedee5
      type: similar
    - id: 15b7abbb-8b40-4d01-9ee2-b51994b1d474
      type: obsolete
status: experimental
description: Detects email forwarding or redirecting activity via ExchangePowerShell Cmdlet
references:
    - https://redcanary.com/blog/email-forwarding-rules/
    - https://github.com/PwC-IR/Business-Email-Compromise-Guide/blob/fe29ce06aef842efe4eb448c26bbe822bf5b895d/PwC-Business_Email_Compromise-Guide.pdf
author: Nasreddine Bencherchali (Nextron Systems), Marco Pedrinazzi (@pedrinazziM) (InTheCyber)
date: 2026-03-01
tags:
    - attack.collection
    - attack.stealth
    - attack.t1114.003
    - attack.t1564.008
    - attack.exfiltration
    - attack.t1020
    - detection.threat-hunting
logsource:
    product: windows
    category: ps_script
    definition: 'Requirements: Script Block Logging must be enabled'
detection:
    selection_cmdlet:
        ScriptBlockText|contains:
            - 'New-InboxRule'
            - 'Set-InboxRule'
            - 'Set-Mailbox'
    selection_params:
        ScriptBlockText|contains:
            - 'ForwardAsAttachmentTo'
            - 'ForwardingAddress'
            - 'ForwardingSmtpAddress'
            - 'ForwardTo'
            - 'RedirectTo' # Also covers "RedirectToRecipients"
    condition: all of selection_*
falsepositives:
    - Legitimate mail forwarding rules created by users or administrators to forward email to designated internal or external recipients for business or operational purposes.
level: medium

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_cmdlet`

selection_cmdlet:
    ScriptBlockText|contains:
        - 'New-InboxRule'
        - 'Set-InboxRule'
        - 'Set-Mailbox'

Stage 2: `selection_params`

selection_params:
    ScriptBlockText|contains:
        - 'ForwardAsAttachmentTo'
        - 'ForwardingAddress'
        - 'ForwardingSmtpAddress'
        - 'ForwardTo'
        - 'RedirectTo'

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
`ScriptBlockText`	match	`ForwardAsAttachmentTo` `ForwardTo` `ForwardingAddress` `ForwardingSmtpAddress` `New-InboxRule` corpus 2 (sigma 2) `RedirectTo` `Set-InboxRule` corpus 2 (sigma 2) `Set-Mailbox`

`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)

Mail Forwarding/Redirecting Activity Via ExchangePowerShell Cmdlet

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_cmdlet`

Stage 2: `selection_params`

Indicators

Keyboard shortcuts

Search operators

Mail Forwarding/Redirecting Activity Via ExchangePowerShell Cmdlet

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_cmdlet

Stage 2: selection_params

Indicators

Stage 0: `condition`

Stage 1: `selection_cmdlet`

Stage 2: `selection_params`