PowerShell script touching the Outlook Home Page WebView setting

Siemphony@siemphonylowT1137.004unverified

AN0502 offers three log sources, and the first two are already spoken for elsewhere in this corpus. The process-creation leg — outlook.exe as parent, a scripting host as child — is already published as data/detections/T1203's selector, which lists outlook.exe among its parents and mshta.exe/wscript.exe among its children; every event that leg would match already matches T1203, with nothing on the event to say which technique produced it. This rule previously took the image-load leg instead, matching outlook.exe loading vbscript.dll/jscript.dll/scrobj.dll — but data/detections/T1137.003 was reworked onto the near-identical selector (outlook.exe loading vbscript.dll and scrobj.dll, same category) to catch a malicious custom Form, and a review confirmed every event satisfying either selector satisfies the other: jscript.dll was the only field that differed, so the two technique-tagged rules fired on the same Sysmon EID7 record with no way to tell a malicious folder Home Page from a malicious Form. That leg is dropped here in favour of AN0502's third, previously-unused source: the PowerShell operational log. A Home Page is not set through a registry edit alone — Ruler-style tooling and the equivalent manual/administrative cleanup both go through Outlook's COM object model, setting a folder's `WebViewURL` and `WebViewOn` properties, which is what actually populates the `Outlook\WebView\<Folder>` registry value the brief has no log source for. Matching PowerShell script block text that references either the COM properties or the registry path they write catches the mechanism itself, on a source neither the T1137.003 nor the T1203 rule reads. Of MITRE's knobs: `ChildProcessName` belongs to the process-creation leg dropped above, so it has no expression here; `TargetFolder` and `HTMLPayloadLocation` may appear as literal substrings inside a matching script block but are not separately addressable fields on a 4104 event, so neither is asserted as its own gate; `TimeWindow` and `FormViewBehavior` describe timing and rendering state that a script-block log does not carry. The event records that a script touched the Home Page mechanism, not what it changed it to, and it cannot distinguish an adversary's Ruler module from a defender's or IT team's own remediation script doing the identical property writes to remove one — both are "execution of a PowerShell script to enumerate or remove malicious Home Page folder config" in MITRE's own words for this source, and this analytic cannot separate the two directions. Prerequisite: EventID 4104 requires PowerShell Script Block Logging, which is off in a default install and in the MS/CIS baselines; an empty result means the telemetry is most likely missing, not that the technique is absent. UNVERIFIED AS AUTHORED — derived from MITRE ATT&CK DET0177, not hand-written and not tested by its author. Any lab result is on this rule's own page.Full description

The 4 SIEM queries below are previews produced by Siemphony’s own translator, not pySigma, and none has been executed against a real backend — review before you deploy.

detection.yml

Sentinel · KQL

Run this as a search.

Event| where (EventID == 4104 and (ScriptBlockText contains "WebViewURL" or ScriptBlockText contains "WebViewOn" or ScriptBlockText contains "Outlook\\WebView"))

Splunk · SPL

Run this as a search.

index=* (EventID="4104" AND (ScriptBlockText="*WebViewURL*" OR ScriptBlockText="*WebViewOn*" OR ScriptBlockText="*Outlook\\WebView*"))

Elastic · ES|QL

Run this as a search.

FROM logs-*| WHERE (event.code == 4104 AND (TO_LOWER(powershell.file.script_block_text) LIKE "*webviewurl*" OR TO_LOWER(powershell.file.script_block_text) LIKE "*webviewon*" OR TO_LOWER(powershell.file.script_block_text) LIKE "*outlook\\\\webview*"))

Wazuh · XML rule

Deploy to your manager — this is a rule, not a search.

<group name="sigma,windows,">  <!-- Rule ids must be unique on your manager; 100000+ is the user range. -->  <rule id="100000" level="5">    <!-- Set <if_sid> to the decoder/base rule for windows so this only evaluates relevant events. -->    <field name="EventID" type="pcre2">(?i)^4104$</field>    <field name="ScriptBlockText" type="pcre2">(?i)(WebViewURL|WebViewOn|Outlook\\WebView)</field>    <description>PowerShell script touching the Outlook Home Page WebView setting</description>    <mitre>      <id>T1137.004</id>    </mitre>  </rule></group>

At a glance

Logsource: powershell / windows
Level: low
Category: office_macro_execution
Sigma status: experimental
Backends: 4 of 4 rendered
Updated: 30 Aug 2026
Rule ID: 6640bb31-0422-4736-8ee7-afaa2cf05c66

ATT&CKT1137.004

T1137.004Outlook Home Page

ATT&CK® is a registered trademark of The MITRE Corporation. These mappings are the rule author’s, and are neither reviewed nor endorsed by MITRE.

Lessonoffice macro execution

office macro execution

01 · What this rule detects

A document application — Word, Excel, PowerPoint, Outlook, Publisher, Visio — starting a shell, a script host or a signed proxy binary. The document is the delivery vehicle and the child process is the first thing the attacker actually controls. It is one of the most durable detections available because the parent-child relationship is a property of how the payload has to run, not a string the attacker chooses.

02 · Telemetry you need

Process creation with ParentImage and the full command line — Sysmon EventID 1, or Security 4688 with Audit Process Creation plus the command-line inclusion policy; both off by default, and 4688 without the second one gives you the child process with no arguments
Sysmon EventID 7 (ImageLoad) for the macro runtime — VBE7.DLL and its companions loading into an Office process tells you a macro ran even when nothing was spawned
Sysmon EventID 11 (FileCreate) under %TEMP%, INetCache and the Outlook secure temp folder, where the attachment lands before it is opened
Sysmon EventID 15 (FileCreateStreamHash) for the Mark of the Web zone identifier, which records that the document arrived from outside
Sysmon EventID 3 (NetworkConnect) initiated by the Office process itself

03 · How the logic works

The rule is ParentImage in the Office set AND Image in a child set — shells, script hosts, the signed proxy binaries, and any process launched from a user-writable path. ParentImage carries the signal; the child list is the part you widen over time. The important weakness to teach is that the parent link is breakable: a macro that goes through WMI or a COM object gets its child reparented away from Office, and the rule silently stops matching for exactly the samples that bothered to evade it. That is why the macro-runtime image load and Office writing an executable file are worth having as separate rules rather than as extra conditions on this one — they survive the reparenting. Outlook deserves its own rule too: as a parent it means something different from Word, and the volume profile is different.

04 · Common false positives

Genuine business macros — finance and reporting workbooks that shell out, refresh external data or drive another application
Office add-ins and vendor integrations that launch helper processes, including document management and e-signature tooling
Mail merge, document conversion and reporting pipelines that automate Office on a server
Helpdesk and repair scripts that invoke Office repair or start a shell from a template
Office spawning ordinary Windows infrastructure processes as part of normal operation

05 · Detection maturity

Tier 1 — starting point: Matches an Office application spawning cmd.exe or PowerShell.
Tier 2 — tuned: Covers the full parent set against a broad child set including script hosts and proxy binaries, with named business macros excluded by document path.
Tier 3 — correlated: Adds the macro runtime load and the Mark of the Web, and correlates the chain with what the child process did on the network or wrote to disk.

Tuning & false positivesexperimental · 4 declared

experimental (current stage)
test
stable

Written and reasoned about, but not yet run against real telemetry. Expect to tune it before trusting it.

Tuning tips

Exclude by the document or template path, or by the host running the automation, rather than by the child process — dropping cmd.exe from the child list to quieten one workbook blinds the rule everywhere
Raise severity when the document carries a Mark of the Web zone identifier, and again when it came from Outlook's temp folder
Keep the child list additive, and include 'anything launched from a user-writable directory' as a condition rather than trying to name every binary
Give outlook.exe as parent its own rule so the two can be tuned and prioritised independently
Where the telemetry has it, capture the grandparent as well, so the alert shows the analyst the whole chain rather than one hop

Declared by the author

IT and security-response scripts that enumerate or strip a malicious Home Page as remediation: by design they set WebViewOn to false and clear WebViewURL through the same COM properties an attacker's tooling used to set them, and on an estate actively responding to this technique they are the highest-volume match by a wide margin.
Legitimate corporate provisioning scripts that set an internal portal, dashboard or knowledge-base page as a folder Home Page through Group Policy logon scripts or an MDM-pushed PowerShell configuration, which is the sanctioned use of the exact same properties.
Outlook profile migration and tenant-to-tenant cutover scripts that reset or reapply WebView settings across mailboxes in bulk ahead of or after a mailbox move.
Diagnostic or inventory scripts run by help desk staff that read WebViewURL/WebViewOn to audit which mailboxes have a Home Page configured, without writing anything — the read appears as the same substring in the script's own text as a write would.

Noise check0 of 4 fired

Fires on none of the 4 ordinary behaviours the baseline holds for this log type. That is not a precision guarantee: it means only not caught by these 4 behaviours, and the next ordinary behaviour nobody wrote down may still match.

Run live against 65 hand-written events of ordinary corporate activity — backup agents, software distribution, patching, helpdesk scripts, EDR. It says nothing about how often the rule would fire in production, which is the number that actually exhausts an analyst.

Verification1 run

Not verified

Siemphony benign baseline
completed
Siemphony IR · 3 Sept 2026
No events matched, so this run proves nothing either way.

Test it yourselfin your browser

Runs here in your browser against public attack recordings. A preview — not the verification badge.

Recordings of T1137.004, evaluated against their — events.

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PowerShell script touching the Outlook Home Page WebView setting

The detection

detection.yml

Sentinel · KQL

Splunk · SPL

Elastic · ES|QL

Wazuh · XML rule

Community

Verdicts from engineers who deployed it