Outlook folder home page or Outlook Today URL set in the registry

Siemphony@siemphonymediumT1137+1unverified

Detects a write to the Outlook registry values that hold a folder home page — the URL Outlook fetches and renders inside its own process whenever the folder is opened after startup, executing whatever script the page carries without any macro setting being involved. This is the registry-side artefact of the Outlook Home Page sub-technique T1137.004, observed here through the parent strategy's analytic AN1116 because AN0502, the analytic under .004 itself, offers only process_creation, image_load and PowerShell log sources and no registry feed at all. It says nothing about the other Office startup mechanisms: T1137.001 covers the Word and Excel template values, T1137.002 the Office test key and T1137.006 the add-in startup folders, none of which touch these keys. MITRE's RegistryPath knob names Outlook Today; the WebView folder URLs and the EnableRoamingFolderHomepages value, which an operator has to set back on to defeat the update that disabled folder home pages, are added here rather than taken from MITRE. The version number between the product name and the Outlook subkey is why nothing left of `\Outlook\` is anchored. The registry is only one of two delivery paths, and the narrower one: Ruler, the tool the technique's own citation names, sets the folder home page as a MAPI property on the mailbox over RPC/HTTP, so the WebView values never appear in HKCU on the victim and this arm sees nothing. Against that path EnableRoamingFolderHomepages is the sole local artefact, and only on a host that took the 2017 update. Sysmon registry monitoring is an explicit allowlist of paths, so confirm the Office subtree is inside the running configuration before reading an empty result as an absence. The level answers to the first false positive rather than to the severity of a true positive — high is appropriate only once an estate has confirmed it carries no legacy folder home pages and has baselined Outlook's own WebView writes away. UNVERIFIED AS AUTHORED — derived from MITRE ATT&CK DET0398, 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.

DeviceRegistryEvents| where (RegistryKey contains "\\Outlook\\WebView\\" or RegistryKey contains "\\Outlook\\Today\\UserDefinedUrl" or RegistryKey contains "\\Outlook\\Security\\EnableRoamingFolderHomepages")

Splunk · SPL

Run this as a search.

index=* (TargetObject="*\\Outlook\\WebView\\*" OR TargetObject="*\\Outlook\\Today\\UserDefinedUrl*" OR TargetObject="*\\Outlook\\Security\\EnableRoamingFolderHomepages*")

Elastic · ES|QL

Run this as a search.

FROM logs-*| WHERE (TO_LOWER(registry.path) LIKE "*\\\\outlook\\\\webview\\\\*" OR TO_LOWER(registry.path) LIKE "*\\\\outlook\\\\today\\\\userdefinedurl*" OR TO_LOWER(registry.path) LIKE "*\\\\outlook\\\\security\\\\enableroamingfolderhomepages*")

Wazuh · XML rule

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

<group name="sigma,windows,registry_set,">  <!-- Rule ids must be unique on your manager; 100000+ is the user range. -->  <rule id="100000" level="7">    <!-- Set <if_sid> to the decoder/base rule for windows so this only evaluates relevant events. -->    <field name="TargetObject" type="pcre2">(?i)(\\Outlook\\WebView\\|\\Outlook\\Today\\UserDefinedUrl|\\Outlook\\Security\\EnableRoamingFolderHomepages)</field>    <description>Outlook folder home page or Outlook Today URL set in the registry</description>    <mitre>      <id>T1137</id>      <id>T1137.004</id>    </mitre>  </rule></group>

At a glance

Logsource: registry_set / windows
Level: medium
Category: office_macro_execution
Sigma status: experimental
Backends: 4 of 4 rendered
Updated: 30 Aug 2026
Rule ID: f12d9869-e8c9-4a79-a212-e4ca13661042

ATT&CKT1137 +1

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 · 5 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

Outlook itself populating its own WebView subkeys during profile creation, mailbox re-cache and shared or delegate mailbox opens. This is a product-generated write with no deployment and no user action behind it, it lands on every new profile, and it is the highest-volume benign source this rule has.
Legacy line-of-business deployments that genuinely use folder home pages. Older CRM and workflow add-ins pointed an Outlook folder at an internal page, and an estate still running one pushes the same values to every mailbox by Group Policy or logon script on every refresh.
An administrator deliberately re-enabling folder home pages after the Outlook update switched them off. Microsoft documents EnableRoamingFolderHomepages for exactly that compatibility case, so the write arrives from a support process with no adversary behind it.
A user customising the Outlook Today pane through its own Customize dialog, which writes UserDefinedUrl. It is uncommon but entirely supported, so a single hit is a question to ask rather than an incident on its own.
Profile roaming, migration and backup-restore tooling replaying an HKCU hive that already contained one of these values, which reproduces the original write on a new host without any new action having taken place.

Noise check0 of 6 fired

Fires on none of the 6 ordinary behaviours the baseline holds for this log type. That is not a precision guarantee: it means only not caught by these 6 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, T1137.004, evaluated against their registry_set events.

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Outlook folder home page or Outlook Today URL set in the registry

The detection

detection.yml

Sentinel · KQL

Splunk · SPL

Elastic · ES|QL

Wazuh · XML rule

Community

Verdicts from engineers who deployed it