Windows InProcServer32 New Outlook Form

name: Windows InProcServer32 New Outlook Form
id: fedb49c4-4bd7-4d42-8fd9-f8c8538c73c4
version: 8
date: '2026-03-10'
author: Michael Haag, Splunk
data_source:
    - Sysmon EventID 13
type: Anomaly
status: production
description: The following analytic detects the creation or modification of registry keys associated with new Outlook form installations, potentially indicating exploitation of CVE-2024-21378. It leverages data from the Endpoint.Registry datamodel, focusing on registry paths involving InProcServer32 keys linked to Outlook forms. This activity is significant as it may signify an attempt to achieve authenticated remote code execution via malicious form objects. If confirmed malicious, this could allow an attacker to create arbitrary files and registry keys, leading to remote code execution and potential full system compromise.
search: '| tstats `security_content_summariesonly` count FROM datamodel=Endpoint.Registry where Registry.registry_path="*\\InProcServer32\\*" Registry.registry_value_data=*\\FORMS\\* by Registry.action Registry.dest Registry.process_guid Registry.process_id Registry.registry_hive Registry.registry_path Registry.registry_key_name Registry.registry_value_data Registry.registry_value_name Registry.registry_value_type Registry.status Registry.user Registry.vendor_product | `drop_dm_object_name(Registry)` |`security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `windows_inprocserver32_new_outlook_form_filter`'
how_to_implement: To successfully implement this search you need to be ingesting information on process that include the name of the process responsible for the changes from your endpoints into the `Endpoint` datamodel in the `Registry` node.
known_false_positives: False positives are possible if the organization adds new forms to Outlook via an automated method. Filter by name or path to reduce false positives.
references:
    - https://www.netspi.com/blog/technical/red-team-operations/microsoft-outlook-remote-code-execution-cve-2024-21378/
drilldown_searches:
    - name: View the detection results for - "$dest$"
      search: '%original_detection_search% | search  dest = "$dest$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$dest$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$dest$") starthoursago=168  | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
rba:
    message: A registry key associated with a new Outlook form installation was created or modified. This could indicate exploitation of CVE-2024-21378 on $dest$.
    risk_objects:
        - field: dest
          type: system
          score: 20
    threat_objects: []
tags:
    analytic_story:
        - Outlook RCE CVE-2024-21378
    cve:
        - CVE-2024-21378
    asset_type: Endpoint
    mitre_attack_id:
        - T1566
        - T1112
    product:
        - Splunk Enterprise
        - Splunk Enterprise Security
        - Splunk Cloud
    security_domain: endpoint
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1566/cve-2024-21378/inprocserver32_windows-sysmon.log
          source: XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
          sourcetype: XmlWinEventLog