Windows XLL File Creation Outside of Typical Location

SPLUNK detection rule by Raven Tait, Splunk · Apache-2.0

Detects the creation of an XLL file outside of typical locations. This can indicate an attempt to abuse Excel COM objects to load and execute a malicious XLL.

name: Windows XLL File Creation Outside of Typical Location
id: 7cdbe5e2-878d-4020-94f3-52938760155e
version: 2
creation_date: '2021-09-02'
modification_date: '2026-05-13'
author: Raven Tait, Splunk
status: production
type: Anomaly
description: |-
    Detects the creation of an XLL file outside of typical locations.
    This can indicate an attempt to abuse Excel COM objects to load and execute a malicious XLL.
data_source:
    - Sysmon EventID 11
search: |-
    | tstats `security_content_summariesonly`
      count min(_time) as firstTime
            max(_time) as lastTime

    from datamodel=Endpoint.Filesystem where

    Filesystem.file_path="*.xll"

    NOT Filesystem.file_path IN (
        "*:\\Program Files\\Microsoft Office\\*",
        "*\\AppData\\Roaming\\Microsoft\\AddIns\\*"
        )

    by Filesystem.dest Filesystem.file_create_time Filesystem.process_path
       Filesystem.process_guid Filesystem.process_id Filesystem.file_path
       Filesystem.action Filesystem.file_name
       Filesystem.user Filesystem.vendor_product

    | `drop_dm_object_name(Filesystem)`
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `windows_xll_file_creation_outside_of_typical_location_filter`
how_to_implement: The detection is based on data that originates from Endpoint Detection and Response (EDR) agents. These agents are designed to provide security-related telemetry from the endpoints where the agent is installed. To implement this search, you must ingest logs that contain the process GUID, process name, and parent process. Additionally, you must ingest complete command-line executions. These logs must be processed using the appropriate Splunk Technology Add-ons that are specific to the EDR product. The logs must also be mapped to the `Processes` node of the `Endpoint` data model. Use the Splunk Common Information Model (CIM) to normalize the field names and speed up the data modeling process.
known_false_positives: Some legitimate Excel add-ins and administrative tools may create XLL files outside typical locations. Review and allow approved applications to reduce false positives.
references:
    - https://www.bleepingcomputer.com/news/security/new-specula-tool-uses-outlook-for-remote-code-execution-in-windows/
    - https://github.com/trustedsec/specula/wiki
drilldown_searches:
    - earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
      name: View the detection results for - "$user$" and "$dest$"
      search: '%original_detection_search% | search  user = "$user$" dest = "$dest$"'
    - name: View risk events for the last 7 days for - "$user$" and "$dest$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$user$", "$dest$") | 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: 7d
      latest_offset: "0"
intermediate_findings:
    entities:
        - field: dest
          type: system
          score: 20
          message: XLL add-in file created outside standard add-in paths at $file_path$ on $dest$.
threat_objects:
    - field: file_path
      type: file_path
analytic_story:
    - Spearphishing Attachments
asset_type: Endpoint
mitre_attack_id:
    - T1129
    - T1059
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1129/snapattack/snapattack.log
          source: XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
          sourcetype: XmlWinEventLog
      test_type: unit

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291cb133-bd0c-454d-9617-4a479ea81c0e

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