Windows AppCertDLL Modification Via Command Line

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

This analytic detects attempts to modify AppCertDLL registry keys via some command line utility. Values under this key are used to specify DLLs loaded by the Windows Session Manager. Such modifications can be abused by attackers to load malicious code early in the system startup process, enabling persistent malware execution with high privileges. If confirmed malicious, this behavior may lead to system compromise, persistence, and the evasion of security controls.

name: Windows AppCertDLL Modification Via Command Line
id: 3ba73a2b-3396-47e4-bdef-c80e6a7895c0
version: 2
creation_date: '2021-05-07'
modification_date: '2026-07-27'
author: Raven Tait, Splunk
status: production
type: Anomaly
description: |-
    This analytic detects attempts to modify AppCertDLL registry keys via some command line utility. Values under this key are used to specify DLLs loaded by the Windows Session Manager.
    Such modifications can be abused by attackers to load malicious code early in the system startup process, enabling persistent malware execution with high privileges.
    If confirmed malicious, this behavior may lead to system compromise, persistence, and the evasion of security controls.
data_source:
    - Sysmon EventID 1
    - Windows Event Log Security 4688
    - CrowdStrike ProcessRollup2
search: |-
    | tstats `security_content_summariesonly`
      count min(_time) as firstTime
            max(_time) as lastTime

    from datamodel=Endpoint.Processes where

    Processes.process="*\\Control\\Session Manager\\AppCertDLLs*"

    by Processes.process Processes.vendor_product Processes.user_id
       Processes.process_hash Processes.parent_process_name
       Processes.parent_process_exec Processes.action Processes.dest
       Processes.process_current_directory Processes.process_path
       Processes.process_integrity_level Processes.original_file_name
       Processes.parent_process Processes.parent_process_path
       Processes.parent_process_guid Processes.parent_process_id
       Processes.process_guid Processes.process_id
       Processes.user Processes.process_name

    | `drop_dm_object_name(Processes)`
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `windows_appcertdll_modification_via_command_line_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: |-
    Modifications to AppCertDLL keys may be made by legitimate software updates or trusted system management tools. Review changes to ensure they are from approved sources.
references:
    - https://www.hexacorn.com/blog/2013/01/19/beyond-good-ol-run-key-part-3/
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: Potential AppCertDLL modification activity observed on $dest$ via $process$.
threat_objects:
    - field: parent_process_name
      type: parent_process_name
    - field: process
      type: process
    - field: process_name
      type: process_name
analytic_story:
    - Windows Persistence Techniques
    - Windows Privilege Escalation
asset_type: Endpoint
mitre_attack_id:
    - T1546.009
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/T1546.009/snapattack/snapattack.log
          source: XmlWinEventLog:Security
          sourcetype: XmlWinEventLog
      test_type: unit

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