Windows Rundll32 Execution With Log.DLL

name: Windows Rundll32 Execution With Log.DLL
id: f9593331-804c-4268-8b4c-2693c5ae786c
version: 3
creation_date: '2026-03-16'
modification_date: '2026-05-13'
author: Michael Haag, Splunk
status: production
type: Anomaly
description: |
    Identifies the execution of rundll32 with a command line argument of "log.dll", as used in the Lotus Blossom Chrysalis backdoor campaign.
    Attackers placed a malicious "log.dll" in "%AppData%\Bluetooth" and invoked it via rundll32.exe "log.dll,LogInit" to decrypt and execute ShellCode.
    The legitimate Bitdefender Submission Wizard (BDSubmit.exe, bdsw.exe) also uses log.dll and is vulnerable to DLL sideloading.
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
      `process_rundll32`
      Processes.process="* log.dll*"
      by Processes.action Processes.dest Processes.original_file_name Processes.parent_process Processes.parent_process_exec Processes.parent_process_guid Processes.parent_process_id Processes.parent_process_name Processes.parent_process_path Processes.process Processes.process_exec Processes.process_guid Processes.process_hash Processes.process_id Processes.process_integrity_level Processes.process_name Processes.process_path Processes.user Processes.user_id Processes.vendor_product
    | `drop_dm_object_name(Processes)`
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `windows_rundll32_execution_with_log_dll_filter`
how_to_implement: |
    Ingest process creation logs (Sysmon EventID 1 or equivalent) and map them to the Endpoint.Processes data model. Ensure command-line arguments are captured so that "log.dll" appears in the process field. The Splunk Add-on for Microsoft Sysmon and CIM are required.
known_false_positives: |
    Legitimate use of rundll32 to load log.dll from trusted locations may trigger this.
    Allowlist known paths or parent processes as needed.
references:
    - https://attack.mitre.org/techniques/T1574/002/
    - https://www.rapid7.com/blog/post/tr-chrysalis-backdoor-dive-into-lotus-blossoms-toolkit/
    - https://attack.mitre.org/groups/G0065/
intermediate_findings:
    entities:
        - field: dest
          type: system
          score: 20
          message: Rundll32 loaded log.dll on $dest$ by user $user$, indicating potential Lotus Blossom-style DLL side loading abuse.
        - field: user
          type: user
          score: 20
          message: Rundll32 loaded log.dll on $dest$ by user $user$, indicating potential Lotus Blossom-style DLL side loading abuse.
threat_objects:
    - field: process
      type: process
    - field: process_name
      type: process_name
analytic_story:
    - Lotus Blossom Chrysalis Backdoor
asset_type: Endpoint
mitre_attack_id:
    - T1574
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint