Windows EFI Bootloader File Modification

name: Windows EFI Bootloader File Modification
id: 28e1dbcd-9fd2-4de0-a637-f2b30db5dbb9
version: 2
creation_date: '2026-05-05'
modification_date: '2026-05-13'
author: Raven Tait, Splunk
status: production
type: TTP
description: |-
    Detects where a process writes to critical EFI bootloader files (bootmgfw.efi or bootx64.efi) within the \EFI\Boot\ directory.
    These files are responsible for initializing the Windows Boot Manager during system startup.
    Modification or replacement of these files is highly unusual in normal operations and may indicate an attempt to install a bootkit, persist malicious code at the firmware level, or otherwise tamper with the system boot process.
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 IN (
        "*\\EFI\\Boot\\bootmgfw.efi",
        "*\\EFI\\Boot\\bootx64.efi"
    )

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

    | `drop_dm_object_name(Filesystem)`
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `windows_efi_bootloader_file_modification_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: Legitimate system updates or authorized IT maintenance may modify EFI bootloader files. Review changes against approved maintenance windows and tools.
references:
    - https://www.bleepingcomputer.com/news/security/new-hybridpetya-ransomware-can-bypass-uefi-secure-boot/
    - https://www.welivesecurity.com/en/eset-research/under-cloak-uefi-secure-boot-introducing-cve-2024-7344/
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"
finding:
    title: Critical EFI bootloader file activity observed at $file_path$ on $dest$.
    entity:
        field: dest
        type: system
        score: 50
threat_objects:
    - field: file_path
      type: file_path
analytic_story:
    - Windows BootKits
asset_type: Endpoint
mitre_attack_id:
    - T1542.003
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/T1542.003/snapattack/snapattack.log
          source: XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
          sourcetype: XmlWinEventLog
      test_type: unit