GitHub Repo

Detection: Windows EFI Bootloader File Modification

Updated Date: 2026-04-13 ID: 28e1dbcd-9fd2-4de0-a637-f2b30db5dbb9 Author: Raven Tait, Splunk Type: TTP Product: Splunk Enterprise Security

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.

 1
 2| tstats `security_content_summariesonly`
 3  count min(_time) as firstTime
 4        max(_time) as lastTime
 5
 6from datamodel=Endpoint.Filesystem where
 7
 8Filesystem.file_path IN (
 9    "*\\EFI\\Boot\\bootmgfw.efi",
10    "*\\EFI\\Boot\\bootx64.efi"
11)
12
13by Filesystem.dest Filesystem.file_create_time Filesystem.process_path
14   Filesystem.process_guid Filesystem.process_id Filesystem.file_path Filesystem.file_name
15   Filesystem.user Filesystem.vendor_product Filesystem.action
16
17
18| `drop_dm_object_name(Filesystem)`
19
20| `security_content_ctime(firstTime)`
21
22| `security_content_ctime(lastTime)`
23
24| `windows_efi_bootloader_file_modification_filter`

Data Source

Name Platform Sourcetype Source
Sysmon EventID 11 Windows icon Windows 'XmlWinEventLog' 'XmlWinEventLog:Microsoft-Windows-Sysmon/Operational'

Macros Used

Name Value
security_content_ctime convert timeformat="%Y-%m-%dT%H:%M:%S" ctime($field$)
windows_efi_bootloader_file_modification_filter search *
windows_efi_bootloader_file_modification_filter is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Annotations

- MITRE ATT&CK
+ Kill Chain Phases
+ NIST
+ CIS
- Threat Actors
ID Technique Tactic
T1542.003 Bootkit Persistence
Exploitation
Installation
DE.CM
CIS 10

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Setting Value
Disabled true
Cron Schedule 0 * * * *
Earliest Time -70m@m
Latest Time -10m@m
Schedule Window auto
Creates Notable Yes
Rule Title %name%
Rule Description %description%
Notable Event Fields user, dest
Creates Risk Event True
This configuration file applies to all detections of type TTP. These detections will use Risk Based Alerting and generate Notable Events.

Implementation

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.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message:

Critical EFI bootloader file activity observed at $file_path$ on $dest$.

Risk Object Risk Object Type Risk Score Threat Objects
dest system 50 file_path

References

Detection Testing

Test Type Status Dataset Source Sourcetype
Validation ✅ Passing N/A N/A N/A
Unit ✅ Passing Dataset XmlWinEventLog:Microsoft-Windows-Sysmon/Operational XmlWinEventLog
Integration ✅ Passing Dataset XmlWinEventLog:Microsoft-Windows-Sysmon/Operational XmlWinEventLog

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 1