Detection: Windows EFI Volume Mount Attempt Via Mountvol

Description

Detects attempts to mount the EFI volume. The EFI system partition (ESP) is a special partition on a data storage device (usually a hard disk drive or solid-state drive) that computers adhering to the UEFI (Unified Extensible Firmware Interface) specification use to store data necessary for the system to boot, such as bootloaders, device drivers, and system utilities. This is used with attacks such as PKFail to modify the system on boot.

Search

 1
 2| tstats `security_content_summariesonly`
 3  count min(_time) as firstTime
 4        max(_time) as lastTime
 5
 6from datamodel=Endpoint.Processes where
 7
 8(
 9    Processes.process_name="mountvol.exe"
10    OR
11    Processes.original_file_name="MOUNTVOL.EXE"
12)
13Processes.process IN ("*-S*", "* /S*")
14
15by Processes.process Processes.vendor_product Processes.user_id Processes.process_hash
16   Processes.parent_process_name Processes.parent_process_exec Processes.action
17   Processes.dest Processes.process_current_directory Processes.process_path
18   Processes.process_integrity_level Processes.original_file_name Processes.parent_process
19   Processes.parent_process_path Processes.parent_process_guid Processes.parent_process_id
20   Processes.process_guid Processes.process_id Processes.user Processes.process_name
21
22
23| `drop_dm_object_name(Processes)`
24
25| `security_content_ctime(firstTime)`
26
27| `security_content_ctime(lastTime)`
28
29| `windows_efi_volume_mount_attempt_via_mountvol_filter`

Data Source

Name	Platform	Sourcetype	Source
CrowdStrike ProcessRollup2	Other	'crowdstrike:events:sensor'	'crowdstrike'
Sysmon EventID 1	Windows	'XmlWinEventLog'	'XmlWinEventLog:Microsoft-Windows-Sysmon/Operational'
Windows Event Log Security 4688	Windows	'XmlWinEventLog'	'XmlWinEventLog:Security'

Macros Used

Annotations

Default Configuration

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

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

Mounting the EFI volume can be part of normal system updates or legitimate maintenance tasks. Monitor for unusual timing or context to reduce false alerts.

Associated Analytic Story

• Compromised Windows Host

Risk Based Analytics (RBA)

Risk Message:

Potential EFI Volume Mount Attempt by $user$ via $process$ observed on $dest$.

References

• https://www.binarly.io/blog/pkfail-untrusted-platform-keys-undermine-secure-boot-on-uefi-ecosystem

Detection Testing

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