Detects an implant used by Bemstour exploitation tool (APT3)

Hunt Hypothesis

This hunt hypothesis identifies the presence of a Bemstour exploitation tool implant associated with APT3 to detect early-stage adversary reconnaissance and persistence activities within the network. Proactively hunting for this specific implant in Azure Sentinel is critical because it enables the SOC team to uncover stealthy, low-severity intrusions that may evade standard signature-based detections before they escalate into significant compromises.

YARA Rule

rule apt3_bemstour_implant_byte_patch
{
meta:

description = "Detects an implant used by Bemstour exploitation tool (APT3)"
author = "Mark Lechtik"
company = "Check Point Software Technologies LTD."
date = "2019-06-25"
sha256 = "0b28433a2b7993da65e95a45c2adf7bc37edbd2a8db717b85666d6c88140698a"

/*

0x41b7e1L C745B8558BEC83                mov dword ptr [ebp - 0x48], 0x83ec8b55
0x41b7e8L C745BCEC745356                mov dword ptr [ebp - 0x44], 0x565374ec
0x41b7efL C745C08B750833                mov dword ptr [ebp - 0x40], 0x3308758b
0x41b7f6L C745C4C957C745                mov dword ptr [ebp - 0x3c], 0x45c757c9
0x41b7fdL C745C88C4C6F61                mov dword ptr [ebp - 0x38], 0x616f4c8c

*/

id = "c30434c3-8949-566c-b6a6-29bffdaf961d"
strings:

$chunk_1 = {

C7 45 ?? 55 8B EC 83
C7 45 ?? EC 74 53 56
C7 45 ?? 8B 75 08 33
C7 45 ?? C9 57 C7 45
C7 45 ?? 8C 4C 6F 61

}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints
• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the “Bemstour exploitation tool (APT3)” detection rule, including suggested filters and exclusions:

• Scenario: Legitimate Backup Operations via Veeam or Commvault

  • Context: Enterprise backup agents often utilize PowerShell scripts or custom executables to stage data before transmission. These processes frequently spawn child processes with similar command-line arguments (e.g., powershell.exe -WindowStyle Hidden -NoProfile) and network behaviors that mimic the Bemstour implant’s staging mechanism.
  • Filter/Exclusion: Exclude process paths matching known backup agent executables (e.g., C:\Program Files\Veeam\Backup and Replication\Agent\... or C:\Program Files\Commvault\...). Additionally, filter out events where the parent process is a recognized service account (e.g., NT SERVICE\VeeamTransportService) running on dedicated backup servers.

• Scenario: Scheduled Patch Management Jobs using SCCM/Intune

  • Context: System Center Configuration Manager (SCCM) or Microsoft Intune often executes scheduled deployment tasks that download and stage update packages. These jobs may invoke temporary scripts or wrappers that match the specific file hash patterns or registry modifications associated with the Bemstour tool, particularly when deploying custom application updates to endpoints.
  • Filter/Exclusion: Create an exclusion for process trees where the root parent is ccmexec.exe (SCCM) or IntuneManagementExtension. Further refine by excluding events occurring during defined maintenance windows (e.g., 02:00–04:00 UTC) on endpoints tagged with “Patch Target” in your asset inventory.

• Scenario: Endpoint Detection and Response (EDR) Self-Scans

  • Context: Modern EDR solutions (such as CrowdStrike Falcon, Microsoft