malware creates a process in the temp directory and performs the sysprep UAC bypass method.

Hunt Hypothesis

This hypothesis posits that adversaries are executing malicious processes within temporary directories to leverage the Sysprep User Account Control (UAC) bypass technique for privilege escalation without triggering standard alerts. Proactive hunting in Azure Sentinel is critical because this specific combination of low-severity indicators often evades signature-based detection, allowing attackers to establish persistence and execute further lateral movement undetected.

YARA Rule

rule Sakurel_backdoor
{
	meta:
		maltype = "Sakurel backdoor"
    ref = "https://github.com/reed1713"
		reference = "http://www.microsoft.com/security/portal/threat/encyclopedia/entry.aspx?Name=Trojan:Win32/Sakurel.A#tab=2"
		description = "malware creates a process in the temp directory and performs the sysprep UAC bypass method."
	strings:
		$type="Microsoft-Windows-Security-Auditing"
		$eventid="4688"
		$data="Windows\\System32\\sysprep\\sysprep.exe" nocase

		$type1="Microsoft-Windows-Security-Auditing"
		$eventid1="4688"
		$data1="AppData\\Local\\Temp\\MicroMedia\\MediaCenter.exe" nocase
	condition:
		all of them
}

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

This rule contains 6 string patterns in its detection logic.

References

• http://www.microsoft.com/security/portal/threat/encyclopedia/entry.aspx?Name=Trojan:Win32/Sakurel.A#tab=2
• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the rule “Malware creates a process in the temp directory and performs the Sysprep UAC bypass method,” along with suggested filters or exclusions:

• Windows Update Deployment via Microsoft Endpoint Configuration Manager (MECM/SCCM)

  • Scenario: During major OS updates or feature packs, SCCM agents often extract installation packages to %TEMP% before execution. The ccmexec.exe service may spawn a temporary process (e.g., msiexec.exe or a custom installer) in the temp folder that invokes sysprep.exe with specific UAC bypass arguments (/unattend, /quiet) to apply system configurations without triggering user prompts during off-hours.
  • Suggested Filter: Exclude processes where the parent process is ccmexec.exe (Microsoft Endpoint Configuration Manager) and the command line contains known SCCM package identifiers or paths under C:\Windows\CCMCache.

• Automated Patch Management with Chocolatey

  • Scenario: In environments using Chocolatey for software deployment, the choco.exe service frequently downloads installers to %TEMP% and executes them. When deploying system-level updates or drivers that require UAC elevation but need to run silently in a background job context, Chocolatey may utilize the Sysprep bypass method to inject configurations into the registry or file system without user interaction.
  • Suggested Filter: Create an exclusion for processes initiated by choco.exe where the working directory is %TEMP% and the command line includes flags such as -noupdate, -y, or specific package names defined in your software inventory (e.g., adobe-acrobat-reader).

• Scheduled System Health Diagnostics via PowerShell

  • Scenario: Enterprise IT teams often run scheduled PowerShell scripts (powershell.exe) to perform system health checks