SierraBravo One

Hunt Hypothesis

This hypothesis posits that adversaries are executing initial reconnaissance or lateral movement activities characterized by the specific file signatures identified in the SierraBravo One YARA rule. Proactive hunting for this behavior within Azure Sentinel is essential to validate the effectiveness of existing detection logic and identify potential low-severity indicators that may precede more critical compromise events.

YARA Rule

rule SierraBravo_One
{
	meta:
		copyright = "2015 Novetta Solutions"
		author = "Novetta Threat Research & Interdiction Group - [email protected]"

	strings:
		/*
			.text:00402A65                 push    8004667Eh       ; cmd
			.text:00402A6A                 push    esi             ; s
			.text:00402A6B                 call    ioctlsocket
			.text:00402A70                 push    32h             ; dwMilliseconds
			.text:00402A72                 mov     [esp+24Ch+writefds.fd_array], esi
			.text:00402A79                 mov     [esp+24Ch+writefds.fd_count], 1
			.text:00402A84                 mov     [esp+24Ch+timeout.tv_sec], 3
			.text:00402A8C                 mov     [esp+24Ch+timeout.tv_usec], 0			
		*/
		$spreaderSetup = {68 7E 66 04 80 5? E8 [4] 6A 32 89 B4 [5] C7 84 [5] 01 00 00 00 C7 44 [2] 03 00 00 00 C7 44 [2] 00 00 00 00 }

	condition:
		$spreaderSetup in ((pe.sections[pe.section_index(".text")].raw_data_offset)..(pe.sections[pe.section_index(".text")].raw_data_offset + pe.sections[pe.section_index(".text")].raw_data_size))
}

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 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the SierraBravo One detection rule in a legitimate enterprise environment, along with suggested filters or exclusions:

• Scenario: Enterprise Endpoint Protection Scans

  • Context: The YARA rule likely flags the execution of complex, multi-threaded scanning binaries. In an environment using CrowdStrike Falcon or SentinelOne, the scheduled daily full-system scan involves the main scanner process spawning multiple worker threads that match the behavioral signature of SierraBravo One.
  • Filter/Exclusion: Create a whitelist rule excluding processes where ImageName equals C:\Program Files\CrowdStrike\fs_hids.exe (or equivalent SentinelOne sensor path) and ParentProcessName is the service host (svchost.exe). Additionally, exclude events occurring during the defined maintenance window (e.g., 02:00–04:00 local time).

• Scenario: Microsoft Office Macro Deployment via GPO

  • Context: During a standard Group Policy Object (GPO) refresh cycle, the Microsoft Office Click-to-Run service (OfficeClickToRun.exe) extracts and executes macro-enabled templates across user workstations. This extraction process often triggers the SierraBravo One signature due to the rapid creation of temporary script files in %TEMP% directories.
  • Filter/Exclusion: Exclude alerts where CommandLine contains parameters related to GPO updates (e.g., /gpo, /update) and the UserAccount is a non-interactive system account such as NT AUTHORITY\SYSTEM or DOMAIN\GPO_Service_Acc.

• Scenario: Automated Software Patching with SCCM/MECM

  • Context: The Microsoft Endpoint Configuration Manager (MECM) agent (ccmexec.exe) frequently deploys