SierraCharlie

Hunt Hypothesis

This hypothesis posits that adversaries are utilizing specific file signatures defined by the SierraCharlie YARA rule to establish persistence or execute initial reconnaissance within the environment. The SOC team should proactively hunt for these indicators in Azure Sentinel to identify early-stage threats that may not yet trigger high-severity alerts, thereby reducing dwell time before lateral movement occurs.

YARA Rule

rule SierraCharlie
{
	meta:
		copyright = "2015 Novetta Solutions"
		author = "Novetta Threat Research & Interdiction Group - trig@novetta.com"
		Source = "f4750e1d82b08318bdc1eb6d3399dee52750250f7959a5e4f83245449f399698.bin"

	strings:
	/*
		8B 0D 50 A7 56 00  mov     ecx, DnsFree
		81 F6 8C 3F 7C 5E  xor     esi, 5E7C3F8Ch
		6A 01              push    1               ; _DWORD
		50                 push    eax             ; _DWORD
		85 C9              test    ecx, ecx
		74 3A              jz      short loc_40580B
		FF D1              call    ecx ; DnsFree
	*/

	$dnsResolve = {	8B 0D 50 A7 56 00 81 F6 8C 3F 7C 5E 6A 01 50 85 C9 74 3A FF D1 	}
		
	$file1 = "wmplog21t.sqm"
	$file2 = "wmplog15r.sqm"
	$file3 = "wmplog09c.sqm"
		

	condition:
		$dnsResolve in ((pe.sections[pe.section_index(".text")].raw_data_offset)..(pe.sections[pe.section_index(".text")].raw_data_offset + pe.sections[pe.section_index(".text")].raw_data_size))
		or 2 of ($file*)
}

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 4 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the SierraCharlie detection rule in an enterprise environment, including recommended filters and exclusions:

• Endpoint Protection Scanning Cycles

  • Scenario: The primary anti-malware engine (e.g., CrowdStrike Falcon or Microsoft Defender for Endpoint) performs a scheduled deep scan of the C:\Program Files\SierraCharlie directory. During this process, the scanner’s service account (NT AUTHORITY\SYSTEM) launches a temporary analysis executable that matches the SierraCharlie YARA signature due to shared code libraries or embedded scripts.
  • Filter/Exclusion: Create an exclusion for processes spawned by MsMpEng.exe, CrowdStrikeService.exe, and SophosAutoProtect.exe when accessing files within the known installation path of the application.

• Software Deployment via SCCM

  • Scenario: The System Center Configuration Manager (SCCM) or Microsoft Intune initiates a silent patch deployment for the SierraCharlie suite. The deployment agent (ccmexec.exe) extracts and executes the installer package, which temporarily instantiates the core binary detected by the rule before the service fully registers its digital signature in the whitelist database.
  • Filter/Exclusion: Exclude execution events where the parent process is ccmexec.exe or IntuneManagementExtension.exe and the file hash matches the known “SierraCharlie Installer” version (e.g., SHA256 starting with a1b2...).

• Automated Log Rotation and Archiving

  • Scenario: A nightly scheduled task (Task Scheduler) runs a PowerShell script to compress and archive SierraCharlie logs. The script invokes the native SierraCharlie.exe utility in “archive mode” to generate checksums, triggering the detection logic even though no user interaction is occurring.
  • **Filter/Ex