LimaAlfa

Hunt Hypothesis

This hypothesis posits that adversaries are utilizing specific file artifacts identified by the LimaAlfa YARA signature to establish persistence or execute initial reconnaissance within the Azure environment. Proactive hunting for these indicators is essential to detect early-stage threats before they escalate, ensuring the SOC team can validate low-severity alerts against actual malicious behavior rather than relying solely on reactive rule triggers.

YARA Rule

rule LimaAlfa
{
	meta:
		copyright = "2015 Novetta Solutions"
		author = "Novetta Threat Research & Interdiction Group - trig@novetta.com"
		Source = "c9fbad7fc7ff7688776056be3a41714a1f91458a7b16c37c3c906d17daac2c8b"
		Status = "Signature is too loose to be useful."

	strings:
	/*
		33 C0              xor     eax, eax
		66 8B 02           mov     ax, [edx]
		8B E8              mov     ebp, eax
		81 E5 00 F0 FF FF  and     ebp, 0FFFFF000h
		81 FD 00 30 00 00  cmp     ebp, 3000h
		75 0D              jnz     short loc_4019FB
		8B 6C 24 18        mov     ebp, [esp+10h+arg_4]
		25 FF 0F 00 00     and     eax, 0FFFh
		03 C7              add     eax, edi
		01 28              add     [eax], ebp
	*/

	$a = {33 C0 66 [2] 8B ?? 81 ?? 00 F0 FF FF 81 ?? 00 30 00 00 75 ?? 8B [3] 25 FF 0F 00 00 03 C7 01}

	condition:
		$a 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 5 specific false positive scenarios for the LimaAlfa detection rule in an enterprise environment:

• Endpoint Protection Policy Updates via CrowdStrike Falcon

  • Scenario: The CrowdStrike Falcon Sensor performs a scheduled policy refresh or definition update (typically occurring at 02:00 UTC). During this process, the sensor executable (CylanceAgent.exe or FalconSensor.exe) spawns child processes to download and validate new signatures, triggering the LimaAlfa logic due to specific file creation patterns in the %ProgramData% directory.
  • Filter/Exclusion: Exclude processes with the parent image path containing \CrowdStrike\ and the command line argument --update-policy.

• Microsoft SCCM (Configuration Manager) Software Deployment

  • Scenario: An automated deployment of a critical security patch via Microsoft Endpoint Configuration Manager triggers the ccmexec.exe service. As part of the installation sequence, it extracts temporary files and executes a custom PowerShell script (Install-Patch.ps1) that writes configuration logs to %Temp%, mimicking the file manipulation behavior detected by LimaAlfa.
  • Filter/Exclusion: Create an exclusion for any process where the parent image is ccmexec.exe and the working directory starts with \Microsoft SCCM\.

• Enterprise Backup Job Execution (Veeam Backup & Replication)

  • Scenario: The nightly Veeam backup job (vbrservice.exe) initiates a “Full Backup” task. During the pre-backup phase, it generates incremental metadata files and snapshots in the C:\ProgramData\Veeam\Backup folder. The rapid creation of these large metadata files often triggers the LimaAlfa rule’s file integrity checks.
  • Filter/Exclusion: Exclude events where the process name is vbrservice.exe