LimaDelta

Hunt Hypothesis

This hypothesis posits that adversaries are utilizing the specific code patterns identified by the LimaDelta YARA signature to establish persistence or execute initial reconnaissance within Azure workloads. The SOC team should proactively hunt for these indicators in Azure Sentinel because early detection of this low-severity activity can reveal stealthy lateral movement before it escalates into a high-impact incident.

YARA Rule

rule LimaDelta
{
	meta:
		copyright = "2015 Novetta Solutions"
		author = "Novetta Threat Research & Interdiction Group - [email protected]"
		Source = "81e6118a6d8bf8994ce93f940059217481bfd15f2757c48c589983a6af54cfcc"

	strings:
	/*
		8B 69 FC           mov     ebp, [ecx-4]
		83 C1 10           add     ecx, 10h
		81 F5 6D 3A 71 58  xor     ebp, 58713A6Dh
		89 2A              mov     [edx], ebp
		33 ED              xor     ebp, ebp
		66 8B 69 F0        mov     bp, [ecx-10h]
		89 6A 04           mov     [edx+4], ebp
		83 C2 08           add     edx, 8
		4F                 dec     edi
		75 E3              jnz     short loc_4026CE
	*/

	$fileDecoder = {8B ?? ?? 83 ?? 10 81 ?? 6D 3A 71 58 89 ?? 33 ?? 66 ?? ?? F0 89 ?? 04 83 ?? 08 4? 75}
		
	/*
		66 81 BC 24 A0 00 00 00 BB 01  cmp     [esp+98h+arg_4], 1BBh
		74 21                          jz      short loc_401BD7
		FF 15 58 30 40 00              call    ds:rand
		99                             cdq
		B9 32 00 00 00                 mov     ecx, 32h
		F7 F9                          idiv    ecx
		8B DA                          mov     ebx, edx
		8D 54 24 5E                    lea     edx, [esp+98h+var_3A]
		53                             push    ebx             ; dwSize
		52                             push    edx             ; pvBuffer
		E8 3F FB FF FF                 call    GenerateRandomBuffer
		83 C4 08                       add     esp, 8
		83 C3 46                       add     ebx, 46h
	*/

	$authenicateBufferGen = {BB 01 74 ?? FF 15 [4] 99 B? 32 00 00 00 F7 ?? 8B ?? 8D [3] 5? 5? E8 [4] 83 C4 08 83 ?? 46}
	
	condition:
		$authenicateBufferGen 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 $fileDecoder 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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the LimaDelta detection rule in an enterprise environment, along with suggested filters or exclusions:

• Antivirus Real-Time Scanning of Large Archives

  • Scenario: The endpoint protection agent (e.g., CrowdStrike Falcon, SentinelOne, or Microsoft Defender for Endpoint) performs a scheduled deep scan on large compressed files (.zip, .7z) in the C:\Data\Archives directory. During this process, the scanner extracts and analyzes file headers, triggering the LimaDelta logic which mimics suspicious delta changes often associated with malware unpacking.
  • Filter/Exclusion: Exclude processes running under the specific AV service accounts (e.g., svc-falcon, sentinel-agent) and paths containing \Archives\ or \Temp\. Additionally, filter out events where the parent process is the AV engine itself rather than a user-interactive application.

• Automated Patch Deployment via SCCM/MECM

  • Scenario: The Microsoft Endpoint Configuration Manager (SCCM) executes a scheduled update task on workstations during business hours. As part of the deployment, the ccmexec.exe process writes temporary installation manifests and modifies registry keys to track patch status. These rapid file system modifications and registry delta updates match the signature pattern defined in the LimaDelta YARA rule.
  • Filter/Exclusion: Create a whitelist for the ccmexec.exe executable path (C:\Windows\CCM\) and its child processes. Furthermore, exclude events occurring during the organization’s defined “Maintenance Window” (e.g., 02:00–04:00 local time) where patching is expected.

• SQL Server Database Backup Jobs

  • Scenario: A nightly SQL maintenance job runs via **SQL Server Agent