RomeoCharlie

Hunt Hypothesis

This hypothesis posits that adversaries are executing specific code patterns identified by the RomeoCharlie YARA signature to establish persistence or execute initial payloads within the environment. The SOC team should proactively hunt for these indicators in Azure Sentinel to validate the rule’s efficacy and uncover potential low-severity threats that may be evolving into more significant incidents before triggering automated alerts.

YARA Rule

rule RomeoCharlie
{
	meta:
		copyright = "2015 Novetta Solutions"
		author = "Novetta Threat Research & Interdiction Group - trig@novetta.com"
		Source = "a82108ef7115931b3fbe1fab99448c4139e22feda27c1b1d29325710671154e8"

	strings:
		$auth1 = "Success - Accept Auth"
		$auth2 = "Fail - Accept Auth"

	/*
		81 E3 FF FF 00 00  and     ebx, 0FFFFh
		8B EB              mov     ebp, ebx
		57                 push    edi
		C1 EE 10           shr     esi, 10h
		81 E5 FF FF 00 00  and     ebp, 0FFFFh
		8B FE              mov     edi, esi
		8B C5              mov     eax, ebp
		81 E7 FF FF 00 00  and     edi, 0FFFFh
		C1 E0 10           shl     eax, 10h
		6A 00              push    0               ; _DWORD
		0B C7              or      eax, edi
		6A 00              push    0               ; _DWORD
		50                 push    eax             ; _DWORD
		68 10 14 11 71     push    offset sub_71111410; _DWORD
		6A 00              push    0               ; _DWORD
		6A 00              push    0               ; _DWORD
		FF 15 5C 8E 12 71  call    CreateThread_0
		C1 E7 10           shl     edi, 10h
	*/

	$startupRelayThreads = {81 ?? FF FF 00 00 8B ?? 5? C1 ?? 10 81 ?? FF FF 00 00 8B ?? 8B ?? 81 ?? FF FF 00 00 C1 ?? 10 6A 00 0B ?? 6A 00 	50 68 [4] 6A 00 6A 00 FF 15 [4] C1 ?? 10 }

	/*
	source: 641808833ad34f2e5143001c8147d779dbfd2a80a80ce0cfc81474d422882adb
		25 00 20 00 00     and     eax, 2000h
		3D 00 20 00 00     cmp     eax, 2000h
		0F 94 C1           setz    cl
		81 E2 80 00 00 00  and     edx, 80h
		33 C0              xor     eax, eax
		80 FA 80           cmp     dl, 80h
		0F 94 C0           setz    al
		03 C8              add     ecx, eax
		33 D2              xor     edx, edx
		83 F9 01           cmp     ecx, 1
	*/

	$crypto = {2? 00 20 00 00 3? 00 20 00 00 0F [2] 81 ?? 80 00 00 00 33 ?? 80 ?? 80 0F [2] 03 ?? 33 ?? 83 ?? 01 }

	condition:
		all of ($auth*) 
		or $startupRelayThreads 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 $crypto 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 4 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the RomeoCharlie detection rule in an enterprise environment, including suggested filters and exclusions:

• Endpoint Protection Policy Updates via Windows Update Service

  • Scenario: The Windows Update service (or a managed agent like Microsoft Intune or SCCM) periodically downloads and installs security definition updates. This process often involves the creation of temporary files, registry modifications, and execution of signed binaries in system directories that mimic the behavioral patterns targeted by the RomeoCharlie rule.
  • Filter/Exclusion: Exclude processes originating from C:\Windows\SoftwareDistribution or those with a digital signature belonging to “Microsoft Corporation” where the parent process is svchost.exe (specifically the wuauserv service). Additionally, filter out events occurring during the standard maintenance window (e.g., 02:00–04:00 UTC) if the rule triggers on file creation in %TEMP%.

• Scheduled Antivirus Full Scans by CrowdStrike Falcon

  • Scenario: During nightly full system scans, the CrowdStrike Falcon Sensor executes deep inspection routines that spawn multiple child processes to analyze memory and disk sectors. These activities generate high-volume I/O operations and process spawning events that can be misidentified as lateral movement or suspicious execution by the RomeoCharlie logic.
  • Filter/Exclusion: Create an exclusion for the specific executable path C:\Program Files\CrowdStrike\falcon-sensor.exe (and its known child processes like falcon.sys). Ensure the filter checks for the “Full Scan” job ID in the event metadata, or explicitly whitelist the hash of the CrowdStrike sensor binary to bypass detection during scan windows.

• Automated Backup Jobs via Veeam Backup & Replication

  • Scenario: The **Vee