AcidCrypt

Hunt Hypothesis

This detection identifies the presence of AcidCrypt ransomware signatures on endpoints to uncover early-stage infection or dormant malware that may evade standard heuristic scans. Proactively hunting for this behavior in Azure Sentinel is critical because it allows the SOC team to isolate affected assets and initiate incident response before encryption activities cause widespread data loss.

YARA Rule

rule AcidCrypt: Packer
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 B9 [3] 00 BA [3] 00 BE [3] 00 02 38 40 4E 75 FA 8B C2 8A 18 32 DF C0 CB }
	$a1 = { BE [4] 02 38 40 4E 75 FA 8B C2 8A 18 32 DF C0 CB }

condition:
		$a0 at pe.entry_point or $a1 at pe.entry_point
}

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 5 specific false positive scenarios for the AcidCrypt detection rule in an enterprise environment, including suggested filters and exclusions:

• Scenario: Microsoft Office Document Macro Execution

  • Context: Users opening Excel workbooks or Word documents containing VBA macros often trigger AcidCrypt because the Office process (EXCEL.EXE or WINWORD.EXE) spawns child processes to compile and execute macro code, mimicking the behavior of file encryption and script injection.
  • Filter/Exclusion: Exclude processes running from the standard Microsoft Office installation directory (e.g., C:\Program Files\Microsoft Office\root\Office16\). Additionally, filter out events where the parent process is a known Office executable with a valid digital signature from “Microsoft Corporation.”

• Scenario: Antivirus Real-Time Scanning and Heuristics

  • Context: Endpoint Protection platforms like CrowdStrike Falcon, SentinelOne, or Microsoft Defender for Endpoint frequently perform real-time scanning that involves temporary file creation, hashing, and memory mapping. These activities can resemble the file manipulation patterns detected by AcidCrypt as potential ransomware behavior.
  • Filter/Exclusion: Create an exclusion list for known security vendor processes (e.g., C:\Program Files\CrowdStrike\csagent.exe, C:\ProgramData\McAfee\Agent\x86\mfevcs.exe). Filter alerts where the parent process is a recognized antivirus engine and the action involves “scan” or “quarantine” rather than bulk file modification.

• Scenario: Scheduled Backup and Archiving Jobs

  • Context: Enterprise backup solutions such as Veeam, Rubrik, or Commvault run scheduled jobs that compress, encrypt, and archive large volumes of data. These tools often spawn multiple threads to process files simultaneously,