EXECryptor2021protectedIAT

Hunt Hypothesis

This detection identifies potential fileless or packed malware execution by monitoring for specific Import Address Table (IAT) protection signatures characteristic of the EXECryptor2021 family, which often indicates an adversary attempting to evade static analysis through runtime encryption. A SOC team should proactively hunt for this behavior in Azure Sentinel because early identification of these protected executables allows analysts to isolate suspicious processes before they establish persistence or exfiltrate data, mitigating risks associated with low-severity alerts that might otherwise be overlooked during routine monitoring.

YARA Rule

rule EXECryptor2021protectedIAT
{
      meta:
		author="malware-lu"
strings:
		$a0 = { A4 [3] 00 00 00 00 FF FF FF FF 3C [3] 94 [3] D8 [3] 00 00 00 00 FF FF FF FF B8 [3] D4 [3] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 6B 65 72 6E 65 6C 33 32 2E 64 6C 6C 00 00 00 00 00 00 47 65 74 4D 6F 64 75 6C 65 48 61 6E 64 6C 65 41 00 00 00 00 4C 6F 61 64 4C 69 62 72 61 72 79 41 00 00 00 00 47 65 74 50 72 6F 63 41 64 64 72 65 73 73 00 00 00 00 00 00 45 78 69 74 50 72 6F 63 65 73 73 00 00 00 [19] 00 60 [3] 70 [3] 84 [3] 00 00 00 00 75 73 65 72 33 32 2E 64 6C 6C 00 00 00 00 4D 65 73 73 61 67 65 42 6F 78 41 }

condition:
		$a0
}

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 4 specific false positive scenarios for the EXECryptor2021protectedIAT detection rule, along with recommended filters and exclusions:

• Scenario: Microsoft Office Click-to-Run Updates

  • Context: The OfficeClickToRun.exe process frequently modifies its own Import Address Table (IAT) during background updates to inject new DLLs or patch security features. This behavior mimics the IAT hooking technique used by EXECryptor2021, triggering a false positive on the update client service.
  • Filter/Exclusion: Add an exclusion for the process name OfficeClickToRun.exe running under the user context SYSTEM or specific service accounts (e.g., NT SERVICE\OfficeClickToRun).

• Scenario: Endpoint Protection Scanning (CrowdStrike Falcon)

  • Context: CrowdStrike Falcon’s real-time protection engine (C-Drive.exe) actively hooks the IAT of monitored applications to inspect file system and process creation events. When scanning protected executables, its hooking mechanism is indistinguishable from the EXECryptor2021 signature logic.
  • Filter/Exclusion: Exclude processes where the parent process name matches C-Drive.exe or CSFalconService.exe, specifically when the file path contains \Program Files\CrowdStrike\.

• Scenario: Scheduled Antivirus Definition Updates (Sophos Intercept X)

  • Context: During scheduled nightly definition updates, the Sophos engine (rtscan64.exe) performs deep integrity checks on protected binaries. It temporarily modifies IAT entries to verify cryptographic signatures of critical system files, which triggers the YARA rule’s detection logic for legitimate protection activities.
  • Filter/Exclusion: Create a time-based exclusion window (e.g., 01:00–04