EXECryptor 2021 protected IAT wwwstrongbitcom

Hunt Hypothesis

This hypothesis targets the presence of EXECryptor 2021 malware variants that utilize protected Import Address Tables (IAT) to obscure their dependencies, a technique often employed by threat actors to evade static analysis and dynamic detection. Proactively hunting for this specific YARA signature in Azure Sentinel allows the SOC to identify compromised endpoints early, preventing the malware from establishing persistence or executing further malicious actions before it is contained.

YARA Rule

rule EXECryptor_2021_protected_IAT_wwwstrongbitcom: PEiD
{
    strings:
        $a = { A4 ?? ?? ?? 00 00 00 00 FF FF FF FF 3C ?? ?? ?? 94 ?? ?? ?? D8 ?? ?? ?? 00 00 00 00 FF FF FF FF B8 ?? ?? ?? D4 ?? ?? ?? 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 }
    condition:
        $a 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 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: A legacy internal application or third-party plugin is compiled using an older version of the StrongBit EXECryptor 2021 packer for code obfuscation or anti-reverse-engineering, and the binary is deployed via a standard software distribution channel (e.g., SCCM, Intune, or Group Policy) to user workstations.

  • Filter/Exclusion: Exclude files located in known application directories (e.g., C:\Program Files\InternalApp\bin\) or created by specific service accounts (e.g., DOMAIN\svc-appdeploy) within the last 30 days.

• Scenario: An IT administrator manually copies a packed executable (such as a diagnostic tool or a small utility) from a network share or USB drive to a local temp folder (%TEMP% or C:\Users\<user>\AppData\Local\Temp) for quick testing or troubleshooting, triggering the rule due to the unusual location and packed state.

  • Filter/Exclusion: Exclude files with a path containing \Temp\ or \AppData\Local\Temp\ if the file size is under 5MB and the parent process is a known administrative tool (e.g., explorer.exe, powershell.exe, or cmd.exe) initiated by a user in the Domain Admins or IT Support group.

• Scenario: A scheduled maintenance job or a custom PowerShell script downloads a packed installer or updater component from an internal repository (e.g., Artifactory, Nexus, or a shared drive) to a working directory (e.g., C:\Scripts\ or C:\Maintenance\) for automated deployment or patching.

  • Filter/Exclusion: Exclude files created by scheduled tasks (check Task Scheduler logs or schtasks output) or by specific service accounts (e.g.,