AVI movie file Hint FILE START

Hunt Hypothesis

This hypothesis targets the presence of AVI video files in locations where they are atypical, such as application data directories or system folders, which may indicate an adversary using steganography or file carving to hide payloads within multimedia containers. Proactively hunting for these files helps the SOC identify potential data staging or covert communication channels that standard file extension monitoring might miss, ensuring that low-severity artifacts are not overlooked in the Azure Sentinel environment.

YARA Rule

rule AVI_movie_file_Hint_FILE_START: PEiD
{
    strings:
        $a = { 52 49 46 46 ?? ?? ?? ?? 41 56 49 ?? 4C 49 53 54 }
    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

• Media Asset Ingestion by Digital Asset Management (DAM) Systems: When marketing or creative teams upload legacy video assets to a central repository (e.g., Bynder, Adobe Experience Manager, or SharePoint), the system may temporarily write .avi files to the application server’s local temp directory or shared drive before transcoding them to modern formats.

  • Filter/Exclusion: Exclude file paths under specific DAM application directories (e.g., C:\Program Files\Adobe\..., \\fileserver\dam\temp\) or exclude processes associated with known DAM services (e.g., w3wp.exe when the parent process is the DAM service, or specific service executables like bynderagent.exe).

• Legacy Video Transcoding via FFmpeg or HandBrake: IT support or media teams may use command-line tools like ffmpeg or HandBrake to convert old AVI files to MP4 or MOV. During this process, the tool may create temporary .avi files in the user’s %TEMP% directory or the source folder to handle intermediate processing steps.

  • Filter/Exclusion: Exclude files located in the user’s %TEMP% directory (C:\Users\<username>\AppData\Local\Temp\) or exclude files created by known transcoding executables (ffmpeg.exe, HandBrakeCLI.exe, mediainfo.exe).

• Backup and Archive Restoration: When restoring old backup sets using tools like Veeam, Commvault, or Windows Server Backup, legacy video files stored in archives may be extracted to a staging area on the backup server or a dedicated restore share. These files often retain their original .avi extension.

  • Filter/Exclusion: Exclude file paths under backup staging directories (e.g., C:\Backup\Staging\, D:\Commvault\Restore\) or