Executable File with Unusual Extension

Identifies the creation or modification of an executable file with an unexpected file extension. Attackers may attempt to evade detection by masquerading files using the file extension values used by image, audio, or document file types.

Rule type: eql
Rule indices:

• logs-endpoint.events.file-*

Rule Severity: low
Risk Score: 21
Runs every: 60m
Searches indices from: now-119m
Maximum alerts per execution: ?
References:

Tags:

• Domain: Endpoint
• OS: Windows
• Use Case: Threat Detection
• Tactic: Defense Evasion
• Rule Type: BBR
• Data Source: Elastic Defend

Version: ?
Rule authors:

• Elastic

Rule license: Elastic License v2

file where host.os.type == "windows" and event.action != "deletion" and

 /* MZ header or its common base64 equivalent TVqQ */
 file.Ext.header_bytes : ("4d5a*", "54567151*") and

 (
   /* common image file extensions */
   file.extension : ("jpg", "jpeg", "emf", "tiff", "gif", "png", "bmp", "fpx", "eps", "svg", "inf") or

   /* common audio and video file extensions */
   file.extension : ("mp3", "wav", "avi", "mpeg", "flv", "wma", "wmv", "mov", "mp4", "3gp") or

   /* common document file extensions */
   file.extension : ("txt", "pdf", "doc", "docx", "rtf", "ppt", "pptx", "xls", "xlsx", "hwp", "html")
  ) and
  not process.pid == 4 and
  not process.executable : "?:\\Program Files (x86)\\Trend Micro\\Client Server Security Agent\\Ntrtscan.exe"

Framework: MITRE ATT&CK

• Tactic:

  • Name: Defense Evasion
  • Id: TA0005
  • Reference URL: https://attack.mitre.org/tactics/TA0005/

• Technique:

  • Name: Masquerading
  • Id: T1036
  • Reference URL: https://attack.mitre.org/techniques/T1036/

• Sub Technique:

  • Name: Masquerade File Type
  • Id: T1036.008
  • Reference URL: https://attack.mitre.org/techniques/T1036/008/