Downrage Implant by APT28

Hunt Hypothesis

The Downrage Implant by APT28 is associated with advanced persistent threat activity that establishes a covert command and control channel, allowing adversaries to exfiltrate data and maintain long-term access to the network. SOC teams should proactively hunt for this behavior in Azure Sentinel to detect and mitigate APT28’s sophisticated espionage and data theft operations before significant damage occurs.

YARA Rule

rule IMPLANT_1_v7 {
   meta:
      description = "Downrage Implant by APT28"
      author = "US CERT"
      reference = "https://www.us-cert.gov/ncas/current-activity/2017/02/10/Enhanced-Analysis-GRIZZLY-STEPPE"
      date = "2017-02-10"
      score = 85
   strings:
      $XOR_FUNCT = { C7 45 ?? ?? ?? 00 10 8B 0E 6A ?? FF 75 ?? E8 ?? ?? FF FF }
   condition:
      (uint16(0) == 0x5A4D) and all of them
}

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.

References

• https://www.us-cert.gov/ncas/current-activity/2017/02/10/Enhanced-Analysis-GRIZZLY-STEPPE
• Source Rule

False Positive Guidance

• Scenario: Scheduled System Maintenance Task
  Description: A legitimate scheduled task runs a script that uses curl or wget to download a file from an internal or external server, mimicking the behavior of a malicious download.
  Filter/Exclusion: Exclude processes initiated by scheduled tasks with known benign command-line arguments (e.g., schtasks.exe or Task Scheduler-triggered processes).

• Scenario: Software Update via Proxy
  Description: A system administrator uses a proxy tool (e.g., curl, wget, or Invoke-WebRequest in PowerShell) to download a software update from a trusted internal repository.
  Filter/Exclusion: Exclude processes that originate from known update servers or internal IP ranges, and filter by command-line arguments containing update, patch, or install.

• Scenario: Admin Debugging with Process Monitor
  Description: An administrator uses ProcMon (Process Monitor) to debug a service, which may include file system access or network activity that resembles malicious behavior.
  Filter/Exclusion: Exclude processes running under ProcMon or with the Process Monitor executable path, and filter by user account (e.g., Administrator or LocalSystem).

• Scenario: PowerShell Script for Log Collection
  Description: A PowerShell script is used to collect system logs or performance data, which may involve network communication or file access similar to the Downrage Implant.
  Filter/Exclusion: Exclude PowerShell scripts executed by known log collection tools (e.g., LogParser, Event Viewer, or PowerShell scripts with Get-EventLog or Get-WinEvent).

• Scenario: Internal Tool for Threat Hunting
  Description: A red team or security team uses an internal tool (e.g., Cobalt Strike, `Met