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, enabling long-term data exfiltration and system compromise. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and neutralize APT28 campaigns before significant data loss or network disruption occurs.

YARA Rule

rule IMPLANT_1_v4 {
   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_LOOP = { 8B 45 FC 8D 0C 06 33 D2 6A 0B 8B C6 5B F7 F3 8A 82 ?? ??
         ?? ?? 32 04 0F 46 88 01 3B 75 0C 7C E0 }
   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 system maintenance task, such as schtasks.exe running a scheduled job to clean temporary files or update system settings.
  Filter/Exclusion: Exclude processes where the command line includes cleanmgr.exe, del, rmdir, or schtasks /run with a known maintenance task name.

• Scenario: Admin Performing Disk Cleanup
  Description: An administrator using cleanmgr.exe or diskcleanup.exe to remove temporary files or system cache.
  Filter/Exclusion: Exclude processes where the command line includes cleanmgr.exe or diskcleanup.exe with no additional suspicious parameters.

• Scenario: Antivirus or EDR Tool Scanning
  Description: A security tool like Windows Defender or CrowdStrike Falcon performing a full system scan, which may trigger file access patterns similar to malware.
  Filter/Exclusion: Exclude processes with parent process svchost.exe or taskeng.exe and command lines containing Scan or FullScan.

• Scenario: PowerShell Script for Log Rotation
  Description: A PowerShell script used by the IT team to rotate or archive log files, which may involve file deletion or renaming.
  Filter/Exclusion: Exclude processes where the command line includes powershell.exe and the script path is known to be from a trusted internal repository (e.g., C:\scripts\logrotate.ps1).

• Scenario: User-Initiated File Deletion
  Description: A user manually deleting files via command prompt or file explorer, which could mimic the behavior of a malware cleanup.
  Filter/Exclusion: Exclude processes where the user account is a regular user and the command line includes del, erase, or rm with known user-deleted