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 mitigate potential APT28 intrusions before significant data loss or network disruption occurs.

YARA Rule

rule IMPLANT_1_v6 {
   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:
      $XORopcodes_eax = { 35 (22 07 15 0e|56 d7 a7 0a) }
      $XORopcodes_others = { 81 (F1|F2|F3|F4|F5|F6|F7) (22 07 15 0E|56 D7 A7 0A) }
   condition:
      (uint16(0) == 0x5A4D or uint16(0) == 0xCFD0 or uint16(0) == 0xC3D4 or
      uint32(0) == 0x46445025) and any 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 2 string patterns in its detection logic.

References

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 with cleanmgr.exe or diskcleanup.exe in the command line, or where the user is a domain admin with a known cleanup task.
Scenario: Malware Analysis Lab Environment
Description: A security analyst manually executing a known benign file (e.g., msiexec.exe or regsvr32.exe) as part of malware analysis.
Filter/Exclusion: Exclude processes where the file path contains C:\Tools\ or C:\Analysis\, or where the user is a lab admin with elevated privileges.
Scenario: Windows Update or Patching Process
Description: Windows Update or a patching tool (e.g., wusa.exe, msiexec.exe) running to install updates or patches.
Filter/Exclusion: Exclude processes where the command line includes wusa.exe, msiexec.exe, or WindowsUpdate.exe, or where the file path includes C:\Windows\ or C:\Program Files\WindowsUpdate.
Scenario: Legitimate Use of PowerShell for System Monitoring
Description: A system administrator using PowerShell scripts (e.g., powershell.exe) to monitor system performance or collect logs.
Filter/Exclusion: Exclude processes where