Downrage Implant by APT28

Hunt Hypothesis

The Downrage Implant by APT28 is associated with advanced persistent threat actors who use it for long-term data exfiltration and command and control, making it a critical indicator of a sophisticated cyber attack. SOC teams should proactively hunt for this implant in Azure Sentinel to detect and mitigate APT28 campaigns early, preventing data breaches and lateral movement within the network.

YARA Rule

rule IMPLANT_1_v3 {
   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:
      $rol7encode = { 0F B7 C9 C1 C0 07 83 C2 02 33 C1 0F B7 0A 47 66 85 C9 75 }
   condition:
      (uint16(0) == 0x5A4D or uint16(0) == 0xCFD0 or uint16(0) == 0xC3D4 or
      uint32(0) == 0x46445025 or uint32(1) == 0x6674725C) 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

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 server.
Filter/Exclusion: Exclude processes initiated by scheduled tasks with known internal URLs (e.g., internal-repo.example.com), or filter by process.parent_process_name = "schtasks.exe".
Scenario: Software Update Deployment
Description: A patch management tool (e.g., Microsoft Endpoint Configuration Manager) downloads updates from a trusted internal repository.
Filter/Exclusion: Exclude processes associated with patch management tools (e.g., ccmexec.exe, mpsvc.exe) or filter by process.parent_process_name = "mpsvc.exe".
Scenario: Admin Debugging Session
Description: A system administrator uses Process Explorer or Procmon to debug a process, which may trigger network activity.
Filter/Exclusion: Exclude processes initiated by administrative tools like procmon.exe or process.explorer.exe.
Scenario: Internal Code Signing Tool Usage
Description: A developer uses an internal code signing tool (e.g., signtool.exe) that may make network requests to sign binaries.
Filter/Exclusion: Exclude processes with process.name = "signtool.exe" or filter by process.parent_process_name = "devenv.exe" (Visual Studio).
Scenario: Database Backup Job
Description: A database backup job (e.g., using sqlbackup.exe) connects to a remote SQL server and transfers data.
Filter/Exclusion: Exclude processes associated with database backup tools (e.g., sqlbackup.exe) or filter by process.parent_process_name = "sqlservr.exe".