Downrage Implant by APT28

Hunt Hypothesis

The Downrage Implant by APT28 is associated with advanced persistent threat actors who use it for long-term system control and data exfiltration. SOC teams should proactively hunt for this implant in Azure Sentinel to detect and mitigate potential espionage and data theft activities by state-sponsored adversaries.

YARA Rule

rule IMPLANT_1_v5 {
   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:
      $drivername = { 6A 30 ?? 6A 33 [5] 6A 37 [5] 6A 32 [5] 6A 31 [5] 6A 77
         [5] 6A 69 [5] 6A 6E [5] 6A 2E [5] 6A 73 [5-9] 6A 79 [5] 6A 73 }
      $mutexname = { C7 45 ?? 2F 2F 64 66 C7 45 ?? 63 30 31 65 C7 45 ?? 6C 6C
         36 7A C7 45 ?? 73 71 33 2D C7 45 ?? 75 66 68 68 66 C7 45 ?? 66 }
   condition:
      (uint16(0) == 0x5A4D or uint16(0) == 0xCFD0 or uint16(0) == 0xC3D4 or
      uint32(0) == 0x46445025 or uint32(1) == 0x6674725C) 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

• 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 server.
  Filter/Exclusion: Check for destination_ip in internal IP ranges or filter by process_name like schtasks.exe or task scheduler.

• Scenario: Software Update via Proxy
  Description: A system uses a proxy to download updates from a known internal repository, which may trigger the rule due to the use of curl or wget.
  Filter/Exclusion: Filter by destination_ip matching internal update servers or check for user_agent containing “update” or “patch”.

• Scenario: Admin Performing Remote Code Execution (RCE) via PowerShell
  Description: An admin uses PowerShell to execute a script that downloads and runs a legitimate tool, such as PowerShell.exe or msbuild.exe, from an internal server.
  Filter/Exclusion: Check for process_name like powershell.exe and verify destination_ip is internal or check for command_line containing known admin tools.

• Scenario: Log Collection via Syslog Server
  Description: A system sends logs to a centralized syslog server using rsyslog or syslog-ng, which may trigger the rule due to network traffic patterns.
  Filter/Exclusion: Filter by destination_port (e.g., 514) or check for process_name like rsyslogd.exe or syslog-ng.

• Scenario: Internal Tool Deployment via Chocolatey
  Description: An admin uses Chocolatey to install a legitimate tool, which may trigger the rule due to the use of choco install or similar commands.
  Filter/Exclusion: Check for `command_line