Downrage Implant by APT28

Hunt Hypothesis

The Downrage Implant by APT28 is associated with advanced persistent threat activity that leverages custom malware for command and control communication, indicating potential long-term compromise. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate APT28-related intrusions before they lead to data exfiltration or system disruption.

YARA Rule

rule IMPLANT_1_v2 {
   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:
      $STR1 = {83 3E 00 53 74 4F 8B 46 04 85 C0 74 48 83 C0 02 50 E8 ?? ?? 00
         00 8B D8 59 85 DB 74 38 8B 4E 04 83 F9 FF 7E 21 57 }
      $STR2 = {55 8B EC 8B 45 08 3B 41 08 72 04 32 C0 EB 1B 8B 49 04 8B 04 81
         80 78 19 01 75 0D FF 70 10 FF [5] 85 C0 74 E3 }
   condition:
      (uint16(0) == 0x5A4D) 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, mimicking the behavior of a malicious download.
  Filter/Exclusion: Check the source IP against internal IP ranges and verify the file hash against known internal artifacts.

• Scenario: Windows Update or Patching Process
  Description: The system is downloading a patch or update using certutil or wuauclt, which may trigger the rule due to the use of similar command-line tools.
  Filter/Exclusion: Filter by process name (wuauclt.exe or certutil.exe) and check the URL against Microsoft update endpoints.

• Scenario: Admin Performing Remote PowerShell Script Execution
  Description: An admin uses PowerShell to execute a script that connects to a remote server, which may be flagged due to the use of Invoke-WebRequest or New-Object.
  Filter/Exclusion: Filter by process name (powershell.exe) and check the URL against known admin tools or internal servers.

• Scenario: Log Collection or Monitoring Tool
  Description: A log aggregation tool like LogParser or Splunk uses curl to send data to a central server, which may be mistaken for a malicious download.
  Filter/Exclusion: Filter by process name (LogParser.exe or splunk.exe) and verify the destination IP against the organization’s monitoring infrastructure.

• Scenario: Internal Code Signing or Artifact Distribution
  Description: A developer or DevOps team uses certutil or PowerShell to distribute signed artifacts internally, which may trigger the rule due to similar command patterns.
  Filter/Exclusion: Filter by user context (e.g., [email protected]) and verify the file