CORESHELL/SOURFACE Implant by APT28

Hunt Hypothesis

The hypothesis is that the detection identifies potential APT28 activity involving the CORESHELL/SOURFACE implant, which is a known tool used for long-term persistence and command and control. SOC teams should proactively hunt for this behavior in Azure Sentinel to detect and mitigate advanced persistent threats before they cause significant damage.

YARA Rule

rule IMPLANT_2_v13 {
   meta:
      description = "CORESHELL/SOURFACE 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 ?? 06 [7-17] fa [0-10] 45 [2-4] 48 [2-4] e8 [2] FF FF
         [6-8] 48 8d [3] 48 89 [3] 45 [2] 4? [1-2] 01}
   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: Legitimate scheduled job running PowerShell scripts for system maintenance
  Filter/Exclusion: ProcessName != "powershell.exe" OR CommandLine NOT LIKE '%-Command%'

• Scenario: Admin using PowerShell to configure Windows Update settings
  Filter/Exclusion: ProcessName != "powershell.exe" OR CommandLine NOT LIKE '%Set-WindowsUpdate'

• Scenario: Security tool (e.g., CrowdStrike or Microsoft Defender) performing a scan or update
  Filter/Exclusion: ProcessName != "msdefender.exe" OR ProcessName != "CrowdStrike.exe"

• Scenario: IT staff using PowerShell to deploy software via Group Policy or SCCM
  Filter/Exclusion: CommandLine NOT LIKE '%sccm%' AND CommandLine NOT LIKE '%gpupdate%'

• Scenario: System using PowerShell for log analysis or monitoring (e.g., using Get-EventLog)
  Filter/Exclusion: CommandLine NOT LIKE '%Get-EventLog%' AND CommandLine NOT LIKE '%Get-Event'