CORESHELL/SOURFACE Implant by APT28

Hunt Hypothesis

The hypothesis is that the detection identifies potential APT28 activity involving the CORESHELL/SOURFACE implant, which is 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_v15 {
   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:
      $XOR_LOOP1 = { 32 1C 02 33 D2 8B C7 89 5D E4 BB 06 00 00 00 F7 F3 }
      $XOR_LOOP2 = { 32 1C 02 8B C1 33 D2 B9 06 00 00 00 F7 F1 }
      $XOR_LOOP3 = { 02 C3 30 06 8B 5D F0 8D 41 FE 83 F8 06 }
   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 3 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: process.name != "powershell.exe" OR process.args NOT LIKE '%-Command%'

• Scenario: Admin using PowerShell to configure system settings via Group Policy
  Filter/Exclusion: process.name != "powershell.exe" OR process.args NOT LIKE '%-File%'

• Scenario: Use of PowerShell for legitimate remote management tasks (e.g., using Invoke-Command)
  Filter/Exclusion: process.name != "powershell.exe" OR process.args NOT LIKE '%-ComputerName%'

• Scenario: Execution of a legitimate script for patch management or software deployment
  Filter/Exclusion: process.name != "powershell.exe" OR process.args NOT LIKE '%-Script%'

• Scenario: Use of PowerShell for log analysis or monitoring by the security team
  Filter/Exclusion: process.name != "powershell.exe" OR process.args NOT LIKE '%-InputFile%'