Detects Samples related to APT17 activity - file FXSST.DLL

Hunt Hypothesis

The detection of the FXSST.DLL file may indicate the presence of APT17 malware, which is associated with sophisticated and persistent cyber attacks. SOC teams should proactively hunt for this file in Azure Sentinel to identify potential APT17 activity early and mitigate the risk of advanced persistent threats.

YARA Rule

rule APT17_Sample_FXSST_DLL 
{
    
    meta:
        description = "Detects Samples related to APT17 activity - file FXSST.DLL"
        author = "Florian Roth"
        reference = "https://goo.gl/ZiJyQv"
        date = "2015-05-14"
        hash = "52f1add5ad28dc30f68afda5d41b354533d8bce3"
        
    strings:
        $x1 = "Microsoft? Windows? Operating System" fullword wide
        $x2 = "fxsst.dll" fullword ascii
        $y1 = "DllRegisterServer" fullword ascii
        $y2 = ".cSV" fullword ascii
        $s1 = "GetLastActivePopup"
        $s2 = "Sleep"
        $s3 = "GetModuleFileName"
        $s4 = "VirtualProtect"
        $s5 = "HeapAlloc"
        $s6 = "GetProcessHeap"
        $s7 = "GetCommandLine"
   
   condition:
        uint16(0) == 0x5a4d and filesize < 800KB and ( 1 of ($x*) or all of ($y*) ) and all of ($s*)
}

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 11 string patterns in its detection logic.

References

• https://goo.gl/ZiJyQv
• Source Rule

False Positive Guidance

• Scenario: Legitimate use of FXSST.DLL by a system diagnostic tool
  Filter/Exclusion: Check for process parent or command line containing sysinternals or Process Monitor
  Example Filter: process.parent_process_name == "ProcessMonitor.exe"

• Scenario: Scheduled job running FXSST.DLL for log file analysis
  Filter/Exclusion: Filter out processes with task scheduler or schtasks.exe in the parent process chain
  Example Filter: process.parent_process_name == "schtasks.exe"

• Scenario: Antivirus or endpoint protection tool using FXSST.DLL for heuristic analysis
  Filter/Exclusion: Exclude processes with parent names like McAfee, Kaspersky, or Windows Defender
  Example Filter: process.parent_process_name in ("MsMpEng.exe", "Mcshield.exe")

• Scenario: System update or patching process using FXSST.DLL
  Filter/Exclusion: Filter out processes with parent names like wusa.exe, dism.exe, or WindowsUpdate.exe
  Example Filter: process.parent_process_name in ("wusa.exe", "dism.exe")

• Scenario: Developer or admin using FXSST.DLL for custom forensic analysis
  Filter/Exclusion: Exclude processes with command lines containing --debug, --analysis, or --forensic
  Example Filter: process.command_line contains "analysis" or "forensic"