Triggers on old and new variants of W32/NionSpy file infector

Hunt Hypothesis

This detection identifies adversary behavior where attackers utilize W32/NionSpy file infectors to compromise executable integrity across both legacy and modern system environments. Proactive hunting for this signature in Azure Sentinel is essential to uncover persistent, low-severity infections that may evade standard real-time alerts but serve as a foothold for lateral movement or data exfiltration campaigns.

YARA Rule

rule NionSpy : win32
{
meta:
description = "Triggers on old and new variants of W32/NionSpy file infector"
reference = "https://blogs.mcafee.com/mcafee-labs/taking-a-close-look-at-data-stealing-nionspy-file-infector"
strings:
$variant2015_infmarker = "aCfG92KXpcSo4Y94BnUrFmnNk27EhW6CqP5EnT"
$variant2013_infmarker = "ad6af8bd5835d19cc7fdc4c62fdf02a1"
$variant2013_string = "%s?cstorage=shell&comp=%s"
condition:
uint16(0) == 0x5A4D and uint32(uint32(0x3C)) == 0x00004550 and 1 of ($variant*)
}

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://blogs.mcafee.com/mcafee-labs/taking-a-close-look-at-data-stealing-nionspy-file-infector
• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the W32/NionSpy detection rule in an enterprise environment, including suggested filters and exclusions:

• Legacy Application Deployment via SCCM/MECM

  • Scenario: The System Center Configuration Manager (SCCM) deploys a legacy line-of-business application that utilizes an older installer framework. During the installation phase on endpoints running Windows 10/11, the installer extracts and executes a compressed archive containing a file named setup.exe or install_agent.dll. This specific binary shares structural similarities with known NionSpy variants due to its use of standard PE headers and embedded resource sections, causing the EDR agent to flag it immediately upon execution.
  • Filter/Exclusion: Create an exclusion rule for the specific installer executable path (e.g., C:\Windows\CCM\InboxApps\LegacyApp\setup.exe) or whitelist the file hash of the known good installer package distributed by SCCM. Additionally, exclude the ccmexec.exe process from triggering this specific alert when spawning child processes within the C:\Program Files (x86)\[VendorName] directory.

• Antivirus Definition Updates via Windows Defender

  • Scenario: During a scheduled nightly maintenance window, Microsoft Defender Antivirus pushes a new definition update package (MpSigStub.exe). As part of this process, the service extracts and scans a temporary file containing signature data that mimics the “file infector” behavior of NionSpy by injecting code into memory to verify integrity. The detection logic interprets this legitimate self-modification and file scanning activity as a potential infection event.
  • Filter/Exclusion: Configure the EDR rule to exclude processes spawned by MsMpEng.exe (Windows Defender Service) or specifically target the MpSigStub.exe executable path (`C:\ProgramData\Microsoft