Defendnot has been developed by an IT researcher known as Arsenii es3n1n. The utility abuses an undocumented WSC API by registering a fake antivirus product in the system that can pass all Windows checks.
Antivirus software uses WSC API to notify Windows that it’s installed and is currently protecting the device in real time. Once the antivirus software is registered, Windows automatically disables Microsoft Defender to avoid conflicts inevitable if multiple security solutions are running on the same device.
The new tool is based on the previous es3n1n’s project called no-defender: it used code borrowed from a third-party antivirus product to spoof WSC registration. The previous tool was removed from GitHub following a complaint filed by the antivirus vendor.
“Then, after a few weeks after the release, the project blew up quite a bit and gained ~1.5k stars, after that the developers of the antivirus I was using filed a DMCA takedown request and I didn’t really want to do anything with that so just erased everything and called it a day,” – es3n1n’s Blog.
Defendnot shouldn’t have any copyright issues since its functionality has been created from scratch and is based on a fake antivirus DLL.
WSC APIs are usually protected with such mechanisms as Protected Process Light (PPL), valid digital signatures, etc. To circumvent such requirements, Defendnot injects its DLL into the Taskmgr.exe system process, which is signed and hence trusted by Microsoft. From this process, it can register a fake antivirus with a fictitious name.
After registering the fake antivirus product, Microsoft Defender is immediately disabled, thus, leaving the device without active protection.
In addition, the utility includes a loader that passes configuration data in the ctx.bin file and makes it possible to specify the name of the allegedly installed antivirus, disable registration, and enable detailed logging.
“Sadly, to keep this WSC stuff even after reboot, defendnot adds itself to the autorun. Thus, you would need to keep the defendnot binaries on your disk :(” – Arsenii es3n1n.
It must be noted that the current Defendnot version is detected by Microsoft Defender and quarantined as Win32/Sabsik.FL.!ml.