Nightmare Eclipse Publishes SignDrop Zero-Day in Windows CLFS Driver
Security researcher Abdelhamid Naseri, operating under the aliases Nightmare Eclipse and MSNightmare, has publicly released details and a proof-of-concept for a new zero-day vulnerability called SignDrop in the Common Log File System (CLFS) driver of Windows.
The flaw targets authentication protections Microsoft introduced in October 2025 to mitigate attacks against the CLFS driver. Although those measures successfully reduced the volume of CLFS-related exploits, they inadvertently created a new attack surface that SignDrop now abuses.
According to the researcher, the vulnerability stems from how CLFS processes the creation of the authentication file (.cnpf) during new log file initialization. Access checks are performed correctly at the start, yet the logic can be misused when a later privileged process opens the log file, resulting in the authentication file being created with SYSTEM-level permissions.
SignDrop can also be weaponized against any antivirus or EDR solution. By forcing a privileged security process to open the crafted log file, an attacker causes the kernel driver to treat the subsequent file creation request as originating from the legitimate antivirus process, thereby bypassing self-protection mechanisms.
Nightmare Eclipse tested the exploit on Windows 11 26H2, Windows 11 26H2 Experimental, and Windows Server 2025 Experimental, estimating that roughly 80 percent of Windows installations remain vulnerable. Users can verify exposure by running the public PoC or checking the registry value HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode; values of 0 or 1 indicate vulnerability.
No mitigation other than applying a future Microsoft patch is recommended, because disabling CLFS authentication would expose systems to prior memory-corruption attacks. Nightmare Eclipse has previously disclosed multiple Windows zero-days, including BigDiskBuster against Microsoft Defender and RoguePlanet, which enabled SYSTEM command execution via a race condition in the same product.
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