Introduction
Security researcher Abdelhamid Naceri has released a new Windows zero-day exploit, named SignDrop, which can give attackers SYSTEM privileges. This exploit targets vulnerabilities in the Common Log File System (CLFS) driver, which Microsoft introduced mitigations to defend against authentication attacks targeting CLFS vulnerabilities.
Background on CLFS and Mitigations
The CLFS driver is responsible for handling log files in Windows. Microsoft introduced mitigations in October 2025 to defend against authentication attacks targeting vulnerabilities in the CLFS driver. These mitigations aimed to prevent attackers from exploiting vulnerabilities in the CLFS driver to gain SYSTEM privileges.
The SignDrop Exploit
Naceri's SignDrop exploit takes advantage of how CLFS handles creating the authentication file (".cnpf") when it creates a new log file. CLFS correctly enforces access checks at the start, but the logic can be abused if a different, privileged process later opens the log file. Privileged processes such as Windows Defender can be forced to open the files, so the authentication file gets created with SYSTEM privileges, and CLFS creates it with the original file's access permissions.
The Vulnerability
The flaw is in how CLFS handles creating the authentication file (".cnpf") when it creates a new log file. CLFS correctly enforces access checks at the start, but the logic can be abused if a different, privileged process later opens the log file. Privileged processes such as Windows Defender can be forced to open the files, so the authentication file gets created with SYSTEM privileges, and CLFS creates it with the original file's access permissions.
Testing and Vulnerability Assessment
Naceri tested the exploit on Windows 11 26H2 Experimental, and Windows Server 2025 Experimental. He estimates that 80% of Windows installations are vulnerable to the SignDrop exploit. Naceri recommends running the proof-of-concept (PoC) exploit to check if a Windows device is vulnerable to SignDrop attacks or to check the CLFS authentication mode under the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode registry value. If the PoC exploit spawns a SYSTEM shell, the machine is vulnerable. Additionally, if the registry value is 0 or 1 (learning mode or enforcement mode), the computer is vulnerable.
Microsoft's Response
While Microsoft has fixed some of the security flaws Naceri disclosed since the start of the year (including ShieldBreak, RoguePlanet, YellowKey, GreenPlasma, and MiniPlasma flaws), the other zero-day bugs still lack an official patch. Naceri recommends waiting for a patch from Microsoft, since disabling the CLFS mitigation would expose the devices to attacks exploiting memory corruption vulnerabilities.
Conclusion
The SignDrop exploit demonstrates the importance of proper logic implementation in mitigations to prevent potential security vulnerabilities. Security researchers like Naceri play a crucial role in identifying and disclosing such vulnerabilities to help protect users and systems from potential attacks.
Related Zero-Day Exploits
In the past, Naceri has disclosed a long list of other zero-day flaws in Microsoft Defender, BitLocker, and other Windows components, including BigDiskBuster, ShieldCrash, ShieldBreak, LegacyHive, RoguePlanet, BlueHammer, RedSun, YellowKey, GreenPlasma, MiniPlasma, and UnDefend.
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