Nightmare Eclipse, a researcher engaged in a public dispute with Microsoft, has revealed a new Windows zero-day vulnerability designated SignDrop. The flaw enables local users with standard privileges to escalate to full System rights on affected Windows installations.
The vulnerability exists within the Common Log File System, a logging service utilized by both user-mode and kernel-mode software clients. The exploit operates by manipulating interactions between CLFS, Windows Defender, and the operating system’s file permissions framework. An attacker can craft a malicious CLFS container and force a privileged process, such as Windows Defender, to open it. The assault leverages a legitimate Defender function called ServerMpUpdateEngineSignature, which handles antivirus signature updates, to execute the privilege escalation.
The technical mechanics involve manipulating Windows Object Manager symbolic links to cause Defender to access the CLFS container under the antimalware engine’s security context. When an associated authentication file is absent, CLFS generates it using Defender’s System-level privileges. Since the newly created file inherits the Discretionary Access Control List from the original file, the attacker gains access to a System-privileged file, enabling abuse of the file-creation operation for privilege escalation.
Notably, the vulnerability stems from a security enhancement Microsoft implemented in October 2025 intended to prevent exploitation of other CLFS vulnerabilities. According to Nightmare Eclipse, the security mitigation itself can be weaponized for privilege elevation. The researcher emphasized that the vulnerability is particularly significant because it can potentially be used against any antimalware engine to circumvent tamper protection mechanisms.
Nightmare Eclipse reported the exploit functions against Windows 11 26H2, Windows 11 26H2 Experimental, and Windows Server 2025 Experimental. Researcher Will Dormann confirmed the exploit’s functionality but noted important limitations: it appears to require Windows 26H2 that has been active for more than 90 days and specific CLFS authentication settings. Of Dormann’s test systems, only one met the vulnerability conditions. Microsoft has not publicly responded to inquiries regarding the vulnerability.
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