A security researcher using the pseudonym Nightmare Eclipse has publicly disclosed a Windows zero-day vulnerability. The flaw, designated SignDrop, allows local users to attain full System rights on certain versions of the operating system.
The Mechanism Behind SignDrop
The security flaw exists within the Common Log File System, which Microsoft defines as a general-purpose logging service utilized by software clients operating in either user-mode or kernel-mode. As reported by Ars Technica, an attacker holding standard user rights can force a privileged process, such as Windows Defender, to open a specially crafted log file container. This manipulation misuses privileged file creation to elevate user privileges directly to the System level.
The attack mechanism relies on the interaction between the Common Log File System, Windows Defender, and operating system file permissions. The exploit forces Windows Defender to open a log file generated by a regular user through a legitimate antivirus signature update function called ServerMpUpdateEngineSignature.
By altering Windows Object Manager symbolic links, the attacker forces Defender to open the log file container within the security context of the antimalware engine. When a necessary authentication file is absent, the Common Log File System generates it using the elevated privileges of Windows Defender. Because the system assigns the new file the Discretionary Access Control List of the original file, the attacker gains access to a file created with System privileges, enabling a privilege-escalation attack.
Origins in Microsoft’s Security Mitigation
Paradoxically, the vulnerability originates from a defense mechanism that Microsoft deployed in October 2025 to block threat actors from exploiting earlier flaws within the Common Log File System.
Nightmare Eclipse stated that when the exploitation chain is set up correctly, this legitimate security mitigation can be turned against itself to facilitate privilege escalation. The researcher noted that while Microsoft implemented the procedures correctly, the underlying logic permits this specific type of abuse.
