A publicly released Windows exploit is turning a safeguard Microsoft added last year into a route to the highest level of control on a PC. The technique, dubbed SignDrop, lets an attacker starting from a standard user account reach NT AUTHORITY\SYSTEM by abusing the Common Log File System (CLFS), the driver Windows applications and services use to manage their logs.
The proof-of-concept was posted to GitHub on Nov. 10 by Abdelhamid Naceri, a former Microsoft employee who publishes unpatched Windows flaws under the handle Nightmare Eclipse. The release landed as Microsoft was distributing its November security updates. No CVE identifier or dedicated patch has been assigned to the issue so far.
The flaw sits in the hardening Microsoft introduced in October 2025, which uses cryptographic authentication codes to confirm that a log file has not been altered before clfs.sys processes its contents. SignDrop instead targets the companion .cnpf authentication files the driver creates alongside each log. Those files inherit their access permissions from the log itself, so whoever prepares the log retains control over the file that gets created.
That is where Microsoft Defender enters the chain. Because the antivirus runs with the highest privileges on the machine, an attacker can force it to open a crafted log, causing the authentication file to be written with SYSTEM rights while the attacker’s inherited permissions remain in place. With further manipulation, an ordinary user session escalates to a SYSTEM-level command prompt, a level of access that outranks even a standard administrator account.
Naceri also contends the method can evade antivirus and endpoint detection and response tamper protection, because the privileged file operations appear to come from legitimate security processes. Cyderes, an independent security firm, said it reproduced the exploit and obtained a SYSTEM shell, confirming the technique works under certain conditions.
The researcher tested SignDrop against Windows 11 26H2, Windows 11 26H2 Experimental and Windows Server 2025 Experimental, and estimated that roughly 80% of Windows installations could be vulnerable. That figure has not been verified, and outside researchers report compatibility and reliability constraints.
Vulnerability analyst Will Dormann told Ars Technica he could reproduce the exploit only on machines running this year’s Windows 11 26H2 update and only where the installation was more than 90 days old, meaning a fresh copy of Windows would not qualify. Among the systems he had on hand, exactly one met the conditions.
Microsoft said it is investigating the reports. In a statement to SecurityWeek, the company said Defender can detect and block exploitation attempts and that it has found no evidence of active exploitation in the wild. The company’s track record with this researcher is nonetheless a concern: three of Naceri’s earlier Defender exploits, including BlueHammer, were used in real attacks after he disclosed them.
Users can check whether the affected authentication mechanism is active in the registry at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode. A value of 0 (enforcement mode) or 1 (learning mode) indicates the mitigation is switched on and the configuration may be exposed, though that alone does not mean an attack will succeed. Microsoft advises against disabling CLFS authentication as a workaround, since doing so could reopen older memory-corruption vulnerabilities.
| Configuration | Registry value | Status |
|---|---|---|
| Enforcement mode | 0 | Mitigation active; potentially vulnerable |
| Learning mode | 1 | Mitigation active; potentially vulnerable |
| Disabled | n/a | Not advised; reopens older flaws |
Note: Registry path is HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode. A qualifying value does not by itself confirm exploitability.
The exposure matters beyond a single machine. Malware that arrives through a dodgy download typically runs with the same limited rights as the user who launched it. SignDrop offers a way to climb to SYSTEM, where it can switch off security tools and establish persistence, turning a minor infection into a full takeover. Because the code is public, any attacker can attempt it.
For IT administrators, the practical guidance is to keep installing Windows and Defender updates and to watch for a fix, which is unlikely before December’s Patch Tuesday on the 8th unless Microsoft issues one out of band. For Microsoft, the episode is an awkward one: a control built to close a long-abused logging component is now the mechanism by which that component can be exploited.