"If everything is chained correctly, funnily, this legit security mitigation can be used against itself and allow an elevation of privileges. Everything is done properly by Microsoft here but the logic allows abuse," the security researcher Abdelhamid Naceri (also known as Nightmare Eclipse) said.
How SignDrop abuses CLFS file-creation logic
Naceri has published a zero-day exploit he calls "SignDrop" that leverages the Common Log File System (CLFS) driver's handling of authentication files (".cnpf") when creating new log files. According to Naceri, CLFS enforces access checks correctly at the start of log-file creation, but the logic can be abused if a different, privileged process later opens the same log file. In that scenario, privileged processes can be forced to open the files so the authentication file is created with SYSTEM privileges while CLFS assigns it the original file's access permissions.
Microsoft's October 2025 mitigation and the unintended consequence
SignDrop targets a mitigation Microsoft introduced in October 2025 to defend against authentication attacks on CLFS driver vulnerabilities. Naceri says the mitigation did reduce attacks exploiting CLFS — "attacks exploiting CLFS vulnerabilities dropped sharply after the mitigation" — but that the mitigation's design introduced a new avenue for abuse. He explicitly stated Microsoft implemented the mitigation properly, yet "the logic allows abuse."
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See what we buildImplications for antimalware engines and tamper protection
Naceri warned the technique is not limited to Microsoft components. He said privileged processes such as Windows Defender can be forced to open the files, and that the method "can be used against any antimalware engine to bypass their self defense (or tamper protection), as you can trigger arbitrary file creation in the context of their own antimalware process." From his description, the file-creation request appears to come from the antimalware engine's legitimate process, so kernel drivers view the operation as originating from the protected process and permit it — a property Naceri calls a factor that makes the vulnerability "unique" because it can be used to turn off "any antivirus/EDR."
Which systems Naceri tested and how to check vulnerability
Naceri tested SignDrop on Windows 11 26H2, Windows 11 26H2 Experimental, and Windows Server 2025 Experimental, and he estimates that 80% of Windows installations are vulnerable. He published a proof-of-concept (PoC) exploit and recommended two ways to check whether a device is susceptible: run the PoC and see whether it spawns a SYSTEM shell, or inspect the registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode. If the registry value is 0 or 1 (learning mode or enforcement mode), Naceri says the computer is vulnerable.
Available mitigations, trade-offs, and recommended action
According to Naceri, there is currently no workaround that eliminates the risk without opening other exposure. He cautioned that disabling the CLFS mitigation would return systems to the earlier state in which memory-corruption vulnerabilities against CLFS could be exploited. As a result, Naceri's guidance is essentially to test for vulnerability with the PoC or the registry check and then wait for a Microsoft patch; he said there is "currently no solution except waiting for a patch from Microsoft, since disabling the CLFS mitigation would expose the devices to attacks exploiting memory corruption vulnerabilities."
Naceri's disclosure history and Microsoft's prior response
The researcher, who says he is a former Microsoft employee, has published a long list of zero-day disclosures since April. He named BigDiskBuster, ShieldCrash, ShieldBreak, LegacyHive, RoguePlanet, BlueHammer, RedSun, YellowKey, GreenPlasma, MiniPlasma, and UnDefend among those disclosures. Microsoft has fixed some of the reported issues, with Naceri noting fixes for ShieldBreak, RoguePlanet, YellowKey, GreenPlasma, and MiniPlasma; other reported zero-days remain without an official Microsoft patch.
Five years ago, Naceri released several zero-day exploits that allowed privilege escalation on various Windows versions. After some of his public disclosures this year — made without first attempting private reporting, according to the timeline he set — Microsoft warned of legal action against "malicious activity causing real harm" to company customers, a move that members of the information-security community interpreted as a direct threat to Naceri.
For now, SignDrop is a recorded demonstration that a security mitigation can, under certain conditions, be manipulated into creating a higher-risk state. Naceri's advice is concrete: use the PoC or the registry check to determine exposure and await a Microsoft patch rather than disable the mitigation and reopen older attack paths.
Source: BleepingComputer — New Windows SignDrop zero-day gives hackers SYSTEM privileges
