SignDrop zero-day lets local users gain SYSTEM privileges

Security researcher Abdelhamid Naceri, also known as Nightmare Eclipse, published a proof of concept called SignDrop that can let a local Windows user escalate to SYSTEM privileges by abusing a Common Log File System, or CLFS, mitigation, according to BleepingComputer. The researcher says the issue stems from how CLFS creates a companion authentication file when a privileged process later opens a prepared log.

What the SignDrop zero-day does and how it works

According to BleepingComputer, the researcher says CLFS enforces access checks when a log is created but that those checks can be abused if a different, privileged process later opens the log file. The attacker prepares a CLFS log and container in paths they control. When a privileged process opens that log, the driver can create the companion .cnpf authentication file in the privileged process security context, and the file inherits the original log's permissions. BleepingComputer reports the researcher added that privileged processes, including Windows Defender, can be forced to open the prepared logs, allowing the attacker to get a file that appears to be created by the privileged process.

The Hacker News reports the public exploit code and repository showed the technique using object manager symbolic links to redirect where the .cnpf file is created, and that the published chain used the Defender antimalware service in the example. The Hacker News also notes Microsoft had added the CLFS authentication check in updates shipped in October 2025, and that the mitigation was intended to limit exploitation of prior CLFS memory corruption issues.

Who is affected and how widespread it may be

BleepingComputer says the researcher tested the exploit on Windows 11 26H2, Windows 11 26H2 Experimental, and Windows Server 2025 Experimental and estimated roughly 80% of Windows installations are vulnerable. BleepingComputer adds the researcher recommended running his proof of concept to check whether a machine is vulnerable, or to inspect the CLFS authentication registry value at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode. The researcher said a PoC that spawns a SYSTEM shell shows the machine is vulnerable, and that values of 0 or 1 indicate a vulnerable configuration.

The Hacker News reports that systems are affected when the CLFS authentication mode is set to 0 (enforced) or 1 (learning), and that a value of 2 or 3 means the mitigation is disabled and the claimed attack surface does not apply. The Hacker News also warns that disabling the mitigation reopens the older memory corruption risks the protection aimed to prevent.

Why this matters to small businesses, nonprofits, and public teams

If the SignDrop technique works on an endpoint, a local user who already has some access could gain SYSTEM privileges and then disable protections or tamper with files that should be restricted, according to the researcher quoted by BleepingComputer. The researcher also said the approach can be used against any antimalware engine's self-defense, because the file creation request looks like it comes from the antimalware process itself. That makes the issue especially relevant where multiple users share machines, where contractors have local accounts, or where devices are used for both admin and non-admin tasks.

There is no confirmed Microsoft patch reported in these articles. The Hacker News says Microsoft has not confirmed the claimed flaw, assigned a CVE, or released a fix. BleepingComputer reports the researcher saying there is currently no solution other than waiting for a patch, since turning the mitigation off would reopen earlier attack paths.

Practical steps to reduce risk now

• Check the CLFS authentication registry value shown at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode and flag devices with values 0 or 1 for further review, as reported by BleepingComputer and The Hacker News. If you are not comfortable inspecting the registry, have your IT staff check it.
• Do not run public exploit code on production machines unless you have an isolated test environment and experienced staff; BleepingComputer notes the researcher published a PoC and recommended it for testing, but that is best left to security teams.
• Restrict who has local admin access and review accounts that can run local code, since the exploit requires local access to start the chain.
• Ensure antimalware and endpoint protection are up to date and monitored, and treat unusual service behavior or unexpected file creation as high priority for investigation, because the technique targets antimalware processes in the published chain.
• Prepare for a vendor patch: inventory affected systems, plan how you will test and deploy a Microsoft fix, and have rollback and communication steps ready.

If you want an external, noninvasive first look at how exposed your site or systems are, start with a free external scan from our security page: /security.html.

What to watch next

Monitor official Microsoft channels for confirmation, a CVE assignment, and a patch. The Hacker News reports the exploit code and writeup appeared publicly on GitHub the same day Microsoft published its November security updates, but the company had not confirmed or fixed the issue in those reports. Also watch your endpoint logs and alerting for signs of local privilege escalation attempts.

Background on the researcher and disclosure context

BleepingComputer says the researcher, Abdelhamid Naceri (also known as Nightmare Eclipse), has previously disclosed multiple zero-day issues in Microsoft Defender, BitLocker, and other Windows components, and that some earlier reported flaws remain unpatched. BleepingComputer also notes that in the past Microsoft warned of legal action over disclosures that cause customer harm, a detail that framed community reaction to prior public disclosures.

We will update this briefing when Microsoft issues guidance or a patch. For now, treat SignDrop as a high-priority detection and access control issue, and have IT review devices flagged by the CLFS registry value and any endpoints where non-admin users can run local code.