A new Windows zero-day exploit dubbed SignDrop allows local attackers to elevate privileges to the SYSTEM level by weaponizing a security mitigation originally designed to protect the operating system. Public exploit code and an accompanying technical write-up were published this week by security researcher Abdelhamid Naceri, also known as MSNightmare and Nightmare Eclipse.

The proof of concept targets the Windows Common Log File System (CLFS) driver, a subsystem frequently abused by threat actors for privilege escalation. According to reports from BleepingComputer and The Hacker News, the exploit turns Microsoft's own log-authentication mechanisms against the platform.

Flaw in Log Authentication Logic

Microsoft rolled out an authentication check for CLFS log files in October 2025 across Windows 11 and Windows Server 2025. The update was introduced to curtail a wave of attacks targeting memory corruption bugs in clfs.sys, an issue prominent enough that 10 CLFS vulnerabilities have been listed in the CISA Known Exploited Vulnerabilities catalog. The mitigation uses a machine-unique hash-based message authentication code to verify log integrity and generates a companion .cnpf file alongside each log to hold recovery data.

SignDrop focuses on the creation process of that companion file. While CLFS performs initial access checks correctly, Naceri discovered that the logic breaks down when a privileged process opens a prepared log. A standard local user can prepare a CLFS log and container paths they control, then coerce a privileged service to open the file.

In the published proof of concept, the researcher used the Windows Defender antimalware service, invoked via its update interface, to trigger the operation. When the privileged process opens the log, the CLFS driver initiates the creation of the .cnpf companion file under that elevated security context. By employing Object Manager symbolic links, an attacker can redirect this file creation to an arbitrary location. Because the resulting file inherits the original log's permissions, a standard user gains write access to a file generated by a SYSTEM process, enabling privilege escalation.

Naceri noted that this technique bypasses the tamper protection and self-defense mechanisms of antimalware software. Because the file creation originates from the legitimate antimalware process itself, kernel drivers treat the request as authorized. The researcher claimed the approach could theoretically impact any antimalware engine running with elevated privileges, though that broader scope remains untested.

Scope and Reliability

The exploit has been tested on Windows 11 26H2, Windows 11 26H2 Experimental, and Windows Server 2025 Experimental builds. Naceri estimated that roughly 80% of Windows installations could be vulnerable to the technique.

Independent analysis confirms the underlying mechanics work, albeit with practical limitations. Sungyup Nam, head of research at cybersecurity firm 78researchlab, reported that researcher Heewon Song tested the exploit code and found that it successfully functions. However, Song assessed that the current exploit is not reliable enough for seamless operational use. Executing the public exploit interferes with User Account Control (UAC) prompts, opening both standard and SYSTEM shells simultaneously and making the attack obvious to any logged-in user.

Microsoft has not assigned a CVE identifier, issued an advisory, or released a patch for the vulnerability. Naceri has previously disclosed multiple zero-days targeting BitLocker, Windows Defender, and other operating system components without prior private coordination, including bugs tracked as CVE-2026-50656 (RoguePlanet) and CVE-2026-69414 (ShieldBreak).

What Defenders Should Do

Currently, there is no official vendor patch or workaround, and no active in-the-wild exploitation has been recorded by defenders. System administrators should be aware of the following factors when evaluating exposure:

  • Check driver authentication mode: Systems are vulnerable when the CLFS authentication registry value at HKLM\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode is set to 0 (enforced) or 1 (learning). Systems older than 90 days routinely default to enforcement mode.
  • Do not disable CLFS mitigation: While changing the registry value to 2 or 3 disables the feature and stops the SignDrop attack vector, both Naceri and Microsoft documentation explicitly warn against this. Disabling the check allows the driver to parse untrusted log files, leaving devices exposed to known memory corruption exploits.
  • Monitor endpoint telemetry: Defenders should monitor for unusual symbolic link creation within the Windows Object Manager, unexpected invocations of antimalware update interfaces, and secondary shells spawning alongside UAC interactions.