Windows users have a new security problem to watch: a publicly released exploit called SignDrop can reportedly turn ordinary local access into full SYSTEM privileges on affected versions of Microsoft’s operating system. The proof of concept was published by security researcher Abdelhamid Naceri, better known online as Nightmare Eclipse, whose ongoing clashes with Microsoft have already produced several high-profile Windows disclosures. SignDrop’s publication gives attackers a working path to the highest level of access on a Windows machine.
The vulnerability is found in the Common Log File System, or CLFS, a Windows subsystem designed to provide transactional logging for software running in both user mode and kernel mode. Microsoft’s documentation describes CLFS as a general-purpose logging service that manages log records, metadata, storage, and recovery across system failures. Microsoft’s CLFS documentation explains that the system supports both dedicated logs and multiplexed logs shared by multiple clients.
SignDrop’s unusual feature is that it makes Windows’ own security tooling part of the escalation chain. A local user creates a specially prepared CLFS log and then persuades Microsoft Defender’s signature-update process to open it through a legitimate update interface. That privileged interaction can cause CLFS to create an authentication-related file with SYSTEM-level permissions while leaving the resulting file accessible to the attacker. BleepingComputer’s technical coverage says the technique abuses hardening Microsoft added after earlier attacks involving CLFS.
Early analysis indicates that the exploit affects some Windows 11 26H2 and Windows Server 2025 configurations, particularly where CLFS authentication is operating in learning or enforcement mode. The researcher has also used several aliases, including MSNightmare and Chaotic Eclipse, making it important for defenders to track the exploit name as well as the author’s changing online identities. Available coverage identifies the affected configurations, but that does not mean every installation of those operating-system versions is equally vulnerable.
As of November 13, Microsoft had not assigned SignDrop a CVE, issued a dedicated advisory, or released a patch specifically addressing it, according to independent technical analysis. Ostorlab’s assessment also said there were no confirmed attacks and that the exploit was not listed in CISA’s Known Exploited Vulnerabilities catalog at that time. That status can change quickly now that the proof of concept is public, especially because SYSTEM access can let an attacker disable protections, install persistent software, or take control of sensitive parts of a machine.
For now, organizations should treat unexpected Defender activity, unusual CLFS file creation, and suspicious privilege escalation from standard accounts as investigation signals rather than assuming that antivirus software alone will block the chain. Microsoft’s regular monthly security updates remain the primary route for a fix; the company’s release calendar says those updates arrive on the second Tuesday of each month, while its Windows message center confirms that the November 2026 security update is already available. The official Windows release-health page does not, however, identify SignDrop as resolved.
