SignDrop Zero‑Day Bypasses Windows CLFS Authentication, Enables SYSTEM Escalation

Security researchers have disclosed a local privilege escalation exploit named SignDrop that can elevate a standard user to NT AUTHORITY\SYSTEM on Windows 11 25H2/26H1/26H2 and Windows Server 2025 installations where the CLFS authentication mitigation is active. The vulnerability was released publicly on 10 Nov 2026 without coordinated disclosure, a CVE identifier, or an official patch.

What the exploit does

CLFS (Common Log File System) authentication adds an HMAC to log files and checks that the driver parsing the log is the original author. SignDrop subverts this by forcing the Defender antimalware service, which runs in SYSTEM context, to create a companion “.cnpf” file when a privileged process opens a prepared CLFS log. The malicious file‑creation request appears to originate from the antimalware engine itself, allowing the attacker to inject code that runs with SYSTEM privileges.

Technical details

• Mitigation introduced in Windows 11 25H2, 26H1 and Windows Server 2025 updates released October 2025 (KB 5056852).
• Exploit leverages object‑manager symbolic links to redirect the Defender service’s file creation.
• Researcher MSNightmare (Abdelhamid Naceri) posted the exploit code and a write‑up on GitHub the same day Microsoft issued its November 2026 security updates.
• Microsoft has not confirmed the flaw, assigned a CVE, or released a fix.

Who is affected

Any Windows device with the CLFS authentication mitigation enabled is potentially vulnerable. This includes most Windows 11 25H2/26H1/26H2 desktops and laptops, as well as Windows Server 2025 deployments in enterprise environments.

Why it matters for IT and security teams

Because the exploit works against any antimalware engine’s self‑defense, compromised endpoints can be used to execute arbitrary code with SYSTEM rights, bypassing traditional defenses. Organizations should treat this as a high‑severity risk until a patch is available.

Mitigation guidance

IT teams should consider the following interim steps:

• Temporarily disable the CLFS authentication mitigation via Group Policy or registry if it is not required for compliance.
• Ensure Defender and other antimalware solutions are up to date and monitor for any unusual .cnpf file creation in system directories.
• Apply strict least‑privilege policies and limit standard‑user accounts from accessing CLFS‑protected logs.
• Monitor Microsoft security bulletins for an official CVE and patch, and apply it as soon as it is released.

For ordinary users, the simplest protection is to keep Windows updates current and avoid running unknown software that could trigger the exploit.

Sources

• CVE-2026-50656 (NVD)
• CVE-2026-69414 (NVD)
• microsoft.com (official)
• learn.microsoft.com (official)
• support.microsoft.com (official)
• cyderes.com (official)
• github.com (original report)
• The Hacker News
• BleepingComputer

This brief was prepared with AI assistance from the sources above and reviewed by our editors.