Windows Vulnerability Allows Privilege Escalation via SignDrop Exploit

A zero-day exploit has been disclosed by Nightmare Eclipse, a researcher who has frequently criticized Microsoft's security practices. The vulnerability, dubbed SignDrop, allows local users to gain full System rights on certain versions of Windows.

The flaw resides in the Common Log File System (CLFS), which is used by various software clients running in either user-mode or kernel-mode. By coercing a privileged process like Windows Defender into accessing a crafted CLFS container, an attacker can exploit file creation privileges and elevate their access to System rights.

SignDrop exploits the interaction between CLFS, Windows Defender, and Windows file permissions. The attack involves using the legitimate ServerMpUpdateEngineSignature function of Windows Defender, which is responsible for processing antivirus signature updates, to open a log file created by a standard user.

To execute this exploit, the attacker manipulates symbolic links within the Windows Object Manager to trick Defender into accessing the CLFS container in its own security context. If an associated authentication file is missing, CLFS creates it using Defender's System privileges, effectively granting access to a file with elevated permissions.

This created file then inherits the Discretionary Access Control List (DACL) of the original file, allowing the attacker to exploit this privilege escalation and perform unauthorized operations with System rights.

This seemingly innocuous security measure introduced by Microsoft to prevent attackers from exploiting vulnerabilities in CLFS has inadvertently created a backdoor that can be exploited by hackers.

Funnily enough, this legitimate security mitigation can be used against itself, allowing an elevation of privileges, according to Nightmare Eclipse's assessment. The logic behind it is sound, but the potential for abuse exists.

What makes this vulnerability unique is its ability to bypass antimalware engines' self-defense mechanisms. By triggering arbitrary file creation in the context of their own processes, attackers can evade detection and turn off any antivirus or EDR system.

This exploit has been successfully tested on Windows 11 26H2, Windows 11 26H2 Experimental, and Windows Server 2025 Experimental, according to Nightmare Eclipse's findings. Users can check if their systems are vulnerable by verifying the CLFS authentication mode, which should be in learning or enforcement mode for this vulnerability to apply.

The vulnerability in Windows 11 requires specific conditions to be met. A system must run on Windows 26H2 and have existed for more than 90 days. According to Dormann, a new installation of Windows 26H2 will not suffice.

To check if a machine is vulnerable, users can verify the CLFS authentication mode value. It should be set to either 0 or 1 in the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode registry key. Dormann's own testing revealed that only one of his many machines met this requirement.

The prevalence of Windows 26H2 is uncertain due to Microsoft's staged rollouts of updates. Users can check their version by running the "winver" command, but a response from Microsoft representatives was not forthcoming.