SignDrop: a Windows zero‑day that turns a standard user into System

What happened

Security researcher Abdelhamid Naceri, who is known online as Nightmare Eclipse, has published another zero-day exploit for Windows. Named SignDrop, it allows a local user to raise their privileges to System by abusing flawed logic in the Common Log File System (CLFS). CLFS parses complex log files in kernel mode and has long been attractive to threat actors. Last year Microsoft added logfile authentication to CLFS, using authentication codes to check file integrity. SignDrop targets the way clfs.sys handles the creation of the authentication file that carries the .cnpf extension.

As described, creating or opening a log file always causes the associated container to be opened. If that container is empty, CLFS tries to sign it whether or not a signature file exists. A standard user can create a log file that points to a container path, then prompt Windows Defender to open it by calling the signature update RPC interface. Defender follows the path in the container, and if no signature file is present, one is created with System privileges. SecurityWeek has not tested the exploit, but understands that the proof-of-concept uses this behaviour to place a signature file at a location the attacker controls, reopen it and insert a payload, which spawns a System shell. Naceri reports testing on Windows 11 26H2, an experimental build of the same release, and an experimental build of Windows Server 2025. He claims it should work on about 80% of Windows installations and says a system is generally vulnerable if it is more than 90 days old. Cyderes compiled and tested the proof-of-concept and says there is no evidence of exploitation in the wild. SecurityWeek has asked Microsoft for a statement, and none had been reported at the time of writing.

Why it matters for organisations in Africa, the Gulf and beyond

SignDrop is a local privilege escalation flaw, so an attacker needs a foothold first. That can look like a reason to rank it below remote code execution bugs. It is the wrong conclusion. Local escalation is usually the second step in an intrusion, and it is the step that turns a single compromised account or workstation into control of the machine, including its security tooling, stored credentials and access to neighbouring systems. DAAKYI's recent analysis of an Ivanti Connect Secure campaign showed the same pattern: attackers chained an authentication bypass, a remote code execution flaw and a local privilege escalation flaw to reach root, then went after credentials and directory information rather than relying on persistent malware.

For organisations in Africa, the Gulf and beyond, the practical risk is the gap between publication and a vendor fix. A working exploit is public, and Cyderes has confirmed the proof-of-concept compiles and runs. Exploitation in the wild has not been reported, but that status can change quickly once code circulates. Estates with shared workstations, terminal servers, contractor access or many standard user accounts have more places where a low-privilege foothold already exists. Regional organisations that run mixed or ageing Windows fleets, often with managed service providers holding delegated access, should assume that a standard user account is not a strong boundary.

The detail that the technique drives Windows Defender to perform the privileged action is also instructive. Security tooling runs with high privilege by design, so any path that lets a low-privilege user steer it deserves attention. Leaders should not read this as a reason to doubt endpoint protection. It is a reminder that no single control contains an intruder, and that the value of identity, segmentation and monitoring lies in limiting what a stolen or low-level credential can reach. Vulnerability scores alone will not settle priority here. Exploit availability, asset context and exposure to untrusted users will.

What leaders should do now

Because no fix or Microsoft statement has been reported yet, the aim is to reduce exposure and improve detection while the position becomes clear.

• Track Microsoft's response to SignDrop and prepare to test and deploy any update quickly, starting with servers and shared systems.
• Rank the issue using exploit availability and asset context, not severity alone, and identify where untrusted or low-privilege users can log on.
• Reduce the supply of footholds by tightening local logon rights, removing unneeded standard accounts and reviewing contractor and shared-workstation access.
• Ask your security operations team to watch for unexpected privilege jumps to System and unusual activity involving Defender signature updates and log file creation.
• Limit what a single compromised account can reach through segmentation and least privilege, and check that credentials on exposed hosts can be rotated quickly.

How DAAKYI can help

DAAKYI's Continuous Threat Exposure Management gives a current view of which systems are reachable and exposed, and layers exploit evidence and asset context on top of severity so that effort goes where an attacker is most likely to use it. Managed Vulnerability Remediation then turns that view into tracked fixes with named owners and rescan evidence, so a vulnerability counts as closed only when the exposure is shown to be gone.

Where a low-privilege foothold could become wider access, our Zero Trust Architecture work focuses on shrinking what a stolen credential can reach, using identity, device state and segmentation. If you want a view of where SignDrop-style escalation would have the greatest effect in your estate, our team can help you establish it and agree the order of work.

• Continuous Threat Exposure Management
• Managed Vulnerability Remediation
• Zero Trust Architecture

Sources

• Nightmare Eclipse Drops Windows Zero-Day Exploit ‘SignDrop’ (SecurityWeek)