SignDrop Zero-Day Exploit Targets Microsoft Windows clfs.sys for SYSTEM Privilege Escalation

SignDrop is a locally exploitable Windows zero-day in the Common Log File System driver (clfs.sys) that permits standard local users to escalate privileges to SYSTEM. The exploit abuses the CLFS log-authentication and mitigation feature by forcing a privileged process, such as Windows Defender, to open a crafted container, triggering privileged companion file creation through path redirection. Successful exploitation requires Windows 11 26H2 with system age exceeding 90 days and the registry value HKLM\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode set to 0 or 1. Microsoft is investigating, reports no confirmed in-the-wild exploitation, and states Windows Defender blocks attempts.

• Vulnerability Analysis: SignDrop and clfs.sys

  • Zero-day vulnerability in Windows Common Log File System driver (clfs.sys).
  • Disclosed by researcher Nightmare Eclipse; independently verified by Will Dormann.
  • Enables local standard user to escalate privileges to SYSTEM.
  • Public proof-of-concept exploit code has been released.

• Exploitation Mechanics: Path Redirection and Companion Files

  • Attacker forces privileged process, such as Windows Defender, to open crafted CLFS container.
  • Abuses CLFS log-authentication/mitigation feature to trigger companion file creation.
  • Path redirection results in arbitrary file writes under SYSTEM context.
  • Requires local access; no remote exploitation vector identified.

• Affected Systems and Environmental Constraints

  • Limited to Windows 11 26H2 installations.
  • Operating system must be active for more than 90 days.
  • Registry key HKLM\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode must equal 0 or 1.
  • Verified reproducibility limited; succeeded on only one of multiple test hosts.

• Exploitation Status and Vendor Response

  • Microsoft confirms active investigation; no evidence of in-the-wild exploitation.
  • Windows Defender reportedly blocks exploitation attempts.
  • Public exploit release increases near-term weaponization risk.
  • No CVE identifier or security patch issued at time of publication.

• Detection and Mitigation Recommendations

  • Monitor clfs.sys and CLFS container operations for anomalous privileged file creation.
  • Audit HKLM\SYSTEM\CurrentControlSet\Services\CLFS\Authentication\Mode registry values.
  • Enforce updated Windows Defender definitions and endpoint tamper protection.
  • Restrict standard local accounts and apply least-privilege principles pending vendor patch.