SignDrop: Windows Local Privilege Escalation Chaining CLFS Authentication Files and Defender Update RPC

1. Basic Information

• Article Title: Nightmare Eclipse Drops Windows Zero-Day Exploit SignDrop
• Publisher: SecurityWeek
• Publication Date: 2026-11-13T03:50:00-05:00
• Update Date: 2026-11-13T16:00:00-05:00
• Collection Date: 2026-11-14T08:00:43+09:00
• Original Article: SecurityWeek
• Related Sources: SignDrop Zero-Day, SignDrop
• Related Malware: SignDrop
• Related Threat Actors: None
• Related CVEs: None
• Related Products and Technologies: Windows 11 26H2, Windows Server 2025, Microsoft Defender
• Response Priority (Author's Assessment): High
• Priority Rationale: Although exploitation requires a local foothold, SYSTEM access was reproduced in Windows 11 and Windows Server 2025 test environments. Microsoft was investigating the issue, and no patch was available at the time of publication. Exploitation in the wild has not been confirmed.
• Vendor Severity / CVSS: Not specified in the reviewed sources.

2. Executive Summary

SecurityWeek reported that the researcher known as Nightmare Eclipse released SignDrop, a proof-of-concept (PoC) that chains CLFS authentication file creation with a Microsoft Defender signature update RPC call to create files and execute a shell as SYSTEM from a standard user account.

3. Attack Flow

SignDrop Local Privilege Escalation

1. An attacker executes code with standard user privileges.
2. The attacker prepares a CLFS log and associated container paths, using Object Manager symbolic links to redirect file access.
3. The attacker invokes the Microsoft Defender signature update RPC. The redirected access causes CLFS to create an authentication file at an attacker-selected path under SYSTEM privileges.
4. The attacker uses the retained access rights to overwrite the CLFS-created file with a DLL payload, then triggers a privileged process to load that DLL.
5. SYSTEM-level payload and shell access are achieved.

4. Attacker Positioning and Execution Environment

• A local attacker who has already obtained standard user privileges on the Windows host.

5. Visibility for Victims and Administrators

Victim Perspective

• The activity resembles a Defender update or temporary file operation, with an additional SYSTEM process appearing upon success.

Administrator Perspective

• The sequence appears as CLFS authentication file creation, a Defender service RPC call, SYSTEM file creation, and shell spawning.

6. Success and Failure Conditions

Success Conditions

• Ability to execute local code as a standard user.
• CLFS authentication must be enabled. The researcher identifies configurations where the Mode value under HKLM\SYSTEM\CurrentControlSet\Services\CLFS\Authentication is 0 (enforcement mode) or 1 (learning mode), but this value alone does not guarantee successful exploitation.
• Completion of the attack chain without interception by security controls such as Defender.

Failure Conditions and Mitigations

• Exploitation fails if Defender actually blocks execution. Keep the Defender platform and signatures up to date. Microsoft states that Defender detects and blocks exploitation, but the versions required for protection and the scope of blocking remain unconfirmed.
• Blocking of the PoC execution through application control and local execution restrictions.

7. Outcome Upon Success

• Privilege escalation from a standard user to SYSTEM.
• Payload execution under SYSTEM privileges with defense evasion.

8. Observable Logs

• Email: No specific items
• Proxy/SWG/DNS: No specific items
• Endpoint/EDR: Creation of .cnpf files, abnormal file operations by the Defender update service, SYSTEM shells, and transitions from user processes to SYSTEM tokens or processes (requires process creation auditing or EDR collection).
• Identity/IdP: No specific items
• SaaS/Cloud: No specific items
• Network: Inference: Investigate outbound communications if a follow-on payload communicates externally.

9. Attack Success Assessment

Initial Execution Confirmed

• Public Information: Researchers and Cyderes compiled and tested the PoC, confirming SYSTEM access.
• Scope of Verification: Conducted in research and test environments; active exploitation in the wild has not been confirmed.

10. Investigation Playbook

• Trigger: CLFS authentication file, Defender update RPC, SYSTEM child process.
• Initial Checks: Verify host build, Defender version, user context, initial process, and PoC artifacts.
• Endpoint: Preserve process trees, .cnpf files, Defender and CLFS logs, and memory.
• Identity/Cloud: Check initial foothold, credential access, and token usage.
• Follow-On: Track SYSTEM persistence, security setting modifications, and lateral movement.
• Containment: Isolate the host, remove payloads, rotate credentials, and apply patches or guidance.
• Classification: Differentiate between PoC detection, successful LPE, and subsequent SYSTEM activity.

11. Defense and Detection Ideas

• Single Event: User-created .cnpf files, SYSTEM files or shells immediately following a Defender update RPC. Alert on abnormal file operations by the CLFS and Defender services.
• Time Series: Correlate user processes leading to CLFS files, the Defender service, and SYSTEM child processes.
• Hunting: Search for pre- and post-execution Defender events, service file operations, and unknown binaries.
• Logging Gaps: Prioritize behavioral detection due to limited specific IOCs in public reporting.
• Priority Controls: Prioritize updating Defender (Microsoft states it detects and blocks the activity, though required versions and blocking scopes are unconfirmed), application control, and reducing local administrative privileges.

12. Facts, Inferences, and Hypotheses

Facts

• The PoC combines the creation of a CLFS .cnpf authentication file with a Microsoft Defender signature update RPC.
• It triggers SYSTEM-privilege file creation from a standard user, and researchers demonstrated obtaining a SYSTEM shell.
• Tested by researchers on Windows 11 26H2, Windows 11 26H2 Experimental, and Windows Server 2025 Experimental.
• Cyderes compiled the PoC and verified its functionality.
• Microsoft is investigating the issue, stated that Defender detects and blocks it, and noted no observed active exploitation in the wild.
• Broad impact estimates by researchers do not constitute independently verified prevalence rates or victim counts.

Inferences

• Because this is used for privilege escalation after an initial foothold, investigations must track user-level intrusion preceding the exploit file.

Hypotheses

• No additional hypotheses

13. MITRE ATT&CK Mapping

14. Uncertainties and Additional Investigation

• Formal CVEs, patch timelines, all affected builds, the completeness of Defender detection, and the presence of active exploitation remain unconfirmed.

15. Impact on SOCs and Organizations

On Windows endpoints, update Defender definitions and the platform, and monitor for the creation of CLFS authentication files, abnormal file operations by the Defender update service, and escalation to SYSTEM.

16. Target-Specific Summaries

• For SOCs: Correlate .cnpf file creation by user processes, Defender update RPC calls, SYSTEM file creation, and subsequent shell activity.
• For Administrators: Update Defender, strengthen local execution controls, enforce least privilege, and track Microsoft guidance.
• For End Users: Do not execute suspicious files or tools, and do not disable EDR warnings.