GoKawiilResearcher discloses new Windows zero-day exploiting CLFS authentication mitigation
Security researcher Abdelhamid Naceri has published a zero-day called SignDrop that abuses a Microsoft mitigation introduced in October 2025 to protect against Common Log File System authentication attacks. Naceri says the flaw lies in how CLFS handles creation of its '.cnpf' authentication file, which can be forced open by a privileged process such as Windows Defender, allowing attackers to gain SYSTEM privileges. He also says the technique can bypass tamper-protection defenses in any antimalware engine because the file-creation request appears to originate from the engine's own trusted process.
GoKawiil's interpretation of the reporting above, not reported fact.
The disclosure suggests that a security fix meant to close off one class of attacks can itself be turned into a new privilege-escalation path, which could undermine confidence in patch-based mitigations generally. Because Naceri describes the technique as effective against any antimalware engine's self-defense mechanism, it could have broad implications for endpoint security products beyond Windows Defender alone, though real-world exploitation would depend on how quickly Microsoft responds.
- SignDrop exploits a flaw in Microsoft's October 2025 CLFS authentication mitigation, not the original CLFS bug.
- The technique can reportedly grant attackers SYSTEM-level privileges by abusing legitimate privileged processes.
- Naceri says the method could bypass tamper-protection defenses across multiple antimalware engines, not just Windows Defender.
Source: bleepingcomputer.com, 2026-11-12
Published there as: “New Windows SignDrop zero-day gives hackers SYSTEM privileges”
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