New SignDrop Exploit Can Abuse Windows Defender to Bypass Security Protections
New Windows zero-day 'SignDrop' allows local attackers to gain SYSTEM privileges by abusing CLFS.
A new Windows zero-day vulnerability named SignDrop allows local attackers to escalate privileges to SYSTEM by abusing the Common Log File System (CLFS) authentication mitigation. The flaw, discovered by Chaotic Eclipse, exploits a staged rollout of Microsoft's CLFS mitigation, leaving approximately 80% of Windows installations vulnerable if they are less than 90 days old or have the mitigation partially enabled. The exploit tricks Windows Defender into creating a signed authentication file with SYSTEM privileges.
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The lesson from this incident
Full lessonThe SignDrop vulnerability exposes a dangerous window created by Microsoft's phased rollout of the CLFS authentication mitigation, leaving approximately 80% of Windows systems vulnerable during the transition period. Attackers can abuse Windows Defender itself — a trusted security component — to forge a signed authentication file with SYSTEM privileges, turning a defensive tool into an attack vector. This highlights how partial or staged security mitigations can introduce new attack surfaces rather than closing them. The 90-day vulnerability window between mitigation deployment phases gives adversaries ample time to exploit systems that are neither fully protected nor fully unprotected.
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