Threat Intel
BigDiskBuster PoC Blocks Windows Defender Updates While Service Runs
A proof‑of‑concept called BigDiskBuster can stop Windows Defender from downloading update packages without exploiting a vulnerability, leaving the service running but blind to new signatures. The technique was demonstrated by LevelBlue after the original PoC was removed from GitHub. Defenders should verify update status, enforce update policies, and consider alternative EDR solutions.
What Happened
A proof‑of‑concept named BigDiskBuster, released by former Microsoft employee Abdelhamid Naceri (MSNightmare), can prevent Windows Defender from receiving update packages. The technique does not exploit a vulnerability; it simply blocks the update traffic while the Defender service continues to run normally. LevelBlue researchers reproduced the PoC after the original GitHub page was taken down.
Impact
Defender remains operational but is unable to download the latest malware signature and behavior detection updates. This creates a silent detection gap that could allow known threats to bypass protection until the service is manually refreshed or the block is removed.
Mitigation Steps
- Verify Defender update status via the Windows Security app or PowerShell (Get-MpComputerStatus).
- Enforce update policies with Group Policy or Intune to force update downloads and prevent blocking.
- Configure Windows Update for Business to ensure update delivery even if Defender is blocked.
- Deploy a secondary EDR or antivirus solution that does not rely on Defender updates.
- Monitor network traffic for blocked update requests and investigate anomalous patterns.
Recommendations
Maintain a layered security posture by combining Defender with additional EDR tools, regularly audit update mechanisms, and keep all endpoint software up to date. Consider enabling Windows Defender Exploit Guard and other built‑in hardening features to reduce the attack surface.
Related articles
Microsoft Defender Zero‑Day Blocks Antivirus Updates on All Windows Versions
Over the weekend, researcher Abdelhamid Naceri released BigDiskBuster, a new zero‑day that stops Defender from downloading signature updates. The exploit works on every supported Windows OS and requires the malicious process to run in the background. Defenders should terminate the process, disable automatic updates, and apply the latest vendor patches.
Read briefing
CrowdStrike Tightens Falcon Sensor to Block Malicious Open‑Source Packages in Real Time
CrowdStrike has upgraded its Falcon platform to embed supply‑chain protection directly into the endpoint sensor. The new capability detects and blocks malicious open‑source packages the moment they’re downloaded, giving defenders a first‑line defense against supply‑chain attacks before code ever runs.
Read briefing
CrowdStrike Unveils the Agentic SOC: AI‑Powered Defense for the Modern Threat Landscape
CrowdStrike’s latest rollout redefines the SOC with a network of autonomous agents that learn, reason, and act on data owned by the customer. Leveraging on‑device AI for instant data classification, the platform now blocks supply‑chain attacks, streamlines incident response, and integrates next‑gen identity controls—all while keeping the human analyst in the loop.
Read briefing
Microsoft Windows Cross Device Service Elevation of Privilege Vulnerability (CVE-2026-66804) – Immediate Defender Actions
Microsoft has acknowledged a high‑severity elevation‑of‑privilege flaw in Windows Cross Device Service (CVE‑2026‑66804). No patch is currently available, but defenders should monitor MSRC releases and harden cross‑device service access until a fix arrives.
Read briefing