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LevelBlue lab screenshot of BigDiskBuster console claiming free space during a Defender update.
Figure: LevelBlue, Figure 1, BigDiskBuster console output detecting Defender update and allocating hidden temp file, Oct 5, 2026 (fair use).

Detection

LevelBlue Shows BigDiskBuster Starves Microsoft Defender Updates Without Disabling the Service

LevelBlue SpiderLabs reproduces MSNightmare's BigDiskBuster proof of concept that watches Defender update staging, fills free disk space with hidden temp files, and leaves real-time protection running while security intelligence goes stale.

Ava Okello, DetectionLondon5 min read

LONDON - LevelBlue SpiderLabs reproduced BigDiskBuster, a proof of concept published on GitHub on Sept. 19, 2026 by the actor known as MSNightmare, and showed how it can stop Microsoft Defender from refreshing security intelligence without turning the product off. In an Oct. 5 report, Serhii Melnyk and Timmy Lister write that the tool watches the C:\ volume for Defender update activity, then creates a hidden file that claims essentially all remaining free space so the update fails for lack of room. Defender cleans up its staging directory, space returns, and BigDiskBuster repeats the race on the next attempt. Real-time protection keeps running. The only obvious symptom in their lab was a stale intelligence version and error code 0x80070643 on the Windows Security Protection Updates pane, a generic fatal installation error that does not point to disk exhaustion.

Windows Security Protection Updates pane with stale intelligence and error 0x80070643.
Figure: LevelBlue, Figure 2, Windows Security Protection Updates pane after blocked update showing 0x80070643, Oct 5, 2026 (fair use).The visible symptom was a stale intelligence version and error 0x80070643.

Unlike ShieldCrash, which relied on a Windows path-resolution flaw, BigDiskBuster needs no vulnerability. LevelBlue says the roughly 300-line C++ sample opens two NtCreateFile handles for the process lifetime: a raw handle to the C:\ volume device, and a relative open of MRT.exe using that volume handle as RootDirectory. It then registers ReadDirectoryChangesW with a recursive watch across the volume and acts when Defender Platform or Definition Updates paths appear, confirming GUID-named definition subdirectories with CLSIDFromString. When a matching staging directory shows up, GetFreeVolumeSize via NtQueryVolumeInformationFile feeds AllocationSize for a GUID-named hidden file under %TEMP% with FILE_ATTRIBUTE_HIDDEN and FILE_DELETE_ON_CLOSE. Every FILE_ACTION_MODIFIED notification while an update is in progress spawns another allocation thread. Process Monitor in their lab showed repeated IRP_MJ_CREATE and related I/O from BigDiskBuster.exe against those temp files. The targeted update chain they document is svchost to wuauclcore to UpdatePlatform.amd64fre to MpSigStub to MpRecovery. At publication, LevelBlue reports no assigned CVE, available patch, or Microsoft advisory for the technique.

Process Monitor showing BigDiskBuster.exe IRP create operations on GUID temp files.
Figure: LevelBlue, Figure 5, Process Monitor I/O from BigDiskBuster.exe against GUID-named temp files, Oct 5, 2026 (fair use).Process Monitor showed repeated creates against the hidden temp files.

For SOC and detection teams, LevelBlue's highest-confidence signal is a non-allowlisted process that holds both the volume device handle and an MRT.exe handle in the same PID. Volume handles alone are noisy because legitimate Windows services can hold them. Persistent MRT.exe handles outside Defender service and TrustedInstaller contexts warrant investigation. Correlate failed Defender update chains that end without a successful refresh with a near-total free-space drop on C:\ during the same window, and look for briefly created then removed braced-GUID hidden files under %TEMP%. LevelBlue also published a SentinelOne example query that joins GUID temp-file creation with MpRecovery.exe process creation within about 30 seconds and returns the creating process. Treat recurring 0x80070643 update failures without matching installer activity as a prompt to pull handle telemetry before the temporary files disappear.

LevelBlue detection view correlating MRT.exe and volume handles in one PID.
Figure: LevelBlue, Figure 12, same-PID correlation of MRT.exe and volume-device handles, Oct 5, 2026 (fair use).The strongest signal is one process holding both handles.

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Ava Okello covers detection engineering, EDR telemetry, and SOC hunting for SOCtember from London.

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