Microsoft has confirmed and resolved a bug that caused its Defender antivirus software to crash on some Windows 10 and Windows 11 systems. The issue, which began on Tuesday afternoon, August 19, 2026, manifested as "Threat service has stopped. Restart it now" error messages and 0xc0000005 access violation errors.
Users reported that quick or full scans would fail, sometimes requiring the Defender service to be restarted. One system administrator noted that they could reliably reproduce the issue by initiating a quick scan, even on clean systems. The widespread nature of the problem led some affected users to consider reinstalling their operating systems.
Microsoft stated that the bug has been addressed in a new signature update. The company recommends that users apply the latest update or ensure automatic updates are enabled. The fix is automatically applied upon installation of Microsoft Defender Antivirus signature update version 1.457.236.0 or later. Users are advised to update their systems via Windows Update and then verify that they have the most recent security intelligence update.
This incident follows other recent issues with Microsoft Defender. In May, system administrators reported that Defender incorrectly flagged DigiCert root certificate entries as Trojan:Win32/Cerdigent.A!dha malware, leading to numerous false-positive alerts and, in some instances, the removal of certificates from the Windows trust store. Prior to that, in December 2025, a widespread outage of the Microsoft Defender portal disrupted access to some Defender XDR portal capabilities and affected threat hunting alerts.
