Microsoft Defender Bug Sparks Numerous False BIOS Security Alerts

Microsoft Defender for Endpoint users, particularly those with Dell devices, are experiencing a widespread issue with false BIOS security alerts caused by a critical software bug, which surfaced on 2 October 2025. The problem has prompted Microsoft to issue a service degradation notice affecting multiple organisations worldwide, with the incident tracked under reference number DZ1163521.

Microsoft has identified the root cause as a code defect in Defender for Endpoint’s vulnerability detection logic, where the data-fetching mechanism for Dell devices incorrectly flags up-to-date BIOS installations as outdated. According to Microsoft, a targeted fix has been developed and deployment is expected to begin by the next scheduled communication window, with the incident still OPEN and under active monitoring and remediation.

IT teams are advised to verify BIOS versions independently through Dell’s official channels before applying any firmware updates to avoid cascading false alerts. The next official update is scheduled for Friday, 3 October 2025 at 9:00 AM UTC, and organisations should monitor Microsoft’s service health dashboard for real-time status and timelines.