System administrators woke up on May 3rd to inboxes flooded with alerts: Microsoft Defender was flagging a DigiCert certificate hash as Trojan:Win32/Cerdigent.A!dha, automatically quarantining files across environments. The post captures the moment of collective panic — and the collective sigh of relief — that accompanies a major false positive from a trusted endpoint protection platform.

False positives from enterprise antivirus are not rare curiosities. They are operational incidents. When Defender quarantines a file signed by one of the world's largest certificate authorities, the blast radius can be enormous:

• Legitimate applications stop working. Any executable signed with the flagged DigiCert certificate gets yanked out of production, potentially breaking line-of-business software, agents, and services.
• Alert fatigue compounds the problem. Hundreds or thousands of identical "malware detected" alerts bury any real threats that might arrive at the same time.
• Remediation creates its own risks. Restoring quarantined files across a fleet of machines is nontrivial, especially if automated remediation has already run and you need to validate that nothing was genuinely malicious in the mix.

This kind of event is a stress test for your incident response process. The administrators who weathered it best likely had a few things in place:

• Exclusion policies and override procedures ready to deploy quickly when a false positive is confirmed.
• Monitoring that distinguishes patterns — seeing the same signature flagged simultaneously across every endpoint is a strong signal that something is wrong with the signature, not with your environment.
• Communication channels to compare notes with peers. In this case, multiple subreddits lit up within minutes, confirming the false positive faster than Microsoft's own advisory could be published.

The incident also highlights a philosophical tension in endpoint protection: aggressive heuristics catch more real threats but inevitably produce more false positives. Certificate-based detection is particularly high-stakes because a single bad signature definition can flag thousands of otherwise trusted binaries in one stroke. Vendors like Microsoft walk a razor's edge between protection and disruption every time they push a definition update.