Level 32

Citrix released an advisory on Thursday about troublesome Windows Defender definition updates that break Delivery Controllers and Cloud Connectors running Microsoft’s antivirus.

The issue is due to Windows Defender misidentifying as malicious and quarantining the main and secondary Citrix broker services (BrokerService.exe and HighAvailabilityService.exe) responsible for tracking current user connections/sessions.

Both files were erroneously detected as the information-stealing malware Agent Tesla and sent to isolation.

On Thursday, several Citrix system administrators reported that the broker service on Delivery Controllers was failing. Problems started with Windows Defender definition 1.321.1319.0.

The trouble persisted in subsequent updates delivered over more than seven hours until Microsoft pushed Antivirus definition 1.321.1341.0 that corrected the issue.