The issue is related to incorrect handling of the com.apple.quarantine attribute by the Thunderbird user interface. This can be exploited by a remote attacker to execute arbitrary code by loading a malicious email attachment. On macOS, when saving or opening an email attachment, Thunderbird failed to set the com.apple.quarantine attribute on the received file. If the file was an application and the user tried to open it, the application would start immediately without prompting the user for confirmation.