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I see this for 1912CU4 https://docs.citrix.com/en-us/citrix-virtual-apps-desktops/1912-ltsr/secure/virtual-channel-security.html By default, the Virtual channel allow list feature is disabled. When enabled, only Citrix virtual channels are allowed to open in virtual apps and desktops sessions. If there is a need to use custom virtual channels, whether homegrown or from a third party, these need to be explicitly added to the allow list.