I have a USB-device with three interfaces: Vendor-Specific Bulk CDC Serial Control CDC Serial Data To configure the vendor specific bulk endpoints I need to send vendor specific control requests to endpoint 0. I'm using libusb for this task. As long as the interfaces for the CDC serial port are present I get an access error when trying to send vendor control requests. If I disable these CDC interfaces I can send vendor control request without any problems: Is this by design or ist there any possibility to send vendor control requests to the USB device while a CDC driver is active?