Software for USBCanConverterBasic(UCCB) writen for STM32F042C6 mcu, see ucandevices web page for full info.
Source code is pure C. Project can be imported direclty to free vesrion of Atollic TrueSTUDIO for ARM 5.5.1. Drivers for CAN and USB layer were generated in stm32CubeMX.
Hardware is open source. See CCBPCB project for souces.
STM32 have embedded DFU bootloader, for more info see st.com