At Dim Sum Labs, people discussed on possible hardware backdoor inside Gnuk Token.  Well, I rely on Seeed Technology's manufacturing process entirely, and I let them get parts, including the MCU.  Thus, I said that if you care, I'd recommend getting STM8S Discovery Kit, instead.  It's expected usage is for STM8S development.  (Ab)using its part of STM32F103 would be best to avoid hardware back door threat.  It's less than USD10 and its intended purpose is education/promotion of another MCU, it is unlikely we have hidden feature with its STM32F103.
And someone noticed that STMicroelectronics is not US company.  Another person discuss that it would be safer if MCU were implemented by FPGA, then its functionality would be under user's control.  My answer was that it would be easier for an attacker to attack FPGA with full of features, than attacking a simpler MCU with limited capability.