Fault injection tools are commonly used in the early evaluation of microprocessor-based systems. These tools are based on introducing faults into a system to evaluate its behavior, making them of great interest to academia and industry. They are widely used to validate the reliability of mission-critical systems and the effectiveness of fault tolerance techniques. In this paper, we present UN-FIT, an open-source fault injection tool adaptable to different architectures. UN-FIT simulates the target architecture using QEMU, which allows code debugging via GDB. In this way, we can modify the content of registers by simulating bit-flips. We validated the functionality and applicability of this tool by carrying out fault injection campaigns for ARM and RISC-V microprocessors using four test programs. The main contribution of this work is the introduction of an open-source fault injection tool, UN-FIT, for ARM and RISC-V processor architectures, which can be adapted to other architectures or customized according to the user's needs.