A modular system for preventing the unauthorized use of a firearm is provided. The system includes a safety module that is configured to attach to a firearm using a standard rail mount found on most firearms. The safety module includes a trigger bar that is configured to extend from the safety module to behind or through the side of the trigger of the firearm. The safety module further includes a locking mechanism that, in a first state, prevents the trigger bar from moving rearward and thereby also prevents the trigger from being pulled. The locking mechanism further includes a second state where the trigger bar is allowed to move with the trigger thereby allowing the trigger to be pulled.