The modern automobile relies on numerous electronic control units communicating over the de facto standard of the controller area network (CAN) bus. This communication network was not developed with cybersecurity in mind. Many methods based on constant time intervals between messages have been proposed to address this lack of security issue with the CAN bus. However, these existing methods may struggle to handle variable time intervals between messages during transitions of vehicle driving modes. This paper proposes a simple and cost-effective method to ensure the security of the CAN bus that is based on constant message frequencies across vehicle driving modes. This proposed method does not require any modifications on the existing CAN bus and it is designed with the intent for efficient execution in platforms with very limited computational resources. Test results with the proposed method against two different vehicles and a frequency domain analysis are also presented in the paper.