Building Automation Systems (BAS) are crucial for monitoring and controlling buildings, ranging from small homes to critical infrastructure, such as airports or military facilities. A major concern in this context is the security of BAS communication protocols and devices. The
building automation and control networking
protocol (BACnet) is integrated into products of more than 800 vendors worldwide. However, BACnet devices are vulnerable to attacks. We present a novel solution for the two most important BACnet layers, i.e. those independent of the data link layer technology, namely the network and the application layer. We provide the first implementation and evaluation of traffic normalization for BAS traffic. Our proof of concept code is based on the open source software Snort.