The exploration of advanced covert timing channel design is important to understand and defend against covert timing channels. In this paper, we introduce a new class of covert timing channels, called model-based covert timing channels, which exploit the statistical properties of legitimate network traffic to evade detection in an effective manner. We design and implement an automated framework for building model-based covert timing channels. Our framework consists of four main components: filter, analyzer, encoder, and transmitter. The filter characterizes the features of legitimate network traffic, and the analyzer fits the observed traffic behavior to a model. Then, the encoder and transmitter use the model to generate covert traffic and blend with legitimate network traffic. The framework is lightweight, and the overhead induced by model fitting is negligible. To validate the effectiveness of the proposed framework, we conduct a series of experiments in LAN and WAN environments. The experimental results show that model-based covert timing channels provide a significant increase in detection resistance with only a minor loss in capacity.