Network connected devices has become inherent part of our lives. These devices have come to be more and more mobile and are target of various malware attacks. An inability to guarantee or check proper security settings of such devices poses a serious risk to network security. In this paper we propose a novel concept of flow based host trustworthiness estimation. The estimated trustworthiness determines a level of the risk to the network security the host posses. This concept enables network operators to identify a potential dangerous host in their network and take an appropriate precautions. Models used for trustworthiness estimation are based on scoring either single events or host characteristics. In order to be able to estimate trustworthiness of a host even in large scale networks, the data used for estimation are reduced only to extended network flows. The research is in its initial phase and will conclude with Ph.D. thesis in three years.