Threat knowledge-bases such as those maintained by MITRE and NIST provide a basis with which to mitigate known threats to an enterprise. These centralised knowledge-bases assume a global and uniform level of trust for all threat and countermeasure knowledge. However, in practice these knowledge-bases are composed of threats and countermeasures that originate from a number of threat providers, for example Bugtraq. As a consequence, threat knowledge consumers may only wish to trust knowledge about threats and countermeasures that have been provided by a particular provider or set of providers. In this paper, a trust management approach is taken with respect to threat knowledge-bases. This provides a basis with which to decentralize and delegate trust for knowledge about threats and their mitigation to one or more providers. Threat knowledge-bases are encoded as Semantic Threat Graphs. An ontology-based delegation scheme is proposed to manage trust across a model of distributed Semantic Threat Graph knowledge-bases.