Authentication and Authorisation Infrastructures (AAIs) support service providers on the internet to outsource security services. Motivations for their usage stem from software engineering and economics. For the latter an assessment of inherent risks is needed. In this work the authors deduct an appropriate, formalistic risk assessment method for AAIs and analyse outsource able security services in comparison to traditional – non AAI involved – service providing. To achieve the assessment of risks various methods for risk management have been analysed and finally a suitable qualitative method has been chosen. As AAIs differ in their potential to cover security services, combinations of these services are compared. The given risk assessment method enables providers to decide on a special infrastructure for their purpose and lets users of AAIs determine if given advantages surpass the immanent risks. This work also enables service providers to estimate costs for such an infrastructure and calculate potential savings.