The fifth generation of mobile networks (5G) will support new business and service models. A particular model of business and technical interest is multi-operator service orchestration, where service chains are created dynamically with coordination across multiple administrative domains. In such a scenario, resource sharing among operators is expected to be enabled by emerging network softwarization technologies such as software-defined networking (SDN) and network functions virtualization (NFV). On top of the inherent security issues of network softwarization, the complex relationships between operators add a unique dimension to the fundamental requirements for 5G networks. It is a key objective for network operators to identify new threats and security issues before deploying novel methods for service orchestration. This chapter elaborates on new security challenges posed by multi-operator service orchestration as defined by the H2020 5G-PPP 5G Exchange project. We revisit current standards and recommendations from ITU-T and ETSI under the scope of SDN and NFV. In addition, we present a method for threat analysis as well as gaps between requirements and current security schemes and standards, opening new research directions.