  The fifth generation (5G) of cellular networks starts a paradigm shift from
the traditional monolithic system design to a Service Based Architecture, that
fits modern performance requirements and scales efficiently to new services.
This paradigm will be the foundation of future cellular core networks beyond
5G. The new architecture splits network functionalities into smaller logical
entities that can be disaggregated logically, physically, and geographically.
This affords interoperability between the mobile network operators and
commercial software and hardware vendors or cloud providers. By making use of
commodity services and products, this system construct inherits the
vulnerabilities in those underlying technologies, thereby increasing its attack
surface and requiring a rigorous security analysis. In this work, we review the
security implications introduced in B5G networks, and the security mechanisms
that are supported by the 5G standard. We emphasize on the support of Zero
Trust Architecture in 5G and its relevance in decentralized deployments. We
revisit the definition of trust in modern enterprise network operations and
identify important Zero Trust properties that are weakened by the nature of
cloud deployments. To that end, we propose a vertical extension of Zero Trust,
namely, Zero Trust Execution, to model untrusted execution environments, and we
provide an analysis on how to establish trust in Beyond-5G network
architectures using Trusted Execution Environments. Our analysis shows how our
model architecture handles the increased attack surface and reinforces the Zero
Trust Architecture principles in the 5G Core, without any changes to the 5G
standard. Finally, we provide experimental results over a 5G testbed using
Open5GS and UERANSIM that demonstrate minimal performance overhead, and a
monetary cost evaluation.
