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.