  The open radio access network (O-RAN) offers new degrees of freedom for
building and operating advanced cellular networks. Emphasizing on RAN
disaggregation, open interfaces, multi-vendor support, and RAN intelligent
controllers (RICs), O-RAN facilitates adaptation to new applications and
technology trends. Yet, this architecture introduces new security challenges.
This paper proposes leveraging zero trust principles for O-RAN security. We
introduce zero trust RAN (ZTRAN), which embeds service authentication,
intrusion detection, and secure slicing subsystems that are encapsulated as
xApps. We implement ZTRAN on the open artificial intelligence cellular (OAIC)
research platform and demonstrate its feasibility and effectiveness in terms of
legitimate user throughput and latency figures. Our experimental analysis
illustrates how ZTRAN's intrusion detection and secure slicing microservices
operate effectively and in concert as part of O-RAN Alliance's containerized
near-real time RIC. Research directions include exploring machine learning and
additional threat intelligence feeds for improving the performance and
extending the scope of ZTRAN.
