Zero Trust in Heterogeneous Multi-Cloud Environments: A Systematic Literature Review and a Decentralized Federated Trust Framework

Implementing Zero Trust across heterogeneous multi-cloud environments is complicated due to incompatible identity models, policy semantics, trust anchors, and credential-lifecycle mechanisms. This article reports a systematic literature review of research published from January 2020 to July 2026. Searches of Scopus, the Web of Science Core Collection, and IEEE Xplore identified 1800 records; 31 distinct studies were retained after deduplication and screening. The synthesis shows that centralized and federated approaches remain operationally mature but continue to depend on identity providers, brokers, and provider-specific policy models. Approaches based on Decentralized Identifiers (DIDs) and Verifiable Credentials (VCs) improve claim portability and local cryptographic verification, yet they do not by themselves resolve issuer governance, credential status, policy mapping, or key compromise. On this basis, the article proposes a decentralized federated trust framework that combines portable credentials, local verification, issuer and credential-lifecycle controls, policy mapping, and resource-local enforcement. The framework is assessed analytically against review-derived requirements and representative architectures; implementation and performance evaluation remain future work.