A Layered Financial-Grade API Security Architecture: Integrating OAuth 2.0, FAPI, Open Banking, and Regulatory Controls for High-Assurance Financial Platforms

Abstract

Financial services platforms increasingly expose core banking, payments, lending, and wealth management capabilities through APIs to enable open banking initiatives, regulated third-party providers (TPPs), fintech partnerships, and mobile-first digital experiences. Unlike conventional web APIs, however, financial APIs operate in a high-risk environment shaped by stringent regulatory mandates (such as PSD2 and Strong Customer Authentication), data protection requirements, payment network rules, and persistent adversarial threats including token replay, credential stuffing, consent abuse, and object-level authorization flaws. This paper proposes a structured, layered Secure API Design Model purpose-built for financial services platforms, synthesizing foundational authorization protocols (OAuth 2.0), federated identity standards (OpenID Connect), hardened financial profiles (FAPI), Open Banking implementation frameworks, OWASP API Security risk guidance, and NIST digital identity assurance principles. Through detailed analysis of protocol flows, cryptographic trust boundaries, consent lifecycle management, and real-world deployment patterns, the model formalizes a reference architecture centered on strong identity assurance, mutual authentication and cryptographic binding (e.g., mTLS and signed request objects), fine-grained least-privilege authorization, secure token handling, continuous monitoring, and operational governance. By integrating regulatory compliance with modern zero-trust and defense-in-depth strategies, the proposed framework offers architects and platform engineers a systematic approach for designing resilient, high-assurance, and regulation-compliant API ecosystems capable of sustaining secure interoperability in rapidly evolving financial environments.