Federated Single Sign-On (SSO) and Multi-Factor Authentication (MFA) systems have become foundational components of modern enterprise identity management infrastructures. Organizations increasingly rely on Security Assertion Markup Language (SAML), OAuth, and OpenID Connect (OIDC) frameworks to provide scalable authentication and authorization across distributed environments. However, the expansion of federated identity architectures has simultaneously increased the complexity of the threat landscape. Attack vectors such as token replay, golden SAML attacks, credential theft, session hijacking, phishing-based MFA bypass, and privilege escalation have exposed critical vulnerabilities within federated authentication ecosystems. This research presents a STRIDE-based threat modeling framework for analyzing attack vectors in SP-initiated SAML and OAuth deployments integrated with MFA and device fingerprinting controls. The study synthesizes existing literature on threat modeling methodologies, attack-centric risk analysis, and security design frameworks to establish a comprehensive analytical model for federated authentication security assessment. The proposed framework combines attack-centric and asset-centric approaches to identify vulnerabilities across authentication workflows, token exchange mechanisms, session management systems, and trust relationships between Identity Providers (IdPs) and Service Providers (SPs). The research further evaluates the role of device fingerprinting, adaptive MFA, behavioral monitoring, and layered defense strategies in mitigating sophisticated attacks. Findings indicate that traditional authentication controls are insufficient against advanced persistent threats targeting federated systems. The study demonstrates that integrating STRIDE analysis with contextual authentication mechanisms significantly improves threat visibility, attack detection capability, and cyber resilience. The research contributes a structured security assessment framework suitable for enterprise federated identity deployments and provides strategic recommendations for future authentication security architectures.