arXiv:2610.41323v1 Announce Type: new Abstract: Single Sign-On (SSO) is a federated authentication mechanism that improves user experience by enabling access to multiple services, or Relying Parties (RPs), through a single authentication at an Identity Provider (IdP). To reduce integration complexity, many RPs rely on third-party Identity Brokers (Brokers) that aggregate numerous IdPs into a unified interface, thereby simplifying protocol management and interoperability. However, while brokers ease development, they introduce a fundamental security and privacy risk: sensitive authentication credentials and user behavioral data are exposed to the broker, creating a high-value single point of failure. Existing SSO solutions provide limited protection against these risks. With the goal of achieving broker obliviousness, we address these challenges at their root and present OrionLink, a comprehensive privacy-enhancing SSO pipeline that is compatible with existing brokered SSO architectures and systematically mitigates broker-induced risks without sacrificing usability. OrionLink incorporates two core components: (a) an anonymous pseudonym derivation scheme that generates unlinkable, per-RP identifiers to prevent cross-service user tracking; (b) an anonymous key exchange protocol that establishes end-to-end encrypted channels to protect user data from broker visibility. We further instantiate OrionLink with two new features, preserving broker functionality and deployability. We implemented a prototype of OrionLink within a brokered SSO environment, and experimental evaluation shows that the framework achieves substantial security and privacy improvements with acceptable performance overhead.