Web Single Sign-On: Understanding WS-Federation

A practical guide explaining why WS-Federation (WS‑Fed) remains critical for enterprise SSO in 2026: it details the Passive Requestor browser flow (302 redirect with `wa=wsignin1.0`/`wtrealm`, STS authentication, `wresult` POST back to the RP), defines key terms (STS, Relying Party, realm, EPR, claims), compares WS‑Fed to SAML and OIDC, and calls out common operational and security pitfalls (infinite redirect loops, token-signing certificate expiry, XML signature wrapping and Golden SAML). The guide recommends using hybrid identity translation layers to provide modern authentication experiences (OIDC/MFA/passwordless) while supporting legacy applications that require WS‑Fed.