# federation

Source: https://startwithidentity.com/glossary/federation/
Last updated: 2026-08-29
License: content by Start with Identity. Cite the source URL.

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A trust relationship between identity providers and service providers that lets users authenticate once at their home IdP and access applications at the other party. Federation is the foundation of cross-organization SSO.

Federation is the reason an employee has one login instead of forty, and also the reason one compromised login opens forty systems. That trade is worth making, but it changes where the security work goes: into the identity provider, into session lifetime, and into what happens when the assertion itself can be forged. Most catastrophic SAML bugs are federation bugs, where a service provider accepted an assertion it should have rejected.

See also: [SSO](https://startwithidentity.com/glossary/sso/), [SAML 2.0](https://startwithidentity.com/standards/saml-2-0/), [OpenID Connect](https://startwithidentity.com/standards/openid-connect/), [SAML vs OIDC](https://startwithidentity.com/guides/fundamentals/saml-vs-oidc/)