Single Sign-On (SSO) lets you log in once and then access multiple applications without re-authenticating. Common protocols include SAML 2.0, OpenID Connect, and OAuth 2.0. SSO improves user experience and reduces the number of weak or reused passwords. At the same time the identity provider (IdP) becomes a central security element: if it is compromised, all connected services are affected. In your ISMS you therefore protect the IdP with strong authentication (MFA), monitoring, and redundancy. SSO is an access-management control per ISO 27001 Annex A 5.16.