Single Sign-On (SSO) - One Login for All Apps

About 2 min read

Single sign-on (SSO: Single Sign-On) is a mechanism that allows access to multiple services and applications with a single authentication. Users no longer need to enter a separate password for each service, greatly improving convenience. It is widely adopted in corporate internal systems and cloud services. The more cloud services one person uses for work, the harder it becomes to manage a separate password for each of them, which is why consolidating authentication in a single place with SSO becomes necessary.

The Difference Between SSO and OAuth

SSO and OAuth are related technologies, but they solve different problems. SSO provides the authentication convenience of "logging in to multiple services with a single authentication." OAuth provides the authorization security of "delegating permissions to an app without handing over your password." On the implementation side, SSO is realized through protocols such as SAML 2.0 and OpenID Connect (OIDC). OIDC adds an authentication layer on top of OAuth 2.0, integrating OAuth's authorization functionality with SSO's authentication functionality. For enterprise SSO, SAML is mainstream, while OIDC is mainstream for consumer-facing social login.

How SSO Works

In SSO, a central authentication server (IdP: Identity Provider) centrally manages user authentication. When a user completes authentication at the IdP, an authentication token is issued, and each linked service verifies this token to grant access. Representative IdPs include Okta, Microsoft Entra ID (formerly Azure AD), and Google Workspace.

Real-World Use Cases

For example, when an organization where internal systems and cloud services each have their own passwords is consolidated onto an SSO platform, the first visible change is usually not the daily login steps but the help desk's handling of password resets. Because account creation for new hires and deactivation for leavers also come together on the IdP side, it becomes far less likely that a departing employee's account is left behind in an individual service. On the other hand, if business tools that do not support SSO remain, the benefit stays partial. The result depends heavily on whether you took stock of which services can actually be covered before the consolidation.

The SSO Flow

Risks and Countermeasures in Practice

SSO is convenient, but it carries the risk of becoming a "single point of failure." If SSO credentials are leaked, all linked services are affected. To mitigate this risk, set an especially strong random password for the SSO account and always enable multi-factor authentication. A common pitfall in practice is to think that "password management is unnecessary" just because SSO has been introduced. Services that do not support SSO still exist, and they require their own strong individual passwords. It is also important to consider the risk of being unable to log in to any service when the SSO provider has an outage, and to secure an emergency bypass method in advance.