What Is Single Sign-On?
Single Sign-On (SSO) is an authentication method that allows a user to log in once with a single set of credentials and gain access to multiple, independent applications or services — without logging in again for each one.
If you've ever clicked "Sign in with Google" to access a third-party app, you've used SSO. It's the backbone of how modern organizations manage digital identity across dozens of tools and platforms.
The Problem SSO Solves
Without SSO, every application requires its own username and password. This creates several problems:
- Users resort to weak, reused passwords to cope with too many credentials
- IT departments face enormous overhead managing individual accounts across tools
- When an employee leaves, deprovisioning dozens of accounts is time-consuming and error-prone
- Every separate login is an additional attack surface for credential theft
How SSO Works: Step by Step
SSO relies on a trusted third party called an Identity Provider (IdP) — such as Okta, Azure Active Directory, or Google Workspace — to authenticate users centrally.
- User requests access to an application (the Service Provider, or SP).
- The SP redirects the user to the Identity Provider for authentication.
- The user logs in once at the IdP using their credentials (password, MFA, biometric, etc.).
- The IdP generates a security token (often a SAML assertion or OAuth/OIDC token) confirming the user's identity.
- This token is sent back to the SP, which validates it and grants access — no second password required.
- Subsequent access to other connected applications uses the same session token automatically.
Key Protocols Behind SSO
| Protocol | Use Case | How It Works |
|---|---|---|
| SAML 2.0 | Enterprise web apps | XML-based assertions passed between IdP and SP |
| OAuth 2.0 | Third-party API authorization | Token-based delegated access |
| OpenID Connect (OIDC) | Consumer and enterprise login | Identity layer built on top of OAuth 2.0 |
Benefits of SSO
- Improved security: Fewer passwords means fewer opportunities for credential-based attacks. Centralizing authentication also makes it easier to enforce MFA across all apps.
- Better user experience: Employees spend less time logging in and resetting forgotten passwords.
- Simplified IT management: Onboarding and offboarding users requires changes in one place rather than dozens of systems.
- Centralized audit logs: All authentication events flow through the IdP, giving security teams a single pane of glass for monitoring.
Risks and Considerations
SSO concentrates trust into a single identity provider. This introduces important considerations:
- Single point of failure: If the IdP goes down, users lose access to everything. High availability and redundancy are critical.
- Credential compromise impact: A breached SSO account potentially exposes all connected systems. This makes strong MFA on the IdP non-negotiable.
- Vendor lock-in: Choosing a proprietary IdP can make migration painful. Open standards (SAML, OIDC) reduce this risk.
Is SSO Right for Your Organization?
SSO is a strong fit for any organization with more than a handful of applications and a need to manage user identities consistently. The larger your application portfolio and workforce, the greater the return on an SSO investment. For smaller setups, a well-managed password manager combined with MFA may be sufficient as a starting point.