How Does SSO Work?
Single sign-on, or SSO, lets a user authenticate once with a trusted identity provider and then access multiple applications without signing in separately to each one. The application trusts the identity provider’s token or assertion, so authentication is centralized instead of repeated inside every app.
How SSO works is straightforward at a high level: a user signs in once with a trusted identity provider, and connected applications rely on that authentication instead of asking for separate logins. In other words, single sign-on centralizes authentication so users can access multiple apps without managing a different password for each one.
The Main Components of SSO
Most SSO deployments involve four basic parts:
User
The person trying to sign in to an application.
Identity provider
The identity provider (IdP) verifies who the user is. This is often a workforce identity platform, directory-backed login service, or cloud identity system.
Service provider
The service provider (SP), also called the relying party, is the application the user wants to access.
Token or assertion
After login, the identity provider sends a signed token or assertion to the application as proof that authentication already happened.
The Basic SSO Workflow
At a high level, SSO works like this:
- The user opens an application.
- The application sees there is no local session.
- The application redirects the user to the identity provider.
- The identity provider authenticates the user with a password, MFA, device checks, or other controls.
- The identity provider sends back a signed token or assertion.
- The application validates that token.
- The application creates its own session and grants access.
If the user then opens another connected app, that second app can redirect to the same identity provider. If the user already has an active session there, the sign-in may happen silently or with very little friction.
That is the practical benefit of single sign-on: one central authentication event can support access to many trusted applications.
What SSO Is Actually Doing
SSO does not usually mean the same password is copied into every application. In a well-designed deployment, the connected apps never need to handle the user’s primary password directly.
Instead, the application trusts the identity provider’s statement that: - the user authenticated successfully - the user has a specific identity - certain claims are true, such as email address, role, or group membership
This trust relationship is what makes federated identity work.
Common Protocols Used in SSO
Several standards are commonly used behind SSO.
SAML
SAML stands for Security Assertion Markup Language. It is widely used in enterprise environments, especially for older SaaS and business applications. It passes identity information in a signed assertion.
OpenID Connect
OpenID Connect (OIDC) is built on OAuth 2.0 and is common in modern web and mobile applications. It uses tokens and is often easier to integrate into newer app architectures.
OAuth 2.0
OAuth 2.0 is mainly an authorization framework, not an authentication standard by itself. It is often mentioned alongside SSO because OIDC builds on it and many SaaS integrations rely on related flows.
If you want to go deeper on the standards, see: - What Is the Difference Between SAML and OIDC? - What Is Federation in Identity and Access Management?
Why Organizations Use SSO
SSO is widely used because it improves both usability and central control.
Better user experience
Users sign in fewer times and manage fewer passwords. That usually means less password fatigue and fewer help desk resets.
Centralized policy enforcement
Security teams can apply controls like: - MFA requirements - sign-in risk policies - device posture checks - geo restrictions - session limits
from one identity layer instead of configuring every app separately.
Faster onboarding and offboarding
When access is tied to a central identity platform, adding or removing user access becomes more consistent. Disabling the user centrally can cut off access to many connected applications at once.
Better audit visibility
Authentication activity is easier to review when it passes through a central control point.
What SSO Does Not Secure by Itself
SSO is useful, but it is not a complete security solution.
It does not automatically give you: - least privilege - well-designed app roles - strong MFA - secure session handling - proper conditional access - safe app integrations - phishing resistance
If the identity provider account is compromised, an attacker may inherit access to multiple applications. That is why SSO increases the importance of hardening the identity layer itself.
The Main Security Tradeoff
SSO reduces password sprawl, which is good. But it also concentrates trust in one place.
That makes the identity provider a high-value target. If authentication is centralized, you should harden: - MFA enrollment - admin roles - break-glass accounts - conditional access policies - token handling - logging and alerting - app consent and federation settings
A password manager can also help users maintain strong, unique credentials for the identity provider and any non-SSO accounts that still exist. If you need one, 1Password is a practical option for managing those credentials safely.
Common Misconceptions About SSO
“SSO means users only need one password forever.”
Not exactly. SSO reduces repeated logins, but secure deployments still need MFA, session controls, and normal identity hygiene.
“SSO and MFA are the same thing.”
False. SSO centralizes authentication across apps. MFA adds extra proof during login. They are different controls that often work together.
“If an app uses SSO, the app is automatically secure.”
False. The app can still have weak authorization, bad role design, or poor session handling. SSO improves authentication, not every part of application security.
“SSO removes all passwords from the environment.”
Usually not. It reduces app-specific passwords, but some legacy systems, fallback workflows, service accounts, and recovery paths may still rely on passwords or other credentials.
Final Takeaway
The practical takeaway is simple: SSO works by having a trusted identity provider authenticate the user once and then vouch for that identity to connected applications. It improves usability and centralizes access control, but its security value depends on how well the identity layer itself is protected.
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