SP-initiated vs. IdP-initiated SSO: key differences explained

A user can initiate SSO from either the service provider or the identity provider’s end. But which is better for your SaaS? 

In this article, we will:

• Compare SP-initiated vs. IdP-initiated SSO.
• Explain how each method works.
• Highlight how WorkOS simplifies the SSO integration process.

Let’s start by breaking down what SP-initiated and IdP-initiated SSO are.

What is SP-Initiated and IdP-Initiated SSO?

Service Provider (SP) initiated SSO starts at the SP’s end. In the context of a SaaS application, SP-initiated authentication happens when the application exposes a login button for users to gain access to the software product. In this case, the SaaS application is the SP, and the application makes the authentication request to the Identity Provider via the browser.

For example, an employee needs to access the company’s Slack workspace. They click "Login" on Slack (the SP), and Slack redirects them to Okta (the IdP). After authenticating with their company credentials in Okta, they're automatically redirected back to Slack, now logged in and ready to work.

Identity Provider (IdP) initiated SSO starts at the IdP’s end. The user logs into the IdP first (e.g., a company’s central login portal), then selects which Service Provider they want to access from a dashboard. Once chosen, the IdP sends an SSO response to the SP, granting the user access without requiring them to log in again.

For example, an employee might log into their company’s Okta dashboard (the IdP). From there, they can click on the icon for Salesforce (the SP), and Okta will log them into Salesforce without needing to re-enter credentials.

SP-initiated vs. IdP-initiated SSO: key differences

• Flow and initiation process: SP-initiated SSO begins when the user starts from the service or app, while IdP-initiated SSO starts at the Identity Provider’s login page. Both flows involve a browser redirect and communication between the SP and IdP using SAML, but the key difference is where the process starts.

• User experience: SP-initiated flows are more intuitive for consumer apps, where users are accustomed to logging in directly from the service they want to access. IdP-initiated flows offer convenience in enterprise settings, especially when users access multiple services from a central IdP portal.

• Security implications: SP-initiated SSO offers more control over the login process since it originates from the application. This allows better session handling and verification, reducing security risks like replay attacks. While IdP-initiated SSO simplifies access across multiple services, it can expose certain risks if not properly secured. For example, an insecure IdP could leave multiple services vulnerable.

How does SP-initiated SSO work?

In SP-initiated SSO, the user first attempts to log into the SaaS product by clicking a login button within the application.

Here’s a step-by-step breakdown of the process:

1. The user begins by clicking the login button in the SaaS app, initiating the authentication process.
2. The SaaS app generates a request ID and a RelayState and stores them for later use.
3. The app sends these values, along with a SAML authentication request, to the IdP via the user's browser. 
4. The user is redirected to the IdP’s login page, where they authenticate. After successfully logging in, the IdP creates a SAML response that includes the original request ID, RelayState, and details about the authentication.
5. The IdP sends back the SAML response to the SaaS app via the browser. 
6. The app checks the stored request ID and RelayState against the response. If they match, the user is authenticated.

For a more detailed explanation of SSO, check out The Developer's Guide to SSO.

SP-initiated SSO is commonly used by consumer-facing apps where individual users can log in to the application themselves to gain access. In fact, it’s the most common form of SSO that new WorkOS customers start with when they are becoming enterprise-ready because most enterprise customers require it.

How does IdP-initiated authentication work?

Here’s how IdP-initiated authentication works:

1. The employee logs into their employer's IdP.
2. Once logged in, the employee navigates to a dashboard displaying various SPs they can access and clicks on the SP they want to access.
3. The IdP generates an SSO response and a SAML 2.0 Assertion, which contains the user's authentication details, user information, and the RelayState.
4. The IdP sends this SSO response to the SP via the browser.
5. The SP then validates the SAML 2.0 Assertion and, if valid, generates a session for the user, granting access to the application.

IdP-initiated flows are common in internal enterprise environments for central access to multiple services.

When to choose SP-initiated SSO vs. IdP-initiated SSO

Choose SP-initiated SSO when:

• You want to maintain control over the login process.
• You prioritize a direct-to-app user experience, especially for consumer-facing products. See common UI/UX practices for creating your sign-in flow.
• You need a simpler implementation that aligns with existing login workflows.

Choose IdP-initiated SSO when:

• Your customers prefer a centralized login experience.
• You want to enforce your customer’s security policies by allowing their IdP to control authentication.

How WorkOS simplifies SP-initiated and IdP-initiated SSO

Integrating SSO with WorkOS allows you to offer both SP-initiated and Identity Provider-initiated authentication flows to your SaaS customers — a pivotal step in becoming an enterprise-ready business. 

• Get started fast: With SDKs in every popular language, easy-to-follow documentation, and Slack-based support, you can implement SSO in minutes rather than weeks.
• Support every protocol: With OAuth 2.0 integrations to popular providers like Google and Microsoft, compatibility with every major IdP, and full support for custom SAML SSO or OIDC connections, WorkOS can support any enterprise customer out of the box.
• Avoid the back-and-forth: WorkOS’s Admin Portal takes the pain out of onboarding your customers’ IT teams and configuring your app to work with their identity provider.
• Pricing that makes sense: Unlike competitors who price by monthly active users, WorkOS charges a flat rate for each company you onboard — whether they bring 10 or 10,000 SSO users to your app.

If you have any questions about either of these authentication flows and how to offer them to your customers with a WorkOS integration, please reach out to us at support@workos.com, and we’ll be happy to help.

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