Understanding the bindings is one thing, but the sequence of events depends on where the user starts. There are two primary patterns for SAML SSO: SP-Initiated and IdP-Initiated.
In this flow, the user attempts to access a resource on the Service Provider (e.g., going to `aws.amazon.com`). The SP sees the user is not logged in and sends them to the IdP with a `SAMLRequest`.
This is the most common flow because it allows the SP to specify exactly what it needs from the IdP using the `SAMLRequest`.
The key advantage here is that the SP controls the initiation and can provide context (like the specific resource the user was trying to access) to the IdP.
In IdP-Initiated SSO, the user starts at a portal (e.g., an Okta dashboard) and clicks an icon. The IdP sends the user to the SP with a `SAMLResponse` *without* a prior request.
# Conceptual IdP-Initiated Trigger
# 1. User clicks 'Salesforce' on Okta Dashboard
# 2. Okta generates SAMLResponse immediately
# 3. Browser POSTs SAMLResponse to Salesforce ACS URL
# 4. User is logged in instantly| Feature | SP-Initiated | IdP-Initiated |
|---|---|---|
| Starting Point | Application (SP) | Portal (IdP) |
| SAMLRequest | Yes | No |
| Use Case | Direct URL access | Corporate Dashboards |
| Security Risk | Standard | High (Prone to Replay/CSRF) |
IdP-initiated SSO is inherently more dangerous because the SP receives a 'Login' command without having asked for one. This can be exploited for 'Login CSRF,' where an attacker forces a victim to log into an account controlled by the attacker.
Because there is no `SAMLRequest`, the SP cannot verify if the response is actually responding to a valid session, making it a prime target for replay attacks.
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