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Difficulty: HardIdentity and Access Management Operations

A security engineer is configuring a Service Provider-initiated SAML 2.0 Single Sign-On (SSO) integration between an enterprise SaaS application (Service Provider) and an external cloud Identity Provider (IdP) with mandatory MFA. In what order should the steps occur to complete a secure authentication and access flow?

  1. 1The user attempts to access a protected resource on the Service Provider (SP), causing the SP to create a SAML AuthnRequest.
  2. 2The client browser is redirected to the Identity Provider (IdP) single sign-on URL containing the encoded AuthnRequest.
  3. 3The IdP authenticates the user's primary credentials and prompts for a secondary Multi-Factor Authentication (MFA) factor.
  4. 4The IdP issues a digitally signed SAML Response assertion and redirects it to the SP Assertion Consumer Service (ACS) endpoint.
  5. 5The SP validates the cryptographic signature and token validity period, then establishes a local user session to grant access.

Answer

The correct operational sequence begins with the user requesting access at the Service Provider, followed by the SP generating a SAML AuthnRequest and redirecting the browser to the Identity Provider. Next, the IdP authenticates the user credentials and MFA challenge. After authentication succeeds, the IdP sends a signed SAML response to the SP Assertion Consumer Service endpoint. Finally, the SP validates the signature and token claims before creating the local session.
In a Service Provider (SP)-initiated SAML 2.0 Single Sign-On sequence, the workflow must start at the resource endpoint (SP), redirect to the identity repository (IdP) for credential and MFA verification, return a signed SAML assertion token to the Assertion Consumer Service (ACS) endpoint, and end with cryptographic validation and local session establishment by the SP.

Step-by-Step Solution

1
Identify the initiation point for Service Provider (SP)-initiated SSO
The unauthenticated client request to the application triggers the creation of a SAML AuthnRequest payload.
In SP-initiated flows, access attempts at the target application must precede redirection.
2
Trace the HTTP redirect sequence
The user's browser transfers the AuthnRequest parameter to the IdP endpoint.
The browser acts as the user-agent bridge carrying authentication requests between decoupled domains.
3
Perform centralized identity verification and policy enforcement
The IdP processes primary credentials and enforces secondary MFA challenges.
Authentication and policy evaluation take place entirely on the trusted Identity Provider.
4
Return the signed security token via browser redirect/POST
The IdP constructs a SAML assertion signed with its private key and sends it to the SP Assertion Consumer Service (ACS).
The ACS endpoint is designed to receive and parse federated identity assertions.
5
Validate cryptographic trust and grant local application authorization
The SP checks the IdP's public certificate signature, expiration timestamps, and recipient claims before granting access.
Verification prevents SAML response spoofing and token replay attacks.

Key Concept

SAML 2.0 SP-Initiated SSO Authentication Flow
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Topics

Identity and Access Management Operations

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