Key Points
- IdP-initiated SSO starts at the identity provider and sends an unsolicited SAML response to the service provider.
- SP-initiated SSO starts at the service provider and uses a SAML AuthnRequest to validate the response.
- IdP-initiated SSO carries greater security risks, including assertion injection, replay and CSRF.
- SP-initiated SSO is preferred for security-sensitive apps, while IdP-initiated SSO fits centralized internal portals.
Enterprise IT teams often notice a gap between the SSO experience employees expect and the security posture security teams demand.
Employees want to launch apps from a central dashboard with one login. Security teams want every authentication event to be traceable to a specific, verified request. Those two goals can pull organizations toward different single sign-on (SSO) approaches.
Choosing between identity provider (IdP)-initiated and service provider (SP)-initiated SSO affects both the user experience and the security of the login process.
This article explains how each SSO flow works, the security tradeoffs between them and when each approach makes the most sense.
What Is IdP-Initiated Login?
IdP-initiated login is a Security Assertion Markup Language (SAML) single sign-on flow in which the identity provider (IdP) starts the authentication sequence.
The user logs into the IdP (Okta, Microsoft Entra ID, Google Workspace or another provider), selects a target application from a dashboard, and the IdP sends a SAML assertion to the service provider (SP) without first receiving an authentication request from the SP.
Because no authentication request originated from the SP, the SAML response the SP receives is called an unsolicited response.
Info: The OASIS SAML 2.0 Profiles specification addresses this scenario directly in section 4.1.5, specifying that a service provider must ensure any unsolicited SAML response does not contain an InResponseTo value.
In practice, IdP-initiated login looks like this from a user’s perspective: one login to a company portal, one dashboard of available apps and a single click to enter any of them without a secondary prompt.
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What Is SP-Initiated SSO?
SP-initiated SSO begins on the service provider (SP) side. A user navigates directly to a protected resource, such as a SaaS application, an internal web app or a VPN portal, and the SP detects that no valid session exists.
The SP constructs a SAML AuthnRequest, assigns it a unique request ID and redirects the user’s browser to the IdP’s single sign-on endpoint.
After the user authenticates at the IdP, the IdP returns a SAML response that carries the original request ID in its InResponseTo attribute. The SP compares that value against the ID it stored when generating the AuthnRequest.
If the values match, the SP has cryptographic confirmation that the response corresponds to a specific, genuine request it initiated.
SAML 2.0 was approved as an OASIS Standard on 1 March 2005, and the specification set was published on 15 March 2005. The SP-initiated Web Browser single sign-on profile is the primary use case the specification was designed around.
IdP-Initiated vs. SP-Initiated SSO: SAML Message Flows
Understanding the message-level difference makes the security distinction clear.
SP-Initiated Flow (Step by Step)
In an SP-initiated flow, the authentication process begins when the user requests access to a protected application. The flow typically follows these steps:
- User requests a protected resource. The browser navigates to the SP application.
- SP generates an AuthnRequest. The SP creates a SAML AuthnRequest containing a unique request ID and an optional RelayState value (a URL to redirect back to after authentication), and stores both.
- SP redirects the user to the IdP. The AuthnRequest is sent to the IdP’s SSO endpoint using either HTTP redirect or HTTP POST binding.
- User authenticates at the IdP. The IdP validates the user’s credentials or an existing session.
- IdP issues a SAML response. The response includes the user’s assertion, the original InResponseTo value (the request ID from step 2), and the RelayState.
- SP validates the response. The SP confirms that InResponseTo matches the stored request ID, verifies the assertion signature, checks audience restrictions and establishes a session.
IdP-Initiated Flow (Step by Step)
In an IdP-initiated flow, the process starts at the identity provider rather than the application. The flow typically follows these steps:
- User logs into the IdP portal. Authentication happens entirely at the IdP.
- User selects a service provider. The user clicks an app on the IdP dashboard.
- IdP generates a SAML response. The IdP creates an assertion containing the user’s identity and, optionally, a RelayState value.
- IdP sends the response to the SP. The browser POSTs the SAML response to the SP’s assertion consumer service (ACS) endpoint.
- SP validates the assertion. Without a stored request ID, the SP can still verify the assertion signature, check audience restrictions, confirm the Recipient matches its own assertion consumer service URL, validate the time window and reject assertion IDs it has already seen — but it has no prior request to bind the response to.
The absence of a request ID in step 5 of the IdP-initiated flow is the root of its security gap.
IdP-Initiated vs. SP-Initiated SSO: Feature Comparison
While both SSO flows authenticate users, they differ in how they start the login process and the security protections they provide.
The table below compares their key differences.
| Factor | SP-Initiated SSO | IdP-Initiated SSO |
| Where Flow Starts | Service provider | Identity provider |
| SAML AuthnRequest Sent | Yes | No |
| InResponseTo Validation | Yes (SP matches request ID) | No (unsolicited response) |
| Replay Protection | Strong (request ID is single use) | Weak (no request anchor) |
| CSRF Protection | Yes (pre-login state stored at SP) | No |
| Assertion Injection Risk | Low | Higher |
| User Experience | User navigates to app directly | User launches app from IdP dashboard |
| Typical Use Case | External, customer-facing, security-sensitive apps | Internal enterprise portals, intranet apps |
| OASIS SAML 2.0 Support | Primary profile (Web Browser SSO) | Permitted via unsolicited response rules |
Security Tradeoffs: Why IdP-Initiated Login Carries More Risk
The biggest security difference is the absence of a request anchor on the SP side.
In SP-initiated SSO, the SP generates a request ID, stores it in a pre-login session, and later confirms that the SAML response references exactly that ID. This binding means a stolen or forged SAML assertion cannot be silently replayed at the SP: the assertion would carry either the wrong InResponseTo value or no value at all.
IdP-initiated login removes that binding entirely. Three specific attack classes become more feasible:
- Assertion injection. An attacker who intercepts a SAML assertion during transit (or obtains one by other means) can POST it to the SP’s ACS endpoint. Because the SP has no stored request ID to check, it has no protocol-level mechanism to distinguish the injected assertion from a legitimate one.
- Replay attacks. Without a request ID, SAML responses in IdP-initiated flows lose the strongest replay protection mechanism available. An attacker who captures a valid response can attempt to present it again before the assertion expires.
- Cross-site request forgery (CSRF). Because the SP has no opportunity to create a pre-login session before receiving the assertion, it cannot protect against CSRF on the authentication flow itself. A crafted request that delivers a SAML assertion to the SP’s ACS endpoint may succeed if the SP does not apply additional validation.
Note: OWASP describes IdP-initiated SSO as “inherently less secure by design” because of the missing login CSRF protection that the request-response binding provides.
Mitigations for IdP-Initiated Flows
If IdP-initiated SSO is a business requirement, the following controls reduce (but do not eliminate) the risk:
- Short assertion validity windows: Limit SAML assertion lifetime to seconds or minutes. Longer windows give attackers more time to replay intercepted assertions.
- Replay detection: Track assertion IDs and session indexes and reject any assertion that has already been used. This closes the replay window, though it does not protect against a first-time injection of a stolen assertion.
- RelayState allowlisting: Validate the RelayState parameter against a known-good URL allowlist to prevent open redirect attacks.
- Reject unsolicited InResponseTo values: The SAML Profiles specification requires that a genuinely unsolicited response must not carry an InResponseTo An assertion that arrives without a prior request but carries an InResponseTo value is a signal of assertion injection and should be rejected.
None of these controls restore the security level of SP-initiated SSO because they do not restore the request-binding mechanism. They reduce risk but are not equivalent to SP-initiated SSO.
When to Use Each SSO Flow
Choosing between flows is primarily a question of who initiates access and what the trust requirements are for the application.
When SP-Initiated SSO Is the Right Choice
SP-initiated SSO is best suited for scenarios where users access an application directly, the application is externally facing, or the security sensitivity of the resource warrants the stronger validation model:
- Customer-facing SaaS applications where users navigate directly to the app URL.
- Externally accessible tools (VPNs, partner portals, developer consoles) where assertion integrity is non-negotiable.
- Environments with compliance requirements that demand verifiable authentication chains.
- New enterprise integrations. Most enterprise customers set up SP-initiated SSO as the default because it is the most widely supported and secure approach.
When IdP-Initiated Login Has a Role
IdP-initiated login is appropriate when access is centrally managed and internal-facing:
- Internal employee portals where the company controls both the IdP and the applications, and all users operate from within the corporate network or a zero-trust access layer.
- Environments where IT requires a gated, unified launch point for all business applications, and user experience is a priority over external-access security.
- Legacy application integrations where the SP does not support generating AuthnRequests and the IdP-initiated path is the only SAML option available.
Most mature enterprise identity deployments support both flows: SP-initiated for external access, IdP-initiated with mitigations applied for internal portals.
How Certificate-Based Authentication Strengthens SSO
Whether an organization leans on IdP-initiated or SP-initiated SSO, the strength of the underlying authentication at the IdP determines how much the rest of the chain can be trusted.
Password-based authentication at the IdP leaves the entire SSO session exposed if credentials are phished or reused across systems. Certificate-based authentication closes that gap.
When users and devices authenticate to the IdP with digital certificates issued by a managed public key infrastructure (PKI), the authentication event itself becomes phishing-resistant.
For a practical introduction to managed PKI, watch the video below. It explains how managed PKI differs from traditional PKI and why organizations are adopting PKI as a service.
The certificate proves the device’s identity cryptographically, and the IdP passes that assurance forward through the SAML assertion. OAuth, OpenID Connect (OIDC) and SAML each play a different role in enterprise identity and authentication.
Note: Organizations that have deployed certificate-based authentication for Azure AD find that combining certificate authentication with SP-initiated SSO produces the most defensible authentication chain: the identity assertion starts with a hardware-bound certificate, passes through a request-anchored SAML flow and lands at the service provider with both credential and flow integrity intact.
Top identity and access management (IAM) platforms integrate with managed PKI to issue and renew certificates automatically, removing the operational overhead that historically made certificate-based authentication difficult to scale.
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Frequently Asked Questions
What is the difference between IdP-initiated and SP-initiated SSO?
In IdP-initiated login, the user authenticates at the identity provider first and is then redirected to the service provider with a SAML assertion that was not requested by the SP.
In SP-initiated SSO, the user attempts to access the service provider directly, and the SP sends a SAML AuthnRequest to the IdP before receiving the assertion. The SP-initiated flow includes a request ID that the SP validates against the response, providing a security anchor that the IdP-initiated flow lacks.
Why is IdP-initiated SSO considered less secure?
IdP-initiated SSO sends an unsolicited SAML response to the service provider. Because the SP never generated a SAML AuthnRequest, it has no stored request ID to validate against the response. This removes the protocol-level protection against assertion injection and replay attacks.
OWASP identifies the missing login CSRF protection as the root design limitation of IdP-initiated flows.
What is an unsolicited SAML response?
An unsolicited SAML response is a SAML authentication response received by a service provider when that SP did not issue a corresponding SAML AuthnRequest. This is the formal description of the IdP-initiated SSO scenario.
The OASIS SAML 2.0 Profiles specification addresses unsolicited responses in section 4.1.5 and requires that they must not contain an InResponseTo attribute.
What is the InResponseTo attribute in SAML?
InResponseTo is a SAML attribute in the authentication response that carries the unique ID of the AuthnRequest the response corresponds to.
In SP-initiated SSO, the SP stores the request ID when generating the AuthnRequest and then verifies the response’s InResponseTo value against it. This binding confirms that the response corresponds to a genuine, SP-initiated request and was not replayed or injected.
Can an organization support both IdP-initiated and SP-initiated SSO?
Yes. Most enterprise identity deployments support both flows.
SP-initiated SSO is typically configured for external-facing applications and any resource with higher security requirements. IdP-initiated login is used for internal employee portals where centralized app launching is a priority.
When supporting both, the SP must apply the mitigations for IdP-initiated flows: short assertion validity windows, replay detection and RelayState allowlisting.
What is SAML and how does it relate to SSO?
Security Assertion Markup Language (SAML) is an XML-based open standard for exchanging authentication and authorization data between an identity provider and a service provider.
OASIS approved SAML 2.0 as an OASIS Standard on 1 March 2005; the specification set is dated 15 March 2005. Single sign-on in enterprise environments commonly uses SAML as the underlying protocol because it allows the IdP to pass authentication assertions to multiple service providers without requiring the user to re-enter credentials at each one.
