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The Challenge of Building SAML Single Logout

SAML single sign-on (SSO) allows the end-user to securely authenticate across multiple applications by logging in once using one set of credentials.

However, authentication is only the first half of the story.

Unless the user manually logs out of each session that they seamlessly accessed, they leave behind a trail of orphaned sessions. This is where single logout (SLO) comes in. SLO allows the end-user to terminate all sessions by initiating the logout process once.

SAML Single Logout (SLO)

End-users rarely log out of every session established during SSO. All these orphaned sessions increase your attack surface, and ideally, this is something that you should attempt to fix for the user.

SLO is a SAML flow that allows the end-user to logout from a single session and be automatically logged out of all related sessions that were established during SSO.

The Protocol

The end-user can initiate the SLO process from within the Identity Provider (IdP) or one of the Service Providers (SP). We typically see SLO initiated from an SP; however, an IdP can also trigger SLO for other reasons, for example, if an agreed global timeout has been exceeded or if the user credentials have been compromised.

SLO can use asynchronous bindings (front-channel), such as the HTTP Redirect, POST, and Artifact bindings, to send logout messages via the user agent (a browser). Alternatively, synchronous bindings (back-channel), via SOAP, can be used for direct server-to-server communication bypassing the user agent.

The Flow

When initiating SLO from a Service Provider, the following flow will take place:

The SAML SLO flow showing an SP initiating an SLO request and an IdP calling other SPs

  • Step 1: The end-user initiates the SLO process by clicking a logout button within an SP
  • Step 2: The SP terminates the user's session and triggers SP-initiated SLO by sending a logout request to the IdP
  • Step 3: Upon receiving a logout request, the IdP first identifies all other SPs that are part of the current session, and iteratively performs the following steps for each SP. This is known as IdP-initiated SLO
    1. IdP sends a logout request to the SP
    2. IdP redirects the user to the SP’s SLO endpoint
    3. IdP waits for a logout response
  • Steps 4-8: Each SP validates the logout request and terminates the user’s session before returning a logout response to the IdP
  • Steps 9-10: Once the IdP receives logout responses from all SPs, it terminates its own user session and sends a logout response to the originating SP. The logout response includes a status code which informs the originating SP whether SLO completed entirely or partially
  • Step 11: The originating SP can then redirect the user to another page, such as a “logged out” page

SAML SLO Request

A SAML logout request follows your typical SAML message structure, with an ID, lifetime data, and information about its origin and destination.

However, it also includes the name ID of the user who is being logged out. This allows the IdP or SP to confirm that they are logging out the correct user. For instance, if a logout request is received for Bob, but Alice is currently logged in, the IdP or SP would deny the request.

The logout request can optionally contain the reason for the logout, such as if it has been initiated by a user or an admin, or if a global timeout was exceeded:

<saml2p:LogoutRequest xmlns:saml2p="urn:oasis:names:tc:SAML:2.0:protocol"  
   xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion" 
   ID="_9088cb8766164b149e63358b92ece1c3" 
   Version="2.0" 
   IssueInstant="2020-05-11T20:24:11Z" 
   Destination="https://idp.identityserver.com/saml/slo" 
   NotOnOrAfter="2020-05-11T20:26:11Z" 
   Reason="urn:oasis:names:tc:SAML:2.0:logout:user"> 
  <saml2:Issuer>https://sp.identityserver.com/saml</saml2:Issuer> 
  <saml2:NameID>d65a1ecb97404a988c0b9c18cc915e3b_scott</saml2:NameID> 
</saml2p:LogoutRequest>

SAML SLO Response

A SAML Logout response also follows your typical SAML message structure, with an ID and information about the message’s origin and destination.

However, the SLO also includes the ID of the original SAML logout request message, which the Idp or SP can use to correlate responses with original requests to confirm that the response was intended for it.

The logout response also contains a status code, which indicates whether SLO failed or completed successfully or partially:

<saml2p:LogoutResponse xmlns:saml2p="urn:oasis:names:tc:SAML:2.0:protocol"  
   xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion" 
   ID="_145745962cdb411d91d80967fb082643" 
   Version="2.0" 
   IssueInstant="2020-05-11T20:26:36Z" 
   Destination="https://sp.identityserver.com/signout-saml" 
   InResponseTo="_9088cb8766164b149e63358b92ece1c3"> 
  <saml2:Issuer>https://idp.identityserver.com</saml2:Issuer> 
  <saml2p:Status> 
    <saml2p:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success" /> 
  </saml2p:Status> 
</saml2p:LogoutResponse> 

Rock Solid Knowledge Implementation

Our SAML component supports both SP-initiated and IdP-initiated SLO, with HTTP Redirect and POST bindings. We do not currently support any SOAP bindings. The SAML specification recommends using front-channel bindings for the initial request as it provides the best chance of successfully propagating the logout request to all involved SPs.

A significant problem with using front-channel redirection to propagate logout requests is that complete control is forwarded to all the target Service Providers iteratively. This means that the entire SLO process will fall over even if a single SP in the chain is unresponsive. The remaining SPs will never know that a failed logout attempt was made, resulting in various orphaned sessions, and the user will be displayed a generic HTTP error with no meaningful details from either the IdP or the SP.

We think this redirect chain approach is a bit old fashioned and not very user friendly. As a result, our implementation solves this problem by using iframes instead of HTTP redirection. Using iframes to make logout requests means that the IdP can send logout requests to all SPs in parallel, preventing the entire SLO chain from falling over due to one unresponsive SP.

You can find out how to enable SP-Initiated SLO and IdP-Initiated SLO in our SAML documentation.

Caveats

In a perfect scenario, SLO can improve the user experience and security by removing the need for end-users to manually log out of all SSO sessions. However, SLO has some significant restrictions and drawbacks, which make it fragile and challenging to implement.

Firstly, for SLO to work, both the IdP and all SPs must be configured to support SLO; otherwise, various sessions may not be terminated. If only partial logout was possible, the user might assume that the entire SLO process was successful and leave unattended sessions to be exploited.

SLO is one of the more opaque parts of the SAML protocol, with only a status code to work from for error messages. If you do not own all Service Providers, then this may be an issue when supporting users.

Lastly, front-channel SLO requires setting session cookies in browsers, which cannot be triggered from an admin portal. Furthermore, front-channel logout may no longer be a feasible solution in the future due to the upcoming browser changes to phase out third-party cookies. As such, back-channel logout is most likely to be the only solution moving forward; however, it requires a lot of orchestration and additional development effort to implement SOAP and SP-level session blacklists, where the SP must maintain a record of all logged out session IDs.

Conclusion

SAML SLO can improve the user experience and security by removing the need for the end-user to manually log out of all sessions established during SSO. However, SLO is by no means a perfect solution, as it has specification restrictions and technical difficulties, which make it fragile and challenging to implement. Therefore, SLO only mitigates issues of orphaned SSO sessions but does allow the user to log out with (more) confidence.

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