Logout and Single Logout (SLO)

There are potentially many active application sessions during a CAS single sign-on session, and the distinction between logout and single logout is based on the number of sessions that are ended upon a logout operation. The scope of logout is determined by where the action takes place:

1. Application logout - ends a single application session
2. CAS logout - ends the CAS SSO session

Note that the logout action in each case has no effect on the other in the simple case. Ending an application session does not end the CAS session and ending the CAS session does not affect application sessions. This is a common cause of confusion for new users and deployers of an SSO system.

The single logout support in CAS attempts to reconcile the dispartity between CAS logout and application logout. When CAS is configured for SLO, it attempts to send logout messages to every application that requested authentication to CAS during the SSO session. While this is a best-effort process, in many cases it works well and provides a consistent user experience by creating symmetry between login and logout.

CAS Logout

Per the CAS Protocol, the /logout endpoint is responsible for destroying the current SSO session. Upon logout, it may also be desirable to redirect back to a service. This is controlled via specifying the redirect link via the service parameter.

The redirect behavior is turned off by default, and is activated via the following setting in cas.properties:

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# Specify whether CAS should redirect to the specified service parameter on /logout requests
# cas.logout.followServiceRedirects=false

The specified url must be registered in the service registry of CAS and enabled.

Single Logout (SLO)

CAS is designed to support single sign out: it means that it will be able to invalidate client application sessions in addition to its own SSO session.
Whenever a ticket-granting ticket is explicitly expired, the logout protocol will be initiated. Clients that do not support the logout protocol may notice extra requests in their access logs that appear not to do anything.

When a CAS session ends, it notifies each of the services that the SSO session is no longer valid, and that relying parties need to invalidate their own session.

This can happen in two ways:

1. CAS sends an HTTP POST message directly to the service ( back channel communication): this is the traditional way of performing notification to the service.
2. CAS redirects (HTTP 302) to the service with a message and a RelayState parameter (front channel communication): This feature is inspired by SAML SLO, and is needed if the client application is composed of several servers and use session affinity. The expected behaviour of the CAS client is to invalidate the application web session and redirect back to the CAS server with the RelayState parameter.

SLO Requests

The way the notification is done (back or front channel) is configured at a service level through the logoutType property. This value is set to LogoutType.BACK_CHANNEL by default. The message is delivered or the redirection is sent to the URL presented in the service parameter of the original CAS protocol ticket request.

A sample SLO message:

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<samlp:LogoutRequest
    xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
    xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
    ID="[RANDOM ID]"
    Version="2.0"
    IssueInstant="[CURRENT DATE/TIME]">
    <saml:NameID>@NOT_USED@</saml:NameID>
    <samlp:SessionIndex>[SESSION IDENTIFIER]</samlp:SessionIndex>
</samlp:LogoutRequest>

The session identifier is the CAS service ticket ID that was provided to the service when it originally authenticated to CAS. The session identifier is used to correlate a CAS session with an application session; for example, the SLO session identifier maps to a servlet session that can subsequently be destroyed to terminate the application session.

Turning Off Single Logout

To disable single logout, adjust the following setting in cas.properties file:

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# To turn off all back channel SLO requests set slo.disabled to true
# slo.callbacks.disabled=false

Single Logout Per Service

Registered applications with CAS have the option to control single logout behavior individually via the Service Managament component. Each registered service in the service registry will include configuration that describes how to the logout request should be submitted. This behavior is controlled via the logoutType property which allows to specify whether the logout request should be submitted via back/front channel or turned off for this application.

Sample configuration follows:

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{
  "@class" : "org.jasig.cas.services.RegexRegisteredService",
  "serviceId" : "testId",
  "name" : "testId",
  "id" : 1,
  "logoutType" : "BACK_CHANNEL"
}

Service Endpoint for Logout Requests

By default, logout requests are submitted to the original service id. CAS has the option to submit such requests to a specific service endpoint that is different from the original service id, and of course can be configured on a per-service level. This is useful in cases where the application that is integrated with CAS does not exactly use a CAS client that supports intercepting such requests and instead, exposes a different endpoint for its logout operations.

To configure a service specific endpoint, try the following example:

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{
  "@class" : "org.jasig.cas.services.RegexRegisteredService",
  "serviceId" : "testId",
  "name" : "testId",
  "id" : 1,
  "logoutType" : "BACK_CHANNEL",
  "logoutUrl" : "https://web.application.net/logout",
}

Aynchronous SLO Messages

By default, backchannel logout messages are sent to endpoint in an asynchronous fashion. To allow synchronous messages, modify the following setting in cas.properties:

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# To send callbacks to endpoints synchronously, set this to false
# slo.callbacks.asynchronous=true

SSO Session vs. Application Session

In order to better understand the SSO session management of CAS and how it regards application sessions, one important note is to be first and foremost considered:

CAS wants to maintain and control the SSO session in the form of the TicketGrantingTicket and a TGT id which is shared between the user-agent and the CAS server in the form of a secure cookie.

CAS is not an application session manager in that it is the responsibility of the applications to maintain and control their own application sessions. Once authentication is completed, CAS is typically out of the picture in terms of the application sessions. Therefore, the expiration policy of the application session itself is entirely independent of CAS and may be loosely coordinated and adjusted depending on the ideal user experience in the event that the application session expires.

In the event that Single Logout is not activated, typically, application may expose a logout endpoint in order to destroy the session and next, redirect the agent to the CAS logout endpoint in order to completely destroy the SSO session as well.

Here’s a brief diagram that demonstrates various application session configuration and interactions with CAS: