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.
Protocol SupportNote that SLO described here specifically deals with the semantics of the CAS protocol. All other available protocols in CAS may offer and behave differently when it comes to handling, receiving and publishing logout requests whether CAS is acting as an identity provider or service provider. SLO support for each protocol implementation may vary and you should always verify the extent of available functionality for each protocol implementation.
The scope of logout is determined by where the action takes place:
- Application logout - ends a single application session
- 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 disparity 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 specified service must be registered in the service registry of CAS and enabled and
CAS must be allowed to follow service redirects.
The following settings and properties are available from the CAS configuration catalog:
cas.logout.confirm-logout=false
Before logout, allow the option to confirm on the web interface.
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cas.logout.follow-service-redirects=false
Whether CAS should be allowed to redirect to an alternative location after logout.
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cas.logout.redirect-parameter=
The target destination to which CAS should redirect after logout is indicated and extracted by a parameter name of your choosing here. If none specified, the default will be used as
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cas.logout.redirect-url=
A url to which CAS must immediately redirect after all logout operations have completed. Typically useful in scenarios where CAS is acting as a proxy and needs to redirect to an external identity provider's logout endpoint in order to remove a session, etc.
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Configuration Metadata
The collection of configuration properties listed in this section are automatically generated from the CAS source and components that contain the actual field definitions, types, descriptions, modules, etc. This metadata may not always be 100% accurate, or could be lacking details and sufficient explanations.
Be Selective
This section is meant as a guide only. Do NOT copy/paste the entire collection of settings into your CAS configuration; rather pick only the properties that you need. Do NOT enable settings unless you are certain of their purpose and do NOT copy settings into your configuration only to keep them as reference. All these ideas lead to upgrade headaches, maintenance nightmares and premature aging.
YAGNI
Note that for nearly ALL use cases, declaring and configuring properties listed here is sufficient. You should NOT have to explicitly massage a CAS XML/Java/etc configuration file to design an authentication handler, create attribute release policies, etc. CAS at runtime will auto-configure all required changes for you. If you are unsure about the meaning of a given CAS setting, do NOT turn it on without hesitation. Review the codebase or better yet, ask questions to clarify the intended behavior.
Naming Convention
Property names can be specified in very relaxed terms. For instance cas.someProperty, cas.some-property, cas.some_property are all valid names. While all
forms are accepted by CAS, there are certain components (in CAS and other frameworks used) whose activation at runtime is conditional on a property value, where
this property is required to have been specified in CAS configuration using kebab case. This is both true for properties that are owned by CAS as well as those
that might be presented to the system via an external library or framework such as Spring Boot, etc.
Note
When possible, properties should be stored in lower-case kebab format, such as cas.property-name=value.
The only possible exception to this rule is when naming actuator endpoints; The name of the
actuator endpoints (i.e. ssoSessions) MUST remain in camelCase mode.
Settings and properties that are controlled by the CAS platform directly always begin with the prefix cas. All other settings are controlled and provided
to CAS via other underlying frameworks and may have their own schemas and syntax. BE CAREFUL with
the distinction. Unrecognized properties are rejected by CAS and/or frameworks upon which CAS depends. This means if you somehow misspell a property definition
or fail to adhere to the dot-notation syntax and such, your setting is entirely refused by CAS and likely the feature it controls will never be activated in the
way you intend.
Validation
Configuration properties are automatically validated on CAS startup to report issues with configuration binding, specially if defined CAS settings cannot be recognized or validated by the configuration schema. Additional validation processes are also handled via Configuration Metadata and property migrations applied automatically on startup by Spring Boot and family.
Indexed Settings
CAS settings able to accept multiple values are typically documented with an index, such as cas.some.setting[0]=value. The index [0] is meant to be
incremented by the adopter to allow for distinct multiple configuration blocks.
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.
Usage Warning!Single Logout is turned on by default.
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. Remember that the callback submitted to each CAS-protected application is a notification; nothing more. It is the responsibility of the application to intercept that notification and properly destroy the user authentication session, either manually, via a specific endpoint or more commonly via a CAS client library that supports SLO.
Also note that since SLO is a global event, all applications that have an authentication record with CAS will by default be contacted, and this may disrupt user experience negatively if those applications are individually distinct from each other. As an example, if user has logged into a portal application and an email application, logging out of one through SLO will also destroy the user session in the other which could mean data loss if the application is not carefully managing its session and user activity.
The following settings and properties are available from the CAS configuration catalog:
cas.slo.asynchronous=true
Whether SLO callbacks should be done in an asynchronous manner via the HTTP client. When true, CAS will not wait for the operation to fully complete and will resume control to carry on.
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cas.slo.disabled=false
Whether SLO should be entirely disabled globally for the CAS deployment.
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cas.slo.logout-propagation-type=AJAX
Logout propagation type determines how SLO requests will be sent to applications. This is specially applicable when SLO requests are processed using a front-channel mechanism. Available values are as follows:
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Configuration Metadata
The collection of configuration properties listed in this section are automatically generated from the CAS source and components that contain the actual field definitions, types, descriptions, modules, etc. This metadata may not always be 100% accurate, or could be lacking details and sufficient explanations.
Be Selective
This section is meant as a guide only. Do NOT copy/paste the entire collection of settings into your CAS configuration; rather pick only the properties that you need. Do NOT enable settings unless you are certain of their purpose and do NOT copy settings into your configuration only to keep them as reference. All these ideas lead to upgrade headaches, maintenance nightmares and premature aging.
YAGNI
Note that for nearly ALL use cases, declaring and configuring properties listed here is sufficient. You should NOT have to explicitly massage a CAS XML/Java/etc configuration file to design an authentication handler, create attribute release policies, etc. CAS at runtime will auto-configure all required changes for you. If you are unsure about the meaning of a given CAS setting, do NOT turn it on without hesitation. Review the codebase or better yet, ask questions to clarify the intended behavior.
Naming Convention
Property names can be specified in very relaxed terms. For instance cas.someProperty, cas.some-property, cas.some_property are all valid names. While all
forms are accepted by CAS, there are certain components (in CAS and other frameworks used) whose activation at runtime is conditional on a property value, where
this property is required to have been specified in CAS configuration using kebab case. This is both true for properties that are owned by CAS as well as those
that might be presented to the system via an external library or framework such as Spring Boot, etc.
Note
When possible, properties should be stored in lower-case kebab format, such as cas.property-name=value.
The only possible exception to this rule is when naming actuator endpoints; The name of the
actuator endpoints (i.e. ssoSessions) MUST remain in camelCase mode.
Settings and properties that are controlled by the CAS platform directly always begin with the prefix cas. All other settings are controlled and provided
to CAS via other underlying frameworks and may have their own schemas and syntax. BE CAREFUL with
the distinction. Unrecognized properties are rejected by CAS and/or frameworks upon which CAS depends. This means if you somehow misspell a property definition
or fail to adhere to the dot-notation syntax and such, your setting is entirely refused by CAS and likely the feature it controls will never be activated in the
way you intend.
Validation
Configuration properties are automatically validated on CAS startup to report issues with configuration binding, specially if defined CAS settings cannot be recognized or validated by the configuration schema. Additional validation processes are also handled via Configuration Metadata and property migrations applied automatically on startup by Spring Boot and family.
Indexed Settings
CAS settings able to accept multiple values are typically documented with an index, such as cas.some.setting[0]=value. The index [0] is meant to be
incremented by the adopter to allow for distinct multiple configuration blocks.
Back Channel
CAS sends an HTTP POST message directly to the service. This is the traditional way of performing notification to the service.
A sample back channel 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>[PRINCIPAL IDENTIFIER]</saml:NameID>
<samlp:SessionIndex>[SESSION IDENTIFIER]</samlp:SessionIndex>
</samlp:LogoutRequest>
Front Channel
CAS issues asynchronous AJAX GET logout requests via JSONP to authenticated services.
The expected behaviour of the CAS client is to invalidate the application web session.
Usage WarningFront channel logout may not be available for all CAS clients. Ensure your CAS client does support this behavior before trying it out.
A sample front channel SLO request submitted by CAS resembles the following format:
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curl 'https://service.url?callback=jQuery112409319555380089843_1509437967792&logoutRequest=[BASE64 encoded request]&_=1509437967793'
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.
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.
Redirecting Logout to Service
Logout requests may be optionally routed to an external URL bypassing the CAS logout screen. In order to to do you will need to specify the target destination typically in form of a service parameter to the CAS logout endpoint per the CAS protocol specification.
Single Logout Per Service
Registered applications with CAS have the option to control single logout behavior individually via
the Service Management 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.apereo.cas.services.CasRegisteredService",
"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 collected at the time of authentication. 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.apereo.cas.services.CasRegisteredService",
"serviceId" : "testId",
"name" : "testId",
"id" : 1,
"logoutType" : "BACK_CHANNEL",
"logoutUrl" : "https://web.application.net/logout"
}
Asynchronous SLO Messages
By default, backchannel logout messages are sent to endpoint in an asynchronous fashion. This behavior can be modified via CAS settings.
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 is NOT a session manager!
Application session is the responsibility of the application.
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:
