Security Guide
CAS is security software that provides secure Web-based single sign-on to Web-based applications. Single sign-on provides a win/win in terms of security and convenience: it reduces password exposure to a single, trusted credential broker while transparently providing access to multiple services without repetitious logins. The use of CAS generally improves the security environment, but there are several CAS configuration, policy, and deployment concerns that should be considered to achieve suitable security.
Reporting IssuesThe security team asks that you please DO NOT create publicly-viewable issues or posts to discuss what you may consider a security vulnerability. To report issues properly and learn about how responses are produced, please see this guide.
System Security Considerations
Infrastructure security matters to consider may include the following.
Secure Transport (https)
All communication with the CAS server MUST occur over a secure channel (i.e. TLSv1). There are two primary justifications for this requirement:
- The authentication process requires transmission of security credentials.
- The CAS ticket-granting ticket is a bearer token.
Since the disclosure of either data would allow impersonation attacks, it’s vitally important to secure the communication channel between CAS clients and the CAS server.
Practically, it means that all CAS urls must use HTTPS, but it also means that all connections from the CAS server to the application must be done using HTTPS:
- when the generated service ticket is sent back to the application on the “service” url
- when a proxy callback url is called.
The following settings and properties are available from the CAS configuration catalog:
cas.http-client.allow-local-urls=false
Whether CAS should accept local URLs. For example
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cas.http-client.async-timeout=PT5S
Indicates timeout for async operations. This settings supports the
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cas.http-client.authority-validation-reg-ex-case-sensitive=true
Whether the regular expression specified with
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cas.http-client.authority-validation-regex=
If specified the regular expression will be used to validate the url's authority. This settings supports regular expression patterns. [?].
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cas.http-client.connection-timeout=PT5S
Connection timeout for all operations that reach out to URL endpoints. This settings supports the
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cas.http-client.default-headers=
The default headers to use for any HTTP connection. This is defined as map, where the key is the header name and the value is the header value that should be sent along with request.
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cas.http-client.host-name-verifier=default
Enable hostname verification when attempting to contact URL endpoints. May also be set to
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cas.http-client.proxy-host=
Send requests via a proxy; define the hostname.
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cas.http-client.proxy-port=0
Send requests via a proxy; define the proxy port. Negative/zero values should deactivate the proxy configuration for the http client.
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cas.http-client.response-timeout=PT5S
Determines the timeout until arrival of a response from the opposite endpoint. A timeout value of zero is interpreted as an infinite timeout. Please note that response timeout may be unsupported by HTTP transports with message multiplexing. This settings supports the
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cas.http-client.socket-timeout=PT5S
Determines the default socket timeout value for I/O operations. This settings supports the
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cas.http-client.truststore.file=
The CAS local truststore resource to contain certificates to the CAS deployment. In the event that local certificates are to be imported into the CAS running environment, a local truststore is provided by CAS to improve portability of configuration across environments.
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cas.http-client.truststore.psw=changeit
The truststore password.
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cas.http-client.truststore.type=
Truststore type used to create a SSL context for http client.
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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.
Connections to Dependent Systems
CAS commonly requires connections to other systems such as LDAP directories, databases, and caching services. We generally recommend to use secure transport (SSL/TLS, IPSec) to those systems where possible, but there may be compensating controls that make secure transport unnecessary. Private networks and corporate networks with strict access controls are common exceptions, but secure transport is recommended nonetheless. Client certification validation can be another good solution for LDAP to bring sufficient security.
As stated previously, connections to other systems must be secured. But if the CAS server is deployed on several nodes, the same applies to the CAS server itself. If a cache-based ticket registry runs without any security issue on a single CAS server, synchronization can become a security problem when using multiple nodes if the network is not protected.
Deployment-Driven Security Features
CAS supports a number of features that can be leveraged to implement various security policies. The following features are provided through CAS configuration and CAS client integration. Note that many features are available out of the box, while others require explicit setup
Forced Authentication
Many CAS clients and supported protocols support the concept of forced authentication whereby a user must re-authenticate to access a particular service. The CAS protocols support forced authentication via the renew parameter. Forced authentication provides additional assurance in the identity of the principal of an SSO session since the user must verify his or her credentials prior to access. Forced authentication is suitable for services where higher security is desired or mandated. Typically forced authentication is configured on a per-service basis, but the service management facility provides some support for implementing forced authentication as a matter of centralized security policy. Forced authentication may be combined with multi-factor authentication features to implement arbitrary service-specific access control policy.
Passive Authentication
Some CAS protocols support passive authentication where access to a CAS-protected service is granted anonymously when requested. The CASv2 and CASv3 protocols support this capability via the gateway feature. Passive authentication complements forced authentication; where forced authentication requires authentication to access a service, passive authentication permits service access, albeit anonymously, without authentication.
Proxy Authentication
Proxy authentication, or delegated authentication, provides a powerful, important, and potentially security-improving feature of CAS. Proxy authentication is supported by the CASv2 and CASv3 protocols and is mediated by proxy tickets that are requested by a service on behalf of a user; thus the service proxies authentication for the user. Proxy authentication is commonly used in cases where a service cannot interact directly with the user and as an alternative to replaying end-user credentials to a service.
However, proxy tickets carry risk in that services accepting proxy tickets are responsible for validating the proxy chain (the list of services through which the end-user’s authentication have been delegated to arrive at the ticket validating service). Services can opt out of accepting proxy tickets entirely (and avoid responsibility for validating proxy chains) by validating tickets against the /serviceValidate validation endpoint, but experience has shown it’s easy to be confused about this and configure to unintentionally use the /proxyValidate endpoint yet not scrutinize any proxy chains that appear in the ticket validation response. Thus proxy authentication requires careful configuration for proper security controls; it is recommended to disable proxy authentication components at the CAS server if proxy authentication is not needed.
Historically any service could obtain a proxy-granting ticket and from it a proxy ticket to access any other service. In other words, the security model is decentralized rather than centralized. The service management facility affords some centralized control of proxy authentication by exposing a proxy authentication flag that can enabled or disabled on a per-service basis. By default registered services are not granted proxy authentication capability.
Credential Caching and Replay
The ClearPass extension provides a mechanism to capture primary authentication credentials, cache them (encrypted), and replay on demand as needed to access legacy services. While proxy authentication is recommended in lieu of password replay, it may be required to integrate legacy services with CAS. See the ClearPass documentation for detailed information.
Service Management
The service management facility provides a number of service-specific configuration controls that affect security policy and provide some support for centralized security policy. (Note that CAS has historically supported the decentralized security policy model.) Some highlights of service management controls:
- Authorized services
- Forced authentication
- Attribute release
- Proxy authentication control
- Theme control
- Service authorization control
- Multi-factor service access policy
The service management facility is comprised of a service registry containing one or more registered services, each of which specifies the management controls above. The service registry can be controlled via static configuration files, a Web user interface, or both. See the Service Management section for more information.
Authorized ServicesAs a security best practice, it is strongly recommended to limit the service management facility to only include the list of known applications that are authorized to use CAS. Leaving the management interface open for all applications may create an opportunity for security attacks.
SSO Cookie Encryption
A ticket-granting cookie is an HTTP cookie set by CAS upon the establishment of a single sign-on session. The cookie value is by default encrypted and signed via settings defined in CAS properties. While sample data is provided for initial deployments, these keys MUST be regenerated per your specific environment. Please see this guide for more info.
Password Management Secure Links
Account password reset requests are handled via a secured link that is sent to the registered email address of the user. The link is available only within a defined time window and the request is properly signed and encrypted by CAS. While sample data is provided for initial deployments, these keys MUST be regenerated per your specific environment.
Please see this guide for more info.
Protocol Ticket Encryption
Protocol tickets that are issued by CAS and shared with other applications such as service tickets may optionally go through a signing/encryption process. Even though the CAS server will always cross check ticket validity and expiration policy, this may be forced as an extra check to ensure tickets in transit to other applications are not tampered with and remain to be authentic. While sample data is provided for initial deployments, these keys MUST be regenerated per your specific environment.
Pay AttentionEncrypting and signing a generated ticket will, depending on the encryption method and algorithm used, increase the generated ticket length. Not all CAS clients are equipped to handle lengthy ticket strings and may get upset with you. Evaluate existing integrations before turning this on and consider whether this feature is truly needed for your deployment.
This CAS feature is able to accept signing and encryption crypto keys. In most scenarios if keys are not provided, CAS will auto-generate them. The following instructions apply if you wish to manually and beforehand create the signing and encryption keys.
Note that if you are asked to create a JWK of a certain size for the key, you are to use the following set of commands to generate the token:
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wget https://raw.githubusercontent.com/apereo/cas/master/etc/jwk-gen.jar
java -jar jwk-gen.jar -t oct -s [size]
The outcome would be similar to:
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{
"kty": "oct",
"kid": "...",
"k": "..."
}
The generated value for k needs to be assigned to the relevant CAS settings. Note that keys generated via
the above algorithm are processed by CAS using the Advanced Encryption Standard (AES) algorithm which is a
specification for the encryption of electronic data established by the U.S. National Institute of Standards and Technology.
Ticket Registry Encryption
Secure ticket replication as it regards clustered CAS deployments may be required to ensure generated tickets by CAS are not tampered with in transit. CAS covers this issue by allowing tickets to be natively encrypted and signed. While sample data is provided for initial deployments, these keys MUST be regenerated per your specific environment. Please see this guide for more info.
Ticket Expiration Policies
Ticket expiration policies are a primary mechanism for implementing security policy. Ticket expiration policy allows control of some important aspects of CAS SSO session behavior:
- SSO session duration (sliding expiration, absolute)
- Ticket reuse
See the Configuring Ticketing Components section for a detailed discussion of the various expiration policies and configuration instructions.
Single Sign-Out
Single sign-out, or single log-out (SLO), is a feature by which CAS services are notified of the termination of a CAS SSO session with the expectation that services terminate access for the SSO session owner. While single sign-out can improve security, it is fundamentally a best-effort facility and may not actually terminate access to all services consumed during an SSO session. The following compensating controls may be used to improve risks associated with single sign-out shortcomings:
- Require forced authentication for sensitive services
- Reduce application session timeouts
- Reduce SSO session duration
SLO can happen in two ways: from the CAS server (back-channel logout) and/or from the browser (front-channel logout).
For back-channel logout, the SLO process relies on the SimpleHttpClient class which has a threads pool: its size must be defined to properly treat all the logout requests.
Additional not-already-processed logout requests are temporarily stored in a queue before being sent: its size is defined to 20% of the global capacity of the threads pool and can be adjusted.
Both sizes are critical settings of the CAS system and their values should never exceed the real capacity of the CAS server.
Login Throttling
CAS supports a policy-driven feature to limit successive failed authentication attempts to help prevent brute force and denial of service attacks. The feature is beneficial in environments where back-end authentication stores lack equivalent features. In cases where this support is available in underlying systems, we encourage using it instead of CAS features; the justification is that enabling support in underlying systems provides the feature in all dependent systems including CAS. See the login throttling configuration section for further information.
Credential Encryption
To learn how sensitive CAS settings can be secured via encryption, please review this guide.
CAS Security Filter
The CAS project provides a number of a blunt security filters suitable for patching-in-place Java CAS server and Java CAS client deployments vulnerable to certain request parameter based bad-CAS-protocol-input attacks. The filters are configured to sanitize authentication request parameters and reject the request if it is not compliant with the CAS protocol in the event that for instance, a parameter is repeated multiple times, includes multiple values, contains unacceptable values, etc.
It is STRONGLY recommended that all CAS deployments be evaluated and include this configuration if necessary to prevent protocol attacks in situations where the CAS container and environment are unable to block malicious and badly-configured requests.
CORS
CAS provides first-class support for enabling HTTP access control (CORS). One application of CORS is when a resource makes a cross-origin HTTP request when it requests a resource from a different domain than the one which the first resource itself serves. This should help more with CAS-enabled applications are accessed via XHR/Ajax requests.
The following settings and properties are available from the CAS configuration catalog:
cas.http-web-request.cors.enabled=false
Whether CORS should be enabled for http requests.
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cas.http-web-request.cors.allow-credentials=true
The Access-Control-Allow-Credentials header Indicates whether or not the response to the request can be exposed when the credentials flag is true. When used as part of a response to a preflight request, this indicates whether or not the actual request can be made using credentials. Note that simple GET requests are not preflighted, and so if a request is made for a resource with credentials, if this header is not returned with the resource, the response is ignored by the browser and not returned to web content.
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cas.http-web-request.cors.allow-headers=
The Access-Control-Allow-Headers header is used in response to a preflight request to indicate which HTTP headers can be used when making the actual request. Default is everything.
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cas.http-web-request.cors.allow-methods=
The Access-Control-Allow-Methods header specifies the method or methods allowed when accessing the resource. This is used in response to a pre-flight request. The conditions under which a request is pre-flighted are discussed above. Default is everything.
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cas.http-web-request.cors.allow-origin-patterns=
Comma-separated list of origin patterns to allow. Unlike allowed origins which only supports
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cas.http-web-request.cors.allow-origins=
The Origin header indicates the origin of the cross-site access request or preflight request. The origin is a URI indicating the server from which the request initiated. When credentials are allowed, '*' cannot be used and origin patterns should be configured instead. It does not include any path information, but only the server name.
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cas.http-web-request.cors.exposed-headers=
The Access-Control-Expose-Headers header lets a server accept headers that browsers are allowed to access.
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cas.http-web-request.cors.max-age=3600
The Access-Control-Max-Age header indicates how long the results of a preflight request can be cached.
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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.
Security Response Headers
As part of the CAS Security Filter, the CAS project automatically provides the necessary configuration to insert HTTP Security headers into the web response to prevent against HSTS, XSS, X-FRAME and other attacks. These settings are presently on by default.
The following settings and properties are available from the CAS configuration catalog:
cas.http-web-request.header.enabled=true
Allow CAS to inject and enforce http security headers via an http filter that are outlined here for caching, HSTS, etc.
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cas.http-web-request.header.cache=true
When true, will inject the following headers into the response for non-static resources. Cache-Control: no-cache, no-store, max-age=0, must-revalidate Pragma: no-cache Expires: 0
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cas.http-web-request.header.cache-control-static-resources=css|js|png|txt|jpg|ico|jpeg|bmp|gif
Files with these extensions are considered static, so they will be cached by browsers. The value is part of a RegEx. This settings supports regular expression patterns. [?].
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cas.http-web-request.header.content-security-policy=
Helps you reduce XSS risks on modern browsers by declaring what dynamic resources are allowed to load via a HTTP Header. Header value is made up of one or more directives. Multiple directives are separated with a semicolon.
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cas.http-web-request.header.hsts=true
When true, will inject the following headers into the response:
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cas.http-web-request.header.hsts-options=max-age=15768000 ; includeSubDomains
Control the value of the
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cas.http-web-request.header.xcontent=true
When true, will inject the following headers into the response:
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cas.http-web-request.header.xframe=true
When true, will inject the following headers into the response:
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cas.http-web-request.header.xframe-options=DENY
Will inject values into the
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cas.http-web-request.header.xss=true
When true, will inject the following headers into the response:
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cas.http-web-request.header.xss-options=1; mode=block
Will inject values into the
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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.
To review and learn more about these options, please visit [this guide][cas-sec-filter].
Spring Webflow Sessions
The CAS project uses Spring Webflow to manage and orchestrate the authentication process. The conversational state of the webflow used by CAS is managed by the client which is then passed and tracked throughout various states of the authentication process. This state must be secured and encrypted to prevent session hijacking. While CAS provides default encryption settings out of the box, it is STRONGLY recommended that all CAS deployments be evaluated prior to production deployments and regenerate this configuration to prevent attacks.
Long Term Authentication
The long term authentication feature, commonly referred to as “Remember Me”, is selected (usually via checkbox) on the CAS login form to avoid re-authentication for an extended period of time. Long term authentication allows users to elect additional convenience at the expense of reduced security. The extent of reduced security is a function of the characteristics of the device used to establish a CAS SSO session. A long-term SSO session established from a device owned or operated by a single user is marginally less secure than a standard CAS SSO session. The only real concern would be the increased lifetime and resulting increased exposure of the CAS ticket-granting ticket. Establishing a long-term CAS SSO session from a shared device, on the other hand, may dramatically reduce security. The likelihood of artifacts from previous SSO sessions affecting subsequent SSO sessions established by other users, even in the face of single sign-out, may increase the likelihood of impersonation. While there is no feasible mitigation for improving security of long-term SSO sessions on a shared device, educating users on the inherent risks may improve overall security.
It is important to note that forced authentication supersedes long term authentication, thus if a service were configured for forced authentication, authentication would be required for service access even in the context of a long-term session.
Long term authentication support must be explicitly enabled through configuration and UI customization during the installation process. Thus deployers choose to offer long-term authentication support, and when available users may elect to use it via selection on the CAS login form.
Warning Before Redirect
CAS supports optional notification of service access during an established SSO session. By default CAS transparently requests tickets needed for service access and presents them to the target service for validation, whereby upon successful validation access to the service is permitted. In most cases this happens nearly instantly and the user is not aware of the CAS authentication process required to access CAS-enabled services. There may be some security benefit to awareness of this process, and CAS supports a warn flag that may be selected by the user on the CAS login screen to provide an interstitial notification page that is displayed prior to accessing a service. By default the notification page offers the user an option to proceed with CAS authentication or abort by navigating away from the target service.