WebFinger Issuer Discovery - OpenID Connect Authentication

OpenID Provider Issuer discovery is the process of determining the location of the OpenID Connect Provider. Issuer discovery is optional; if a Relying Party knows the OP’s Issuer location through an out-of-band mechanism, it can skip this step.

Issuer discovery requires the following information to make a discovery request:

Parameter	Description
`resource`	Required. Identifier for the target End-User that is the subject of the discovery request.
`host`	Server where a WebFinger service is hosted.
`rel`	URI identifying the type of service whose location is being requested: `http://openid.net/specs/connect/1.0/issuer`

To start discovery of OpenID endpoints, the End-User supplies an Identifier to the Relying Party. The RP applies normalization rules to the Identifier to determine the Resource and Host. Then it makes an HTTP GET request to the CAS WebFinger endpoint with the resource and rel parameters to obtain the location of the requested service. The Issuer location MUST be returned in the WebFinger response as the value of the href member of a links array element with rel member value http://openid.net/specs/connect/1.0/issuer.

Example invocation of the webfinger endpoint follows:

curl https://sso.example.org/cas/oidc/.well-known/webfinger?resource=acct:casuser@somewhere.example.org

The expected response shall match the following example:

{
  "subject": "acct:casuser@somewhere.example.org",
  "links": [
    {
      "rel": "http://openid.net/specs/connect/1.0/issuer",
      "href": "https://sso.example.org/cas/oidc/"
    }
  ]
}

WebFinger Resource UserInfo

To determine the correct issuer, resources that are provided to the webfinger discovery endpoint using the acct URI scheme can be located and fetched using external user repositories via email or username.

Usage Warning!

The default repository implementation will echo back the provided email or username, etc as it is ONLY relevant for demo/testing purposes.

The following user-info repository choices are available for configuration and production use.

Groovy UserInfo Repository

The task of locating accounts linked to webfinger resources can be handled using an external Groovy script whose outline would match the following:

def findByUsername(Object[] args) {
    def (username,logger) = args
    return [username: username]
}

def findByEmailAddress(Object[] args) {
    def (email,logger) = args
    return [email: email]
}

The expected return value from the script is a Map that contains key-value objects, representing user account details. An empty Map would indicate the absence of the user record, leading to a 404 response status back to the relying party.

The following settings and properties are available from the CAS configuration catalog:

cas.authn.oidc.webfinger.user-info.groovy.locationThe location of the resource.

no default

Required

The location of the resource. Resources can be URLs, or files found either on the classpath or outside somewhere in the file system.

In the event the configured resource is a Groovy script, especially if the script is set to reload on changes, you may need to adjust the total number of inotify instances. On Linux, you may need to add the following line to /etc/sysctl.conf: fs.inotify.max_user_instances = 256.

You can check the current value via cat /proc/sys/fs/inotify/max_user_instances.

In situations and scenarios where CAS is able to automatically watch the underlying resource for changes and detect updates and modifications dynamically, you may be able to specify the following setting as either an environment variable or system property with a value of false to disable the resource watcher: org.apereo.cas.util.io.PathWatcherService.

Type: Resource
Default: none
Defined by: Groovy

Required settings may be needed to activate or affect the feature; review them even when they have a default. Optional settings only need to be set to change a default or to turn on the behavior they control. Third party settings belong to libraries such as Spring Boot that CAS builds on; their own documentation may have more detail.

Groovy scripting

CAS takes advantage of Apache Groovy in forms of either embedded or external scripts that allow one to, by default, dynamically build constructs, attributes, access strategies and a lot more. To activate the functionality described here, you may need to prepare CAS to support and integrate with Apache Groovy.

Please review this guide to configure your build.

Notes on configuration

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, especially 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 prepare CAS to support and integrate with Apache Groovy, please review this guide.

REST UserInfo Repository

The REST repository allows the CAS server to reach to a remote REST endpoint via the configured HTTP method to fetch user account information.

Query data is passed via either email or username HTTP headers. The response that is returned must be accompanied by a 200 status code where the body should contain Map representing the user account information. All other responses will lead to a 404 response status back to the relying party.

The following settings and properties are available from the CAS configuration catalog:

cas.authn.oidc.webfinger.user-info.rest.urlThe endpoint URL to contact and retrieve attributes.

no default

RequiredSpEL

The endpoint URL to contact and retrieve attributes.

Type: String
Default: none
Defined by: Rest
Supports: Spring Expression Language

cas.authn.oidc.webfinger.user-info.rest.basic-auth-passwordIf REST endpoint is protected via basic authentication, specify the password for authentication.

no default

If REST endpoint is protected via basic authentication, specify the password for authentication.

Type: String
Default: none
Defined by: Rest

cas.authn.oidc.webfinger.user-info.rest.basic-auth-usernameIf REST endpoint is protected via basic authentication, specify the username for authentication.

no default

If REST endpoint is protected via basic authentication, specify the username for authentication.

Type: String
Default: none
Defined by: Rest

cas.authn.oidc.webfinger.user-info.rest.headersHeaders, defined as a Map, to include in the request when making the REST call.

no default

Headers, defined as a Map, to include in the request when making the REST call. Will overwrite any header that CAS is pre-defined to send and include in the request. Key in the map should be the header name and the value in the map should be the header value.

Type: Map<String,String>
Default: none
Defined by: Rest

cas.authn.oidc.webfinger.user-info.rest.maximum-retry-attemptsWhen attempting to reach the endpoint, this setting controls the number of retry attempts that CAS should execute Setting this value to a zero or negative value will disable the retry policy.

3

When attempting to reach the endpoint, this setting controls the number of retry attempts that CAS should execute Setting this value to a zero or negative value will disable the retry policy.

Type: Integer
Default: 3
Defined by: Rest

cas.authn.oidc.webfinger.user-info.rest.methodHTTP method to use when contacting the rest endpoint.

GET

HTTP method to use when contacting the rest endpoint. Examples include GET, POST, etc.

Type: String
Default: GET
Defined by: Rest

Notes on configuration

Configuration Metadata

Be Selective

YAGNI

Naming Convention

Note

Validation

Indexed Settings

Custom UserInfo Repository

It is possible to design and inject your own version of webfinger user repositories into CAS. First, you will need to design a @AutoConfiguration class to contain your own OidcWebFingerUserInfoRepository implementation:

@AutoConfiguration
@EnableConfigurationProperties(CasConfigurationProperties.class)
public class CustomWebFingerUserInfoConfiguration {

    @Bean
    public OidcWebFingerUserInfoRepository oidcWebFingerUserInfoRepository() {
        ...
    }
}

Your configuration class needs to be registered with CAS. See this guide for better details.