Proxy Authentication
Proxy authentication support for CAS v1+ protocols is enabled by default, thus it is entirely a matter of CAS client configuration to leverage proxy authentication features.
Service ConfigurationNote that each registered application in the registry must explicitly be configured to allow for proxy authentication. See this guide to learn about registering services in the registry.
Disabling proxy authentication components is recommended for deployments that wish to strategically avoid proxy authentication as a matter of security policy.
Use Case
One of the more common use cases of proxy authentication is the ability to obtain a ticket for a back-end [REST-based] service that is also protected by CAS. The scenario usually is:
- User is faced with application A which is protected by CAS.
- Application A on the backend needs to contact a service S to produce data.
- Service S itself is protected by CAS itself.
Because A contacts service S via a server-to-service method where no browser is involved, service S would not be able to recognize that an SSO session already exists. In these cases, application A needs to exercise proxying in order to obtain a proxy ticket for service S. The proxy ticket is passed to the relevant endpoint of service S so it can retrieve and validate it via CAS and finally produce a response.
The trace route may look like this:
- Browser navigates to A.
- A redirects to CAS.
- CAS authenticates and redirects back to A with an
ST. - A attempts to validate the
ST, and asks for aPGT. - CAS confirms
STvalidation, and issues a proxy-granting ticketPGT. - A asks CAS to produce a
PTfor service S, supplying thePGTin its request. - CAS produces a PT for service S.
- A contacts the service S endpoint, passing along
PTin the request. - Service S attempts to validate the
PTvia CAS. - CAS validates the
PTand produces a successful response. - Service S receives the response, and produces data for A.
- A receives and displays the data in the browser.
See the CAS Protocol for more info.
Handling SSL-enabled Proxy URLs
By default, CAS ships with a bundled HTTP client that is partly responsible to callback the URL for proxy authentication. Note that this URL need also be authorized by the CAS service registry before the callback can be made. See this guide for more info.
If the callback URL is authorized by the service registry, and if the endpoint is under HTTPS and protected by an SSL certificate, CAS will also attempt to verify the validity of the endpoint’s certificate before it can establish a successful connection. If the certificate is invalid, expired, missing a step in its chain, self-signed or otherwise, CAS will fail to execute the callback.
The HTTP client of CAS does present a local trust store that is similar to that of the Java platform. It is recommended that this trust store be used to handle the management of all certificates that need to be imported into the platform to allow CAS to execute the callback URL successfully. While by default, the local trust store to CAS is empty, CAS will still utilize both the default and the local trust store. The local trust store should only be used for CAS-related functionality of course, and the trust store file can be carried over across CAS and Java upgrades, and certainly managed by the source control system that should host all CAS configuration.
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.
PGT in Validation Response
In situations where using CAS20ProxyHandler may be undesirable, such that invoking a callback url to receive the proxy granting ticket is not feasible,
CAS may be configured to return the proxy-granting ticket id directly in the validation response. In order to successfully establish trust between the
CAS server and the application, private/public key pairs are generated by the client application and then the public key distributed and
configured inside CAS. CAS will use the public key to encrypt the proxy granting ticket id and will issue a new attribute <proxyGrantingTicketId>
in the validation response, only if the service is authorized to receive it.
Note that the return of the proxy granting ticket id is only carried out by the CAS validation response, provided the client
application issues a request to the /p3/serviceValidate endpoint (or /p3/proxyValidate). Other means of returning attributes to CAS, such as SAML1
will not support the additional returning of the proxy granting ticket.
Note If CAS is configured to return the proxy-granting ticket id directly in the validation
response, the pgtIou parameter is omitted from the response and no callback to the application is performed.Register Service
Once you have received the public key from the client application owner, it must be first registered inside the CAS server’s service registry. The service that holds the public key above must also be authorized to receive the PGT as an attribute for the given attribute release policy of choice.
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{
"@class" : "org.apereo.cas.services.CasRegisteredService",
"serviceId" : "^https://.+",
"name" : "test",
"id" : 1,
"evaluationOrder" : 0,
"attributeReleasePolicy" : {
"@class" : "org.apereo.cas.services.ReturnAllowedAttributeReleasePolicy",
"authorizedToReleaseProxyGrantingTicket" : true
},
"publicKey" : {
"@class" : "org.apereo.cas.services.RegisteredServicePublicKeyImpl",
"location" : "classpath:public.key",
"algorithm" : "RSA"
}
}
The keypair must be generated by the application itself that wishes to obtain the PGT. The public key is shared with CAS. The private key is used by the application to decrypt the PGT. Sample instructions to generate the keypair follow:
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openssl genrsa -out private.key 4096
openssl rsa -pubout -in private.key -out public.key -inform PEM -outform DER
openssl pkcs8 -topk8 -inform PER -outform DER -nocrypt -in private.key -out private.p8
Note that a large key size of 4096 may be required in order to allow CAS to encrypt
lengthy proxy-granting tickets. Choosing a small key size will may prevent CAS to correctly
encrypt the ticket as there is a limit to the lengths the encryption algorithm of a particular size can handle.
Decrypt Proxy-Granting Ticket
Once the client application has received the proxyGrantingTicket id attribute in the CAS validation response, it can decrypt it
via its own private key. Since the attribute is base64 encoded by default, it needs to be decoded first before
decryption can occur. Here’s a sample code snippet:
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var attributes = new HashMap<>();
var encodedPgt = (String) attributes.get("proxyGrantingTicket");
var privateKey = ...
var cipher = Cipher.getInstance(privateKey.getAlgorithm());
var cred64 = decodeBase64(encodedPgt);
cipher.init(Cipher.DECRYPT_MODE, privateKey);
var cipherData = cipher.doFinal(cred64);
return new String(cipherData);