WORKERS AHEAD!
You are viewing the development documentation for the Apereo CAS server. The functionality presented here is not officially released yet. This is a work in progress and will be continually updated as development moves forward. You are most encouraged to test the changes presented.
X.509 Authentication
CAS X.509 authentication components provide a mechanism to authenticate users who present client certificates during
the SSL/TLS handshake process. The X.509 components require configuration outside the CAS application since the
SSL handshake happens outside the servlet layer where the CAS application resides. There is no particular requirement
on deployment architecture (i.e. Apache reverse proxy, load balancer SSL termination) other than any client
certificate presented in the SSL handshake be accessible to the servlet container as a request attribute named
jakarta.servlet.request.X509Certificate
. This happens naturally for configurations that terminate SSL connections
directly at the servlet container and when using Apache/mod_jk
; for other architectures it may be necessary to do
additional work.
CAS can be configured to extract an X509 certificate from a header created by a proxy running in front of CAS.
Overview
Certificates are exchanged as part of the SSL (also called TLS) initialization that
occurs when any browser connects to an https
website.
A certain number of public CA certificates are preinstalled in each browser. It is assumed that:
- Your organization is already able to generate and distribute certificates that a user can install in their browser
- Somewhere in that certificate there is a field that contains the Principal name or can be easily mapped to the Principal name that CAS can use.
The remaining problem is to make sure that the browsers, servers and Java are all prepared to support these institutional certificates and, ideally, that these institutional certificates will be the only ones exchanged when a browser connects to CAS.
Flow
When a browser connects to CAS over an https: URL, the server identifies itself by sending its own certificate. The browser must already have installed a certificate identifying and trusting the CA that issued the CAS Server certificate. If the browser is not already prepared to trust the CAS server, then an error message pops up saying the server is not trusted.
After the Server sends the certificate that identifies itself, it then can then send a list of names of Certificate Authorities from which it is willing to accept certificates. Ideally, this list will include only one name; the name of the internal institutional CA that issues internal intranet-only certificates that internally contain a field with the CAS Principal name.
A user may install any number of certificates into the browser from any number of CA’s. If only one of these certificates comes from a CA named in the list of acceptable CA’s sent by the server, then most browsers will automatically send that one certificate without asking, and some can be configured in to not ask when there is only one possible choice. This presents a user experience where CAS becomes transparent to the user after some initial setup and the login happens automatically. However, if the server hosting CAS sends more than one CA name in the list and that matches more than one certificate on the browser, then the user will get prompted to choose a Certificate from the list. A user interaction defeats much of the purpose of certificates in CAS.
Note that CAS does not control this exchange. It is handled by the underlying server. You may not have the control to require the server to vend only one CA name when a browser visits CAS. So if you want to use X.509 certificates in CAS, you should consider this requirement when choosing the hosting environment. The ideal situation is to select a server that can identify itself with a public certificate issued by something like VeriSign or InCommon but then require the client certificate only be issued by the internal corporate/campus authority.
When CAS gets control, a user certificate may have been presented by the browser and be stored in the request. The CAS X.509 authentication machinery examines that certificate and verifies that it was issued by the trusted institutional authority. Then CAS searches through the fields of the certificate to identify one or more fields that can be turned into the principal identifier that the applications expect.
While an institution can have one certificate authority that issues certificates to employees, clients, machines, services, and devices, it is more common for the institution to have a single “root” certificate authority that in its entire existence only issues a handful of certificates. Each of these certificates identifies a secondary Certificate Authority that issues a particular category of certificates (to students, staff, servers, etc.). It is possible to configure CAS to trust the root Authority and, implicitly, all the secondary authorities that it creates. This, however, makes CAS only as secure as the least reliable secondary Certificate Authority created by the institution. At some point in the future, some manager will buy a product that requires a new class of certificates. He will ask to create a Certificate Authority that vends these certificates to the machines running this new product. He will then turn administration of this mess over to a junior programmer or consultant. If CAS trusts any certificate issued by any Authority created by the root, it will trust a fraudulent certificate forged by someone who has acquired control of what was intended to be a special purpose, isolated CA. Therefore, it is better to configure CAS to only accept certificates from the one secondary CA specifically expected to issue credentials to individuals, instead of trusting the institutional root CA.
Configuration
X.509 support is enabled by including the following dependency in the WAR overlay:
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<dependency>
<groupId>org.apereo.cas</groupId>
<artifactId>cas-server-support-x509-webflow</artifactId>
<version>${cas.version}</version>
</dependency>
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implementation "org.apereo.cas:cas-server-support-x509-webflow:${project.'cas.version'}"
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dependencyManagement {
imports {
mavenBom "org.apereo.cas:cas-server-support-bom:${project.'cas.version'}"
}
}
dependencies {
implementation "org.apereo.cas:cas-server-support-x509-webflow"
}
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dependencies {
/*
The following platform references should be included automatically and are listed here for reference only.
implementation enforcedPlatform("org.apereo.cas:cas-server-support-bom:${project.'cas.version'}")
implementation platform(org.springframework.boot.gradle.plugin.SpringBootPlugin.BOM_COORDINATES)
*/
implementation "org.apereo.cas:cas-server-support-x509-webflow"
}
The X.509 handler technically performs additional checks after the real SSL client authentication process performed by the Web server terminating the SSL connection. Since an SSL peer may be configured to accept a wide range of certificates, the CAS X.509 handler provides a number of properties that place additional restrictions on acceptable client certificates.
The following settings and properties are available from the CAS configuration catalog:
Please review this guide to configure your build.
Authentication handlers that generally deal with username-password credentials can be configured to transform the user id prior to executing the authentication sequence. Each authentication strategy in CAS provides settings to properly transform the principal. Refer to the relevant settings for the authentication strategy at hand to learn more.
Authentication handlers as part of principal transformation may also be provided a path to a Groovy script to transform the provided username. The outline of the script may take on the following form:
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String run(final Object... args) {
def (providedUsername,logger) = args
return providedUsername.concat("SomethingElse")
}
To prepare CAS to support and integrate with Apache Groovy, please review this guide.
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.

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.