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
javax.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:
cas.authn.x509.principal-transformation.groovy.location=
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, specially if the script set to reload on changes, you may need to adjust the total number ofinotify 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.
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cas.authn.x509.principal.principal-transformation.groovy.location=
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, specially if the script set to reload on changes, you may need to adjust the total number ofinotify 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.
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cas.authn.x509.principal-type=SUBJECT_DN
Indicates the type of principal resolution for X509. Available values are as follows:
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cas.authn.x509.principal-transformation.blocking-pattern=
A regular expression that will be used against the username to match for blocking/forbidden values. If a match is found, an exception will be thrown and principal transformation will fail.
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cas.authn.x509.principal-transformation.case-conversion=NONE
Indicate whether the principal identifier should be transformed into upper-case, lower-case, etc. Available values are as follows:
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cas.authn.x509.principal-transformation.pattern=
A regular expression that will be used against the provided username for username extractions. On a successful match, the first matched group in the pattern will be used as the extracted username.
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cas.authn.x509.principal-transformation.prefix=
Prefix to add to the principal id prior to authentication.
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cas.authn.x509.principal-transformation.suffix=
Suffix to add to the principal id prior to authentication.
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cas.authn.x509.principal.active-attribute-repository-ids=
Activated attribute repository identifiers that should be used for fetching attributes if attribute resolution is enabled. The list here may include identifiers separated by comma.
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cas.authn.x509.principal.attribute-resolution-enabled=
Whether attribute repositories should be contacted to fetch person attributes. Defaults to true if not set.
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cas.authn.x509.principal.principal-attribute=
Attribute name to use to indicate the identifier of the principal constructed. If the attribute is blank or has no values, the default principal id will be used determined by the underlying authentication engine. The principal id attribute usually is removed from the collection of attributes collected, though this behavior depends on the schematics of the underlying authentication strategy.
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cas.authn.x509.principal.principal-resolution-conflict-strategy=last
In the event that the principal resolution engine resolves more than one principal, (specially if such principals in the chain have different identifiers), this setting determines strategy by which the principal id would be chosen from the chain. Accepted values are:
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cas.authn.x509.principal.principal-resolution-failure-fatal=
When true, throws an error back indicating that principal resolution has failed and no principal can be found based on the authentication requirements. Otherwise, logs the condition as an error without raising a catastrophic error.
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cas.authn.x509.principal.principal-transformation.blocking-pattern=
A regular expression that will be used against the username to match for blocking/forbidden values. If a match is found, an exception will be thrown and principal transformation will fail.
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cas.authn.x509.principal.principal-transformation.case-conversion=NONE
Indicate whether the principal identifier should be transformed into upper-case, lower-case, etc. Available values are as follows:
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cas.authn.x509.principal.principal-transformation.pattern=
A regular expression that will be used against the provided username for username extractions. On a successful match, the first matched group in the pattern will be used as the extracted username.
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cas.authn.x509.principal.principal-transformation.prefix=
Prefix to add to the principal id prior to authentication.
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cas.authn.x509.principal.principal-transformation.suffix=
Suffix to add to the principal id prior to authentication.
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cas.authn.x509.principal.return-null=
Return a null principal object if no attributes can be found for the principal.
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cas.authn.x509.principal.use-existing-principal-id=
Uses an existing principal id that may have already been established in order to run person directory queries. This is generally useful in situations where authentication is delegated to an external identity provider and a principal is first established to then query an attribute source.
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