SPNEGO Authentication

SPNEGO is an authentication technology that is primarily used to provide transparent CAS authentication to browsers running on Windows running under Active Directory domain credentials. There are three actors involved: the client, the CAS server, and the Active Directory Domain Controller/KDC.

  1. Client sends CAS: HTTP GET to CAS for cas protected page
  2. CAS responds: HTTP 401 - Access Denied WWW-Authenticate: Negotiate
  3. Client sends ticket request: Kerberos(KRB_TGS_REQ) Requesting ticket for HTTP/cas.example.com@REALM
  4. Kerberos KDC responds: Kerberos(KRB_TGS_REP) Granting ticket for HTTP/cas.example.com@REALM
  5. Client sends CAS: HTTP GET Authorization: Negotiate w/SPNEGO Token
  6. CAS responds: HTTP 200 - OK WWW-Authenticate w/SPNEGO response + requested page.

The above interaction occurs only for the first request, when there is no CAS SSO session. Once CAS grants a ticket-granting ticket, the SPNEGO process will not happen again until the CAS ticket expires.

Requirements

  • Client is logged in to a Windows Active Directory domain.
  • Supported browser.
  • CAS is running MIT kerberos against the AD domain controller.
:information_source: JCE Requirement

It's safe to make sure you have the proper JCE bundle installed in your Java environment that is used by CAS, specially if you need to consume encrypted payloads issued by ADFS. Be sure to pick the right version of the JCE for your Java version. Java versions can be detected via the java -version command.

:information_source: Large Kerberos Tickets

If organization users have large kerberos tickets, likely cause by being a member of a large number of groups, the Tomcat connector will need to have the maxHttpHeaderSize value increased from the default amount to allow the ticket to be passed to the CAS Server application.

Components

SPNEGO 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-spnego-webflow</artifactId>
    <version>${cas.version}</version>
</dependency>





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implementation "org.apereo.cas:cas-server-support-spnego-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-spnego-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-spnego-webflow"
}

JCIFS SDK

Note that jcifs-ext library is no longer published to a public Maven repository. This means that you will need to download the necessary JAR files and include them in your build configuration. The SDK may be downloaded from the CAS codebase. Then, assuming the SDK is placed inside a lib directory of the WAR overlay directory, it can be referenced in the build configuration as such:



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implementation files("${projectDir}/lib/jcifs-ext.jar")

Configuration

The following steps are required to turn on the SPNEGO functionality.

Create SPN Account

Create an Active Directory account for the Service Principal Name (SPN) and record the username. Password will be overwritten by the next step.

Create Keytab File

The keytab file enables a trust link between the CAS server and the Key Distribution Center (KDC); an Active Directory domain controller serves the role of KDC in this context. The ktpass tool is used to generate the keytab file, which contains a cryptographic key. Be sure to execute the command from an Active Directory domain controller as administrator (a member of domain administrators will not be able to use ktpass successfully).

Example:



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C:\Users\administrator.DOMAIN>ktpass /out myspnaccount.keytab /princ HTTP/cas.example.com@REALM /pass * /mapuser domain-account@YOUR.REALM /ptype KRB5_NT_PRINCIPAL /crypto RC4-HMAC-NT
Targeting domain controller: DC.YOUR.REALM
Successfully mapped HTTP/cas.example.com to domaine-account.
Type the password for HTTP/cas.example.com:
Type the password again to confirm:
Password succesfully set!
Key created.
Output keytab to myspnaccount.keytab:
Keytab version: 0x502
keysize 69 HTTP/cas.example.com@REALM ptype 1 (KRB5_NT_PRINCIPAL) vno 3 etype 0x17 (RC4-HMAC) keylength 16
(0x00112233445566778899aabbccddeeff)

Using ktpass requires Active Directory admin permissions. If that is not an option, you may be able to use ktab.exe from %JAVA_HOME%\bin\ktab.exe that is provided by the JDK:



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%JAVA_HOME%\bin\ktab.exe -a service_xxx -n 0 -k cas.keytab

-k specifies key tab output file name and -n 0 specifies the KNVO number if available and found for the user account. This value may match the msDS-KeyVersionNumber on the user account.

Also note that the keytab file must be regenerated after password changes, if any.

Test SPN Account

Install and configure MIT Kerberos V on the CAS server host(s). The following sample krb5.conf file may be used as a reference.



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[logging]
 default = FILE:/var/log/krb5libs.log
 kdc = FILE:/var/log/krb5kdc.log
 admin_server = FILE:/var/log/kadmind.log

[libdefaults]
 ticket_lifetime = 24000
 default_realm = YOUR.REALM.HERE
 default_keytab_name = /home/cas/kerberos/myspnaccount.keytab
 dns_lookup_realm = false
 dns_lookup_kdc = false
 default_tkt_enctypes = rc4-hmac
 default_tgs_enctypes = rc4-hmac

[realms]
 YOUR.REALM.HERE = {
  kdc = your.kdc.your.realm.here:88
 }

[domain_realm]
 .your.realm.here = YOUR.REALM.HERE
 your.realm.here = YOUR.REALM.HERE

It is important to note that myspnaccount.keytab is declared as default keytab, otherwise CAS may not be able to find it and will raise an exception similar to



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KrbException: Invalid argument (400) - Cannot find key of appropriate type to decrypt AP REP -RC4 with HMAC`

Then verify that your are able to read the keytab file:



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klist -k
Keytab name: FILE:/home/cas/kerberos/myspnaccount.keytab
KVNO Principal
---- --------------------------------------------------------------------------
   3 HTTP/cas.example.com@REALM

Then verify that your are able to use the keytab file:



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kinit -k HTTP/cas.example.com@REALM
klist
Ticket cache: FILE:/tmp/krb5cc_999
Default principal: HTTP/cas.example.com@REALM

Valid starting       Expires              Service principal
12/08/2016 10:52:00  12/08/2016 20:52:00  krbtgt/REALM@REALM
    renew until 12/08/2016 20:52:00

Browser Configuration

  • Internet Explorer - Enable Integrated Windows Authentication and add the CAS server URL to the Local Intranet zone.
  • Firefox - Set the network.negotiate-auth.trusted-uris configuration parameter in about:config to the CAS server URL, e.g. https://cas.example.com.

Authentication Configuration

Make sure you have at least specified the JCIFS Service Principal in the CAS configuration.

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

The configuration settings listed below are tagged as Required in the CAS configuration metadata. This flag indicates that the presence of the setting may be needed to activate or affect the behavior of the CAS feature and generally should be reviewed, possibly owned and adjusted. If the setting is assigned a default value, you do not need to strictly put the setting in your copy of the configuration, but should review it nonetheless to make sure it matches your deployment expectations.

The configuration settings listed below are tagged as Optional in the CAS configuration metadata. This flag indicates that the presence of the setting is not immediately necessary in the end-user CAS configuration, because a default value is assigned or the activation of the feature is not conditionally controlled by the setting value. In other words, you should only include this field in your configuration if you need to modify the default value or if you need to turn on the feature controlled by the setting.

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.

LDAP Scriptable Search Filter

LDAP search filters can point to an external Groovy script to dynamically construct the final filter template.

The script itself may be designed as:



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import org.ldaptive.*
import org.springframework.context.*

def run(Object[] args) {
    def (filter,parameters,applicationContext,logger) = args

    logger.info("Configuring LDAP filter")
    filter.setFilter("uid=something")
}

The following parameters are passed to the script:

Parameter Description
filter FilterTemplate to be updated by the script and used for the LDAP query.
parameters Map of query parameters which may be used to construct the final filter.
applicationContext Reference to the Spring ApplicationContext reference.
logger The object responsible for issuing log messages such as logger.info(...).

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

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.

:information_source: 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.

You may provide a JAAS login.conf file:



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jcifs.spnego.initiate {
   com.sun.security.auth.module.Krb5LoginModule \
    required storeKey=true useKeyTab=true keyTab="/home/cas/kerberos/myspnaccount.keytab";
};
jcifs.spnego.accept {
   com.sun.security.auth.module.Krb5LoginModule \
    required storeKey=true useKeyTab=true keyTab="/home/cas/kerberos/myspnaccount.keytab";
};

Client Selection Strategy

CAS provides a set of components that attempt to activate the SPNEGO flow conditionally, in case deployers need a configurable way to decide whether SPNEGO should be applied to the current authentication/browser request. The state that is available to the webflow is evaluateClientRequest which will attempt to start SPNEGO authentication or resume normally, depending on the client action strategy chosen below.

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

The configuration settings listed below are tagged as Optional in the CAS configuration metadata. This flag indicates that the presence of the setting is not immediately necessary in the end-user CAS configuration, because a default value is assigned or the activation of the feature is not conditionally controlled by the setting value. In other words, you should only include this field in your configuration if you need to modify the default value or if you need to turn on the feature controlled by the setting.

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.

:information_source: 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.

By Remote IP

Checks to see if the request’s remote ip address matches a predefine pattern.

By Hostname

Checks to see if the request’s remote hostname matches a predefine pattern.

By LDAP Attribute

Checks an LDAP instance for the remote hostname, to locate a pre-defined attribute whose mere existence would allow the webflow to resume to SPNEGO.

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

The configuration settings listed below are tagged as Required in the CAS configuration metadata. This flag indicates that the presence of the setting may be needed to activate or affect the behavior of the CAS feature and generally should be reviewed, possibly owned and adjusted. If the setting is assigned a default value, you do not need to strictly put the setting in your copy of the configuration, but should review it nonetheless to make sure it matches your deployment expectations.

The configuration settings listed below are tagged as Optional in the CAS configuration metadata. This flag indicates that the presence of the setting is not immediately necessary in the end-user CAS configuration, because a default value is assigned or the activation of the feature is not conditionally controlled by the setting value. In other words, you should only include this field in your configuration if you need to modify the default value or if you need to turn on the feature controlled by the setting.

LDAP Scriptable Search Filter

LDAP search filters can point to an external Groovy script to dynamically construct the final filter template.

The script itself may be designed as:



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import org.ldaptive.*
import org.springframework.context.*

def run(Object[] args) {
    def (filter,parameters,applicationContext,logger) = args

    logger.info("Configuring LDAP filter")
    filter.setFilter("uid=something")
}

The following parameters are passed to the script:

Parameter Description
filter FilterTemplate to be updated by the script and used for the LDAP query.
parameters Map of query parameters which may be used to construct the final filter.
applicationContext Reference to the Spring ApplicationContext reference.
logger The object responsible for issuing log messages such as logger.info(...).

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.

:information_source: 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.

Logging

To enable additional logging, configure the log4j configuration file to add the following levels:



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...
<Logger name="jcifs.spnego" level="debug" additivity="false">
    <AppenderRef ref="casConsole"/>
    <AppenderRef ref="casFile"/>
</Logger>
...

Troubleshooting

  • Failure unspecified at GSS-API level


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Caused by: GSSException: Failure unspecified at GSS-API level (Mechanism level: Invalid argument (400) - Cannot find key of appropriate type to decrypt AP REP - AES256 CTS mode with HMAC SHA1-96)
        at sun.security.jgss.krb5.Krb5Context.acceptSecContext(Unknown Source)
        at sun.security.jgss.GSSContextImpl.acceptSecContext(Unknown Source)
        at sun.security.jgss.GSSContextImpl.acceptSecContext(Unknown Source)
        ... 280 more
Caused by: KrbException: Invalid argument (400) - Cannot find key of appropriate type to decrypt 
AP REP - AES256 CTS mode with HMAC SHA1-96
        at sun.security.krb5.KrbApReq.authenticate(Unknown Source)
        at sun.security.krb5.KrbApReq.<init>(Unknown Source)
        at sun.security.jgss.krb5.InitSecContextToken.<init>(Unknown Source)
        ... 283 more

It’s very likely that you have the wrong path to .keytab file. The KVNO in the keytab file must be identical with the KVNO stored in the Active Directory. Active Directory is raising the KVNO with every execution of ktpass, as part of its msDS-KeyVersionNumber.

Other possible causes include:

  1. The service principal in the in the CAS configuration is not identical with that from the keytab. (param /princ from ktpass)
  2. There is no key for the enctype sent with the ticket by Active Directory. (param /crypto from ktpass and set in the krb5.conf/permitted_enctypes+default_tkt_enctypes).
  3. The KVNO from the ticket is different than the KVNO in the keytab (param /kvno from ktpass).