Multifactor Authentication (MFA)

CAS provides support for a variety of multifactor authentication providers and options, while allowing one to design their own. The secondary authentication factor always kicks in after the primary step and existing authentication sessions will be asked to step-up to the needed multifactor authentication factor, should the request or trigger require it. The satisfied authentication context is communicated back to the application as well to denote a successful multifactor authentication event.

At a minimum, you need answer the following questions:

  • Which provider(s) are we using for multifactor authentication?
  • How and for whom are we triggering multifactor authentication?

Supported Providers

The following multifactor providers are supported by CAS.

Provider Id Instructions
Duo Security mfa-duo See this guide.
Authy Authenticator mfa-authy See this guide.
YubiKey mfa-yubikey See this guide.
RSA/RADIUS mfa-radius See this guide.
WiKID mfa-radius See this guide.
Google Authenticator mfa-gauth See this guide.
FIDO U2F mfa-u2f See this guide.
FIDO2 WebAuthN mfa-webauthn See this guide.
CAS Simple mfa-simple See this guide.
Inwebo mfa-inwebo See this guide.
Custom Custom See this guide.
:information_source: Azure Multifactor

Microsoft has removed the ability for external SSO servers to use Azure MFA. To use Azure MFA, you must also have all your users authenticate using Azure AD SSO. You may want to route authentication requests to Azure AD SSO using the delegated authentication features of CAS.

Core 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.

  • cas.authn.mfa.core.provider-selector-groovy-script.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 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.

    org.apereo.cas.configuration.model.SpringResourceProperties.

    How can I configure this property?

    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.authn.mfa.core.authentication-context-attribute=authnContextClass

    • Attribute returned in the final CAS validation payload that indicates the authentication context class satisfied in the event of a multifactor authentication attempt.

    org.apereo.cas.configuration.model.support.mfa.MultifactorAuthenticationCoreProperties.

    How can I configure this property?

  • cas.authn.mfa.core.content-type=application/cas

    • Content-type that is expected to be specified by non-web clients such as curl, etc in the event that the provider supports variations of non-browser based MFA. The value is treated as a regular expression.

    This settings supports regular expression patterns. [?].

    org.apereo.cas.configuration.model.support.mfa.MultifactorAuthenticationCoreProperties.

    How can I configure this property?

  • cas.authn.mfa.core.global-failure-mode=

    • Defines the global failure mode for the entire deployment. This is meant to be used a shortcut to define the policy globally rather than per application. Applications registered with CAS can still define a failure mode and override the global. Available values are as follows:

    • OPEN: Disallow MFA, proceed with authentication but don't communicate MFA to the RP.
    • CLOSED: Disallow MFA, block with authentication.
    • PHANTOM: Disallow MFA, proceed with authentication and communicate MFA to the RP.
    • NONE: Do not check for failure at all.
    • UNDEFINED: The default one indicating that no failure mode is set at all.

    org.apereo.cas.configuration.model.support.mfa.MultifactorAuthenticationCoreProperties.

    How can I configure this property?

  • cas.authn.mfa.core.provider-selection-enabled=false

    • In the event that multiple multifactor authentication providers are determined for a multifactor authentication transaction, this setting will allow one to interactively choose a provider out of the list of available providers. A trigger may be designed to support more than one provider, and rather than letting CAS auto-determine the selected provider via scripts or ranking strategies, this method puts the choice back onto the user to decide which provider makes the most sense at any given time.

    org.apereo.cas.configuration.model.support.mfa.MultifactorAuthenticationCoreProperties.

    How can I configure this property?

    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. The validation process is on by default and can be skipped on startup using a special system property SKIP_CONFIG_VALIDATION that should be set to true. 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.

    Triggers

    Multifactor authentication can be activated via a number of triggers. To learn more, please see this guide.

    Bypass Rules

    Each multifactor provider is equipped with options to allow for MFA bypass. To learn more, please see this guide.

    Failure Modes

    CAS will consult the current configuration in the event that the provider being requested is unreachable to determine how to proceed. To learn more, please see this guide.

    Multiple Provider Selection

    In the event that multiple multifactor authentication providers are determined for a multifactor authentication transaction, by default CAS will attempt to sort the collection of providers based on their rank and will pick one with the highest priority. This use case may arise if multiple triggers are defined where each decides on a different multifactor authentication provider, or the same provider instance is configured multiple times with many instances.

    Provider selection may also be carried out using Groovy scripting strategies more dynamically. The following example should serve as an outline of how to select multifactor providers based on a Groovy script:

    

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    import java.util.*

class SampleGroovyProviderSelection {
    String run(final Object... args) {
        def service = args[0]
        def principal = args[1]
        def providersCollection = args[2]
        def logger = args[3]
        ...
        return "mfa-duo"
    }
}

    The parameters passed are as follows:

    Parameter Description
    service The object representing the incoming service provided in the request, if any.
    principal The object representing the authenticated principal along with its attributes.
    providersCollection The object representing the collection of candidate multifactor providers qualified for the transaction.
    logger The object responsible for issuing log messages such as logger.info(...).

    Ranking Providers

    At times, CAS needs to determine the correct provider when step-up authentication is required. Consider for a moment that CAS already has established an SSO session with/without a provider and has reached a level of authentication. Another incoming request attempts to exercise that SSO session with a different and often competing authentication requirement that may differ from the authentication level CAS has already established. Concretely, examples may be:

    • CAS has achieved an SSO session, but a separate request now requires step-up authentication with DuoSecurity.
    • CAS has achieved an SSO session with an authentication level satisfied by DuoSecurity, but a separate request now requires step-up authentication with YubiKey.

    In certain scenarios, CAS will attempt to rank authentication levels and compare them with each other. If CAS already has achieved a level that is higher than what the incoming request requires, no step-up authentication will be performed. If the opposite is true, CAS will route the authentication flow to the required authentication level and upon success, will adjust the SSO session with the new higher authentication level now satisfied.

    Ranking of authentication methods is done per provider via specific properties for each in CAS settings. Note that the higher the rank value is, the higher on the security scale it remains. A provider that ranks higher with a larger weight value trumps and override others with a lower value.

    Trusted Devices/Browsers

    CAS is able to natively provide trusted device/browser features as part of any multifactor authentication flow. While certain providers tend to support this feature as well, this behavior is now put into CAS directly providing you with exact control over how devices/browsers are checked, how is that decision remembered for subsequent requests and how you might allow delegated management of those trusted decisions both for admins and end-users.

    See this guide for more info.

    2FA vs. MFA

    Multifactor authentication in CAS mostly presents itself in form of two-factor authentication when deployed. The framework however is designed in such a way to allow additional chaining of other providers into an existing authentication experience. If you have a need to string along multiple factors together one after another, it is likely that you may need to adjust and extend the existing authentication workflows to deliver the use case.