Multifactor Authentication (MFA)

CAS provides a framework for multifactor authentication (MFA). The design philosophy for MFA support follows from the observation that institutional security policies with respect to MFA vary dramatically. We provide first class API support for authenticating multiple credentials and a policy framework around authentication. The components could be extended in a straightforward fashion to provide higher-level behaviors such as Webflow logic to assist, for example, a credential upgrade scenario where a SSO session is started by a weaker credential but a particular service demands re-authentication with a stronger credential.

The authentication subsystem in CAS natively supports handling multiple credentials. While the default login form and Webflow tier are designed for the simple case of accepting a single credential, all core API components that interface with the authentication subsystem accept one or more credentials to authenticate.

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.
Microsoft Azure mfa-azure See this guide.
FIDO U2F mfa-u2f See this guide.
Custom Custom See this guide.

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. Once the provider is chosen to honor the authentication request, bypass rules are then consulted to calculate whether the provider should ignore the request and skip MFA conditionally.

Bypass rules allow for the following options for each provider:

  • Skip multifactor authentication based on designated principal attribute names.
  • …[and optionally] Skip multifactor authentication based on designated principal attribute values.
  • Skip multifactor authentication based on designated authentication attribute names.
  • …[and optionally] Skip multifactor authentication based on designated authentication attribute values.
  • Skip multifactor authentication depending on method/form of primary authentication execution.

A few simple examples follow:

  • Trigger MFA except when the principal carries an affiliation attribute whose value is either alum or member.
  • Trigger MFA except when the principal carries a superAdmin attribute.
  • Trigger MFA except if the method of primary authentication is SPNEGO.
  • Trigger MFA except if credentials used for primary authentication are of type org.example.MyCredential.

Note that in addition to the above options, some multifactor authentication providers may also skip and bypass the authentication request in the event that the authenticated principal does not quite “qualify” for multifactor authentication. See the documentation for each specific provider to learn more.

To see the relevant list of CAS properties, please review this guide.

Note that ticket validation requests shall successfully go through if multifactor authentication is bypassed for the given provider. In such cases, no authentication context is passed back to the application and additional attributes are supplanted to let the application know multifactor authentication is bypassed for the provider.

Applications

MFA Bypass rules can be overridden per application via the CAS service registry. This is useful when MFA may be turned on globally for all applications and services, yet a few selectively need to be excluded. Services whose access should bypass MFA may be defined as such in the CAS service registry:

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{
  "@class" : "org.apereo.cas.services.RegexRegisteredService",
  "serviceId" : "^(https|imaps)://.*",
  "id" : 100,
  "multifactorPolicy" : {
    "@class" : "org.apereo.cas.services.DefaultRegisteredServiceMultifactorPolicy",
    "multifactorAuthenticationProviders" : [ "java.util.LinkedHashSet", [ "mfa-duo" ] ],
    "bypassEnabled" : "true"
  }
}

Failure Modes

The authentication policy by default supports fail-closed mode, which means that if you attempt to exercise a particular provider available to CAS and the provider cannot be reached, authentication will be stopped and an error will be displayed. You can of course change this behavior so that authentication proceeds without exercising the provider functionality, if that provider cannot respond.

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{
  "@class" : "org.apereo.cas.services.RegexRegisteredService",
  "serviceId" : "^(https|imaps)://.*",
  "id" : 100,
  "multifactorPolicy" : {
    "@class" : "org.apereo.cas.services.DefaultRegisteredServiceMultifactorPolicy",
    "multifactorAuthenticationProviders" : [ "java.util.LinkedHashSet", [ "mfa-duo" ] ],
    "failureMode" : "CLOSED"
  }
}

The following failure modes are supported:

Field Description
CLOSED Authentication is blocked if the provider cannot be reached.
OPEN Authentication proceeds yet requested MFA is NOT communicated to the client if provider is unavailable.
PHANTOM Authentication proceeds and requested MFA is communicated to the client if provider is unavailable.
NONE Do not contact the provider at all to check for availability. Assume the provider is available.

A default failure mode can also be specified globally via CAS properties and may be overriden individually by CAS registered services. To see the relevant list of CAS properties, please review 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 {
    def String run(final Object... args) {
        def service = args[0]
        def principal = args[1]
        def providersCollection = args[2]
        def logger = args[3]
        ...
        return "mfa-duo"
    }
}

To see the relevant list of CAS properties, please review this guide.

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.

Settings

To see the relevant list of CAS properties, please review this guide.