CAS Multifactor Authentication with Duo Security


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As a rather common use case, the majority of CAS deployments that intend to turn on multifactor authentication support tend to do so via Duo Security. This is a quick and simplified guide to demonstrate an approach to that use case along with some additional explanations regarding specific multifactor triggers supported in CAS today.

Our task list is rather short:

  1. Configure LDAP authentication with CAS
  2. Trigger Duo Security for users who belong to the mfa-eligible group, indicated by the memberOf attribute on the LDAP user account.

Environment

Configuring Authentication

Prior to configuring multiple factors of authentication, we need to first establish a primary mode of validating credentials. To kill two birds with one stone [1], we are going to o address yet another common use case and keep things simple by sticking with LDAP authentication. The strategy here, as indicated by the CAS documentation, is to declare the intention/module in the build script and then configure the relevant cas.authn.ldap[x] settings for the directory server in use. Most commonly, that would translate into the following settings:

cas.authn.ldap[0].type=AUTHENTICATED
cas.authn.ldap[0].ldapUrl=ldaps://ldap1.example.org 
cas.authn.ldap[0].baseDn=dc=example,dc=org
cas.authn.ldap[0].userFilter=cn={user}
cas.authn.ldap[0].bindDn=cn=Directory Manager,dc=example,dc=org
cas.authn.ldap[0].bindCredential=...

Note that the method of authentication, whether on its own or using separate attribute repositories and queries must have the ability to resolve the needed attribute which will be used later by CAS to trigger multifactor authentication. For this context, the simplest way would be to let LDAP authentication retrieve the attribute directly from the directory server. The following setting allows us to do just that:

cas.authn.ldap[0].principalAttributeList=memberOf

At this point in the authentication flow, we have established an authenticated subject that would be populated with fetched attribute memberOf.

Configuring Duo Security

Here, our task is to enable Duo Security in CAS. Practically, similar to the LDAP authentication configuration, this involves declaring the right module in the build and then providing specific Duo Security settings to CAS properties. Things such as the secret key, integration key, etc which should be provided by your Duo Security subscription. Most commonly, that would translate into the following settings:

cas.authn.mfa.duo[0].duoSecretKey=
cas.authn.mfa.duo[0].duoApplicationKey=
cas.authn.mfa.duo[0].duoIntegrationKey=
cas.authn.mfa.duo[0].duoApiHost=

At this point, we have enabled Duo Security and we just need to find a way to instruct CAS to route the authentication flow over to Duo Security in the appropriate condition. This is where triggers come into place.

Configuring Multifactor Authentication Triggers

The entire purpose of a trigger here is to detect a condition by which the authentication flow should be rerouted. There are a large number of triggers supported by CAS, all of which kick into action and behave all the same regardless of the multifactor authentication provider. Our task here is to build a special condition that activates multifactor authentication if any of the values assigned to the attribute memberOf contain the value mfa-eligible:

cas.authn.mfa.globalPrincipalAttributeNameTriggers=memberOf
cas.authn.mfa.globalPrincipalAttributeValueRegex=mfa-eligible

Notice that the conditions above do not indicate anything about Duo Security. If the above condition holds true, how does CAS know that the authentication flow should be routed to Duo Security?

Per the CAS documentation:

Trigger MFA based on a principal attribute(s) whose value(s) matches a regex pattern. Note that this behavior is only applicable if there is only a single MFA provider configured since that would allow CAS to know what provider to next activate.

In other words, if the above condition holds true and CAS is to route to a multifactor authentication flow, that would obviously be one supported and provided by Duo Security since that’s the only provider that is currently configured to CAS. Of course, if there are multiple providers available at runtime (i.e. Duo Security, YubiKey, etc) then we would need massage the condition since the automatic detection of the multifactor provider would not be immediately obvious…and that sort of thing would be outside the scope of this tutorial.

Summary

I hope this review was of some help to you and I am sure that both this post as well as the functionality it attempts to explain can be improved in any number of ways. Please feel free to engage and contribute as best as you can.

Misagh Moayyed

[1] No birds were harmed during the production of this blog post.

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