Right now there is in fairly wide use an LDAP authentication module, and recently a shibboleth authentication module has been contributed but is yet in the main webwork code repository. (But you could probably use it if you want.) There's also one for "Cosign," but I think that may be specific to Michigan?
Unless you can use one of those protocols, you'll have to write your own authenication submodule. It shouldn't be too hard; maybe this would help:
Also, if you've got your CAS system already hooked up for Moodle (or, soon, Blackboard) you could use that as a frontend for WeBWorK and have it do the authentication.