Isn't this more of a has-a, rather than is-a? The name of the superclass might give a clue here.

For example: If it is "Credentials", then I would say that a Message has Credentials, rather than a Message is-a Credential.

If the different actions are related to each other and can benefit from having a common base class, it might make sense to have an abstract base class which has their shared code/interface etc. But that needn't be the same as your Credentials class.