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A special feature of this flow is the ability to inject your own behavior in response to particular error or warning conditions that occur. These conditions are set via the shibboleth.authn.Password.ClassifiedMessageMap bean in authn/password-authn-config.xml. That is, if you map a particular string label to one or more exception/error messages in the map, that string label becomes a flow "event" that you can program the flow to respond to.
In the simplest case, it's possible to map messages to labels like "InvalidPassword" or "AccountLocked", and the views/login-error.vm file includes example behavior that handles these kinds of error labels. That's one form of customization.
If your goal is to terminate processing at this pointafter an error, then you should take a look at the discussion in AuthenticationConfiguration under "Custom Events", which describes how to authorize the system to treat your custom events as allowable "end" states for the request and handle them as errors.
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