In PAM, what is the implication of marking a module as "required"?

Marking a module as "required" in PAM (Pluggable Authentication Module) means that the module must always succeed for the associated application to continue processing. If the "required" module fails, the overall authentication process will fail too, regardless of the results of other modules that may be checked thereafter. This ensures that certain authentication criteria are met before allowing access to a system or application.

For instance, if a system administrator marks a password authentication module as "required," a user will not be granted access unless they provide a correct password, thereby enforcing a critical security measure. This is different from other potential configurations, such as "sufficient," where a module's success could be enough on its own to allow access, or "optional," where its success or failure does not influence the overall outcome. Therefore, the distinction of marking a module as "required" fundamentally impacts the authentication flow and overall security posture of the application.