What happens if a PAM module returns a fail status code after all modules have run?

When using Pluggable Authentication Modules (PAM) for handling authentication, each module can return status codes indicating whether the authentication process was successful, failed, or that further action is required. If one or more modules return a fail status code, PAM will ultimately propagate that failure up the stack.

If all the modules have run and at least one of them returned a fail status, a final fail status will be returned to the calling application. This ensures that the authentication process does not succeed if any single module indicates a failure, adhering to the principle that successful authentication requires all parts of the process to validate the user’s identity.

This behavior is crucial for maintaining security, as it prevents unauthorized access by ensuring that failure at any step results in an overall authentication failure.