Is there a way to disallow external applicants from applying to internal postings?

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We have internal and external career centers setup.  The internal career centre is part of our company intranet and thus can only be accessed by internal employees, however, an internal employee sent a job posting to a non-employee and the non-employee was able to access the link and apply for the job. Is this 'normal" functionality or is there a way to configure something so that an external candidate can not apply to an internal job posting? We are not currently using any sort of single sign on solution.

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TBE