Can prospective employer contact an old boss?
A prospective employer can contact anyone they wish, including but not limited to your former or old boss, a co-worker, or clients or customers, about you.
We would all like to be able to control the information released about us and who can comment about us, especially in an employment contexts--and particularly so when there is a former job from which we were fired, or a former boss with whom we had a bad relationship. We'd like to be able, for example, to prevent prospective employers from contacting that prior boss whose employment we left on bad terms. Unfortunately, we can't--at least, not unless there is some written agreement (such as Separation and Release [SRA]) or severance agreement, which, as part of its terms, restricts the former employer from discussing our employment. In the absence of a written agreement, a prospective or potential employer can contact ANYONE he or she wants: a former boss; former co-workers; clients or customers; even the barista at your local Starbucks. The simple fact is, we can't control who talks about us, or who someone inquires of about us.
That's not to say that there aren't some things you can't do to protect yourself from bad reviews.
---First: don't list a former job or boss you don't want contacted. This may not be possible, of course. If you just left or are leaving a job after six years, for example, you'll have a large, unexplained gap in your resume if you don't list this job. But if you were only there briefly, or if it's several jobs ago, leave it off your resume. Who your prospective employer does not know about, they cannot contact. There is no central repository or listing of your jobs. A prospective employer will only know about your former jobs to the extent you disclose them.
---Second: when leaving a job, if there is a SRA, negotiate a clause either restricting your soon-to-be-ex-job from speaking to anyone about you, or at least restricting what they can say. You can't always get such a clause or term into an SRA (and, of course, there often is no SRA), but when you can, it is as enforceable as any other contract.
--Third: if the former boss says anything provably false about you (for example: they say you were fired for absenteeism when you in fact resigned), you could sue them for defamation, to get compensation for their harmful and untrue statement about you.
But while these options can help to protect you, unfortunately you cannot stop your new or potential boss from contacting an old boss without a written agreement tying your old boss’s hands in regard to what they say about you.