What are is the law regarding harboring a fugitive?

If done knowingly, it's against the law and is itself a crime. The issue is--as with any crime--intention or knowledge; so if someone harbors a fugitive, knowing that he or she is a fugitive, that person has committed a crime. On the other hand, if the person has no knowledge--and no reasonable way or likelihood of knowing--that the fugitive is in fact a fugitive (is fleeing arrest, prosecution, probation, etc.), then it would not be a criminal act to "harbor" them--at least until the person becomes aware of the situation. In addition, there may be civil penalties as well as criminal liability for harboring a fugitive in some cases; for example, public housing leases will usually make it a basis for eviction to harbor a fugitive.