Question: Who is bound to secrecy by the "seal of Confession"?
Answer: In the general sense, everyone is. The Sacrament of Confession, and the confidence that people can receive that Sacrament without having their sins made public, is of great importance to the entire society. This is recognized by civil as well as church law. So, anyone who becomes aware of what is mentioned in Confession is obligated not to repeat it to anyone else.
In the more specific sense some distinctions can be made. The priest is bound by canon law to keep the Sacramental seal inviolate. He may not reveal the sins confessed by a specific penitent, nor may he use knowledge gained from the confession to the detriment of the penitent.1 This obligation extends to an interpreter if one must be employed, and to anyone else who comes by knowledge of the confession by any way (e.g. by overhearing it, or by finding a written list of the penitent's sins).2 The priest who violates the seal of the Sacrament automatically incurs an excommunication that only the Pope can absolve under normal conditions. A variety of other penalties may be imposed, including reduction to the lay state.3 No evil is too great to be suffered and no benefit is so good that avoiding its loss would be an excuse for violating the Sacramental seal. Priests must be prepared for martyrdom rather than compromise this essential principle.
The penitent is also obligated not to discuss with others the advice given by the priest in Confession. This arises from the inability of the priest to refute claims made against his wisdom or reputation by citing the circumstances of his remarks. To gossip about the quality of advice received from a confessor would constitute, at least, the sins of detraction and betrayal of confidence.
A penitent may, however, approach another priest if he feels he has been assigned an excessive penance. This can be done without naming the first priest, for the purpose of receiving a lesser penance. Only for the most serious reasons should a penitent ever name the priest in a complaint, and then only to the priest's lawful superior. There is an obligation to lodge a complaint with the bishop (or the Holy Office) if the confessor tries to induce the penitent to sin against the sixth Commandment (or to have the penitent induce another to do so).4
Saint John Nepomucen (c. 1345-1393; feastday, May 16) is the patron saint of confessional secrecy, having been martyred by the jealous King Wenceslaus IV of Bohemia for refusing to reveal the contents of the Queen's confessions.