Canon law is very explicit on this...
The priest can't betray the person confessing (983.1)
The sacramental seal is inviolable; therefore it is absolutely forbidden for a confessor to betray in any way a penitent in words or in any manner and for any reason.
Sacramentale sigillum inviolabile est; quare nefas est confessario verbis vel alio quovis et quavis modo de causa aliquatenus prodere paenitentem.
Considering these rules, if the priest confessing sexual attacks only ever discloses it in the sacrament of reconciliation, he is free from any disciplinary action from the bishop.
If he were to disclose such abuses outside the sacrament of reconciliation, (or get caught), only then is he vulnerable to disciplinary actions.
Let's say the Bishop was suspicious about a priest and decided to tell a random person to eavesdrop on the confession...
Canon Law 983.2 covers that
The interpreter, if there is one, and all others who in any way have knowledge of sins from confession are also obliged to observe secrecy.
The priest facing disciplinary actions as a result of such abuses (hopefully) either got caught or disclosed it outside the sacrament of reconciliation.
The priest hearing this confession can encourage him to give himself up.
What if the Priest told the guy confessing that his penance is giving himself up?
Using the example of another type of sin, (murder) Fr. Charles Grondin answered this question saying...
To require that someone turn himself in to the police would be to require him to reveal his confession to a third party. While the sinner is free to do so if he chooses, to require it would appear to be the equivalent of the priest revealing the contents of the confession via a secondary source. In light of canon 984, the priest only knows who the murderer is based on the confession. To require the penitent to turn himself in to the police would be to use that information to the detriment of the penitent. And if the absolution is essentially conditional upon this requirement it would seem to also violate canon 980. If someone is not sufficiently contrite the confessor can simply withhold absolution. If the penitent is sufficiently contrite the priest can most certainly encourage him to turn himself in to the police but to require someone to reveal their confession to another would violate canon law.
Can. 980 If the confessor is in no doubt about the penitent's disposition and the penitent asks for absolution, it is not to be denied or delayed.
Can. 984 §1. A confessor is prohibited completely from using knowledge acquired from confession to the detriment of the penitent even when any danger of revelation is excluded.