2. According to ecclesiastical law, however, no penalty is specified for cardinals who break their solemn oath. The small number of support staff who are permitted into conclave precincts are bound by oath to observe secrecy and their oath is enforceable by penalty, but even there the penalty is not excommunication. Rather, it is that “which the future Supreme Pontiff will see fit to adopt, in accordance with Canon 1399 of the Code of Canon Law”(UDG ¶ 48).The sanction enabled by 1983 CIC 1399 is “a just penalty” which might or might not extend all the way to excommunication. There is also ¶ 55 that threatens "grave penalties according to the judgment of the future Pope" for those who violate secrecy, but the context of ¶ 55 is electronic eavesdropping, and in any case, the penalty need not be excommunication. Per ¶ 71, elector notes are to be burned, but again, no specific penalty (excommunication or otherwise) is attached to the violation of this norm.
In brief, if this sad story is true and if the offending cardinal is identified, Pope Benedict XVI may deal with the situation any way he sees fit, for no specific response to this particular offense is dictated by the words of Church law.+++ The Next Papal Conclave: Current Eligible Electors
Read read all of Dr. Peters' comments here.
So excommunication is not the penalty, but rather it is at the discretion of the Supreme Pontiff himself. Can one strip a cardinal of his cardinal robes?