Objection 1. It would seem that, outside a case of necessity, no one but a priest may hear the confession of venial sins. For the dispensation of a sacrament is committed to a layman by reason of necessity. But the confession of venial sins is not necessary. Therefore it is not committed to a layman.
Objection 2. Further, Extreme Unction is ordained against venial sin, just as Penance is. But the former may not be given by a layman, as appears from James 5:14. Therefore neither can the confession of venial sins be made to a layman.
On the contrary, is the authority of Bede (on James 5:16, "Confess... one to another") quoted in the text (Sent. iv, D, 17).
I answer that, By venial sin man is separated neither from God nor from the sacraments of the Church: wherefore he does not need to receive any further grace for the forgiveness of such a sin, nor does he need to be reconciled to the Church. Consequently a man does not need to confess his venial sins to a priest. And since confession made to a layman is a sacramental, although it is not a perfect sacrament, and since it proceeds from charity, it has a natural aptitude to remit sins, just as the beating of one's breast, or the sprinkling of holy water (cf. III, 87, 3).
This suffices for the Reply to the First Objection, because there is no need to receive a sacrament for the forgiveness of venial sins. and a sacramental, such as holy water or the like, suffices for the purpose.
Reply to Objection 2. Extreme Unction is not given directly as a remedy for venial sin, nor is any other sacrament.
Source: Summa Theologica (New Advent)