Traditionally, all 13 of the Pauline epistles have been attributed to St. Paul.
The notion that these may be pseudo-epigraphia (that is, written by someone else and then attributed to St. Paul) is a very recent one, hardly a century and a half old. The arguments for this position can get quite abstruse, and are based on word usage, writing style, perceived theological differences, and so on.
However, these arguments are not conclusive. Even liberal Bible commentators (such as the annotators of the Jerusalem Bible) have remarked that Pauline authorship is still both possible and probale. And there have been critiques of the “historical-critical” method.
Personally, I hold to Pauline authorship. Each of the letters was written to a particular church or person, and their specific troubles or concerns could have been quite different. (For example, 1 and 2 Corinthians deal with the troubling issue of excommunication and reconciliation - it’s natural that these are more “emotional” than Romans, which is a systematic series of theological and doctrinal points.) The Pontifical Biblical Commission (if I’m not mistaken) also said the same some time ago, and the Church has never “officially” endorsed the “pseudo-epigraphia” theory, even if such views appear in the notes to some Catholic Bibles.