As in many issues, the Catholic Chuch has no official position on the authorship of Paul. It is something for biblical scholars to peruse. What the Church does insist upon is the apostolic authorship of all the NT writings. Traditionally, the authorship of Hebrews has been ascribed to Paul.
The letter to the Hebrews appears in the New Testament after the thirteen Pauline letters and before the seven catholic letters. Early tradition, in the main, attributed this text to Paul, but the western Church did not accept its Pauline authorship until the fourth century; and even in the east some (including Clement of Alexandria and Origen) had reservations about whether its literary style coincided with Paul’s.
Internal examination of the text does show that it is in many ways different from the rest of Paul’s writings. For example, it is more elegant, more eloquent, it does not carry the usual greeting and introduction, and it does not quote Scripture in the way Paul does. Its doctrine is Pauline but the way it is expounded makes it difficult to attribute its direct authorship to Paul. The letter’s canonicity is not in doubt; it was included in the canon by the Council of Trent (8 April 1546) among the other writings of Paul, although the Council chose not to state categorically that it was written by Paul.
The Pontifical Biblical Communion, in a decree issued on April 24, 1914, reaffirmed its canonicity. It answered the question, “Has the apostle Paul to be regarded as the author of this letter in the sense that not only must one hold that he conceived it and expressed it under the inspiration of the Holy Spirit, but that he gave it the form in which it has come down to us?” Its reply was, “No, not unless the Church decides so in the future.” This is probably why there is no direct reference to Paul as author of this letter in recent liturgical books. However, Paul can be regarded as the indirect author of Hebrews. Researchers are free to explore this matter.
Some scholars think it may have been written by Barnabas or Silas, disciples of Paul; others suggest Apollos, an Alexandrian Jew noted for his eloquence (ct. Acts 18 24:28), in view of the way it quotes the Old Testament and its beautiful style and language. In any event, this is a secondary question which has nothing to do with matters of faith.
To me, since it can’t be ruled out, and tradition usually has at least some basis in fact, I always refer to the writer of Hebrews as Paul. Unless the Church says I can’t (an unlikely prospect), I’ll continue to do so.