2 Timothy 3: 8. Now as Jannes and Jambres withstood Moshe, so also do these withstand the truth: men whose mind is corrupted and (are) reprobates from the faith. 9. But they will not make progress, for their infatuation will be understood by everyone, as theirs also was understood. (AENT)
This is a very interesting reference by Rav Shaul as Jannes and Mambres are never directly named in any ancient version of the Tanakh as the magicians who tried to compete with Moshe. However, they are referred to in some Jewish targums and even in Roman sources such as Pliny the Elder’s Natural History. Also the late Second century Church Father Origen made reference to “the Book of Jannes and Mambres” but no copies of it have come down to us intact. What this shows is that Rav Shaul is comfortable using both canonical and non-canonical sources to make his points with what he knows his audience is familiar with.
While we may never know if Rav Shaul considered these names as authentic, it very much goes to acquitting other parts of the Renewed Covenant writings as spurious simply because they quoted from resources that we today don’t give any authority to. We have already witnessed other places where Rav Shaul quotes from even pagan poets to make his point if need be. For more information, please consult the footnotes for Titus 1:11 and Yehuda 1:14. The important distinction is, as verse 16 shows, the ultimate authority on proper behavior remains with Tanakh.