As one might guess, a humblebrag is one who uses a pretense of humility as a vehicle for boasting. This word can be found at Urban Dictionary but, it appears, nowhere more authoritative. Yet, of course, the concept is neither new nor esoteric; and one would expect there to be a word to cover it. Of course, we have the idea of false humility and could speak of one who is falsely humble or who is guilty of false modesty.
But isn't there a noun to denote the person who behaves in this way, and an adjective to describe him?