It seems like plain unflavored coffee does not need a kosher certification.
Does flavored drip/ground coffee require a hechsher?
Mi Yodeya is a question and answer site for those who base their lives on Jewish law and tradition and anyone interested in learning more. It only takes a minute to sign up.Sign up to join this community
From The Chicago Rabbinical Council website
A general rule in kashrus is that any item which is flavored requires kosher certification whether the flavor is labeled as natural or artificial, and flavored coffee is no exception to the rule.
And from the Star-K -The Kashrus of Coffee
Flavors are complex chemical products which contain various ingredients, some natural and others artificial. There are more than three thousand flavor ingredients to choose from in formulating a flavor. Some are clearly not kosher. To complicate matters further, food labels are only required to mention that flavors were added without listing the specific flavor. Based on this it is easy to see why all flavored coffees must be kosher certified.
Unflavored coffee is a fruit according to halacha, like any fruit outside of israel it does not require a hechsher ( I believe that coffee is not being grown in Israel, but if it were, it would need a hechsher for orlah, shmittah, trumah etc). However flavoring has many ingredients, a chemist for the OU Rebbetzin Leff told me coffee flavor has over 200 ingredients, which although batel in quantity to the main ingredient, cant be batel lechatchila.