Recently, Yasser Allam, known by the pseudonym inzo_, and I, decided to team up for some research. We discussed potential targets and chose to begin by focusing on Next.js (130K stars on github, currently downloaded + 9,4 million times per week), a framework I know quite well and with which I already have fond memories, as evidenced by my previous work. Therefore, the “we” throughout this paper will naturally refer to the two of us.

Next.js is a comprehensive javascript framework based on React, packed with numerous features — the perfect playground for diving into the intricacies of research. We set out, fueled by faith, curiosity, and resilience, to explore its lesser-known aspects, hunting for hidden treasures waiting to be found.

It didn’t take long before we uncovered a great discovery in the middleware. The impact is considerable, with all versions affected, and no preconditions for exploitability — as we’ll demonstrate shortly.