Next.js is a React framework for building full-stack web applications. From 13.0.0 until 15.5.28 and 16.4.1, crafted requests targeting App Router applications with at least one Server Action can cause excessive CPU consumption and block processing of further requests in the same process. Applications using the Pages Router and App Router applications without Server Actions are not affected. This issue is fixed in versions 15.5.28 and 16.4.1.