A flaw was found in Next.js. A remote attacker could exploit a vulnerability in the server-side fetch mechanism, where a request with a body might receive a cached response intended for a different request to the same URL but different body. This could lead to the unauthorized disclosure of confidential data from POST request response bodies to an unprivileged user.

Next.js is a React framework for building full-stack web applications. In versions 12.0.0 through 15.5.20 and 16.0.0 through 16.2.10, a server-side fetch with a request body may return a cached response body from a different request to the same URL but different body. Confidential data in the POST's response body would then leak to unauthorized requests. Though the request itself will not be deduped. This only applies to fetch calls with a request that has a different init than the one passed to fetch. A safe request would be: fetch(new Request(init), init). An unsafe request would be: fetch(new Request(init), aDifferentInit). This issue has been fixed in versions 15.5.21 and 16.2.11.

Next.js: Cache confusion of response bodies for requests with bodies