A flaw was found in Next.js. This vulnerability allows a remote attacker to potentially access confidential data. When a server-side fetch processes a request with a body using a non-UTF-8 character set, it may incorrectly return a cached response body from a different request to the same URL but different body. This can lead to the unauthorized disclosure of sensitive information.

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 is only an issue when receiving request bodies with a content type charset other than UTF-8. For example, the UTF-16 byte sequences for 삃삃 and 섄섄 in the request body would share the same cache. 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 containing invalid UTF-8 byte sequences