Next.js has a Middleware / Proxy bypass in Pages Router applications using i18n

Impact

Applications using the Pages Router with i18n configured and middleware/proxy-based authorization can allow unauthorized access to protected page data through locale-less /_next/data/<buildId>/<page>.json requests. In affected configurations, middleware does not run for the unprefixed data route, allowing an attacker to retrieve SSR JSON for protected pages without passing the intended authorization checks.

Fix

The matcher logic was updated to perform the same match as it would on a non-i18n data route.

Workarounds

If you cannot upgrade immediately, enforce authorization in the page's server-side data path instead of relying solely on middleware.