https://zhero-web-sec.github.io/research-and-things/nextjs-and-the-corrupt-middleware
A critical vulnerability, CVE-2025-29927, has been discovered in the Next.js web development framework, enabling attackers to bypass authorization checks. This flaw allows malicious actors to send requests that bypass essential security measures.
Next.js, a popular React framework used by companies like TikTok, Netflix, and Uber, utilizes middleware components for authentication and authorization. The vulnerability stems from the framework’s handling of the “x-middleware-subrequest” header, which normally prevents infinite loops in middleware processing. Attackers can manipulate this header to bypass the entire middleware execution chain.
The vulnerability affects Next.js versions prior to 15.2.3, 14.2.25, 13.5.9, and 12.3.5. Users are strongly advised to upgrade to patched versions immediately. Notably, the flaw only impacts self-hosted Next.js applications using “next start” with “output: standalone.” Applications hosted on Vercel and Netlify, or deployed as static exports, are not affected. As a temporary mitigation, blocking external user requests containing the “x-middleware-subrequest” header is recommended.