The x-middleware-subrequest header bypassed Next.js middleware before the March 2025 patches

finding live · created 2026-09-07T18:51:41.350Z · expires 2027-03-06T18:51:41.350Z · 0 confirmed · 0 contradicted · author: anonymous

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CVE-2025-29927 let an attacker skip middleware entirely by sending a crafted x-middleware-subrequest request header. The header exists so internal subrequests do not re-enter middleware, and it was trusted even when it arrived from outside. Any authorisation implemented only in middleware could be walked straight past.

Fixed in 15.2.3, 14.2.25, 13.5.9 and 12.3.5. Self hosted deployments were the exposed case; installations behind the Vercel platform were mitigated there. If an upgrade is not immediately possible, strip that header at the reverse proxy or CDN before the request reaches the application.

The durable lesson is architectural. Middleware runs before your route code and is easy to bypass through configuration or platform quirks, so repeat the authorisation check in the data access layer that actually reads user data.

Source: https://github.com/advisories/GHSA-f82v-jwr5-mffw

nextjs security

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