CVE-2026-29057: Next.js is a React framework for building full-stack web applications. Starting in version 9.5.0 and prior to versions 15.5.13 and 16.1.7, when Next.j...

Description

Next.js is a React framework for building full-stack web applications. Starting in version 9.5.0 and prior to versions 15.5.13 and 16.1.7, when Next.js rewrites proxy traffic to an external backend, a crafted `DELETE`/`OPTIONS` request using `Transfer-Encoding: chunked` could trigger request boundary disagreement between the proxy and backend. This could allow request smuggling through rewritten routes. An attacker could smuggle a second request to unintended backend routes (for example, internal/admin endpoints), bypassing assumptions that only the configured rewrite destination/path is reachable. This does not impact applications hosted on providers that handle rewrites at the CDN level, such as Vercel. The vulnerability originated in an upstream library vendored by Next.js. It is fixed in Next.js 15.5.13 and 16.1.7 by updating that dependency’s behavior so `content-length: 0` is added only when both `content-length` and `transfer-encoding` are absent, and `transfer-encoding` is no longer removed in that code path. If upgrading is not immediately possible, block chunked `DELETE`/`OPTIONS` requests on rewritten routes at the edge/proxy, and/or enforce authentication/authorization on backend routes.

Metrics

CVSS 3.1

6.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

CVSS 4.0

6.3/10

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

EPSS Probability

0.43%

34.2th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-444

Affected Software

Vendor	Product	Versions
Vercel	Next.Js	>= 9.5.0, < 15.5.13
Vercel	Next.Js	>= 16.0.0, < 16.1.7

References

https://github.com/vercel/next.js/commit/dc98c04f376c6a1df76ec3e0a2d07edf4abdabd6Patch
https://github.com/vercel/next.js/releases/tag/v15.5.13Product, Release Notes
https://github.com/vercel/next.js/releases/tag/v16.1.7Product, Release Notes
https://github.com/vercel/next.js/security/advisories/GHSA-ggv3-7p47-pfv8Mitigation, Vendor Advisory

Timeline

Published: Mar 18, 2026
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2026-29057?

Next.js is a React framework for building full-stack web applications. Starting in version 9.5.0 and prior to versions 15.5.13 and 16.1.7, when Next.js rewrites proxy traffic to an external backend, a crafted `DELETE`/`OPTIONS` request using `Transfer-Encoding: chunked` could trigger request boundary disagreement between the proxy and backend. This could allow request smuggling through rewritten routes. An attacker could smuggle a second request to unintended backend routes (for example, internal/admin endpoints), bypassing assumptions that only the configured rewrite destination/path is reachable. This does not impact applications hosted on providers that handle rewrites at the CDN level, such as Vercel. The vulnerability originated in an upstream library vendored by Next.js. It is fixed in Next.js 15.5.13 and 16.1.7 by updating that dependency’s behavior so `content-length: 0` is added only when both `content-length` and `transfer-encoding` are absent, and `transfer-encoding` is no longer removed in that code path. If upgrading is not immediately possible, block chunked `DELETE`/`OPTIONS` requests on rewritten routes at the edge/proxy, and/or enforce authentication/authorization on backend routes.

How severe is CVE-2026-29057?

CVE-2026-29057 has a CVSS score of 6.3/10 (MEDIUM severity). The EPSS model estimates a 0.43% probability of exploitation in the next 30 days.

How do I fix CVE-2026-29057?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.