In the process of remediating CVE-2025-29927, we looked at other possible exploits of Middleware. We independently verified this low severity vulnerability in parallel with two reports from independent researchers.

Link to headingSummary

To mitigate CVE-2025-29927, Next.js validated the x-middleware-subrequest-id which persisted across multiple incoming requests:

 const randomBytes = new Uint8Array(8) 
 crypto.getRandomValues(randomBytes) 
 const middlewareSubrequestId = Buffer.from(randomBytes).toString('hex') 
 ;(globalThis as any)[Symbol.for('@next/middleware-subrequest-id')] = 
   middlewareSubrequestId 

However, this subrequest ID is sent to all requests, even if the destination is not the same host as the Next.js application.

 init.headers.set( 
   'x-middleware-subrequest-id', 
   (globalThis as any)[Symbol.for('@next/middleware-subrequest-id')] 
 ) 

Initiating a fetch request to a third-party within Middleware will send the x-middleware-subrequest-id to that third party.

Link to headingImpact

While the exploitation of this vulnerability is unlikely due to an attacker requiring control of the third-party, we want to be proactive. We were already planning on removing this recursion prevention logic from Middleware—it was not supported in newer updates to Middleware to support the Node.js runtime—this disclosure expedited our efforts to bring parity between runtimes.

Vercel customers are protected with mitigations already implemented within our platform environment. We still encourage teams to update to the latest Next.js patch version or their chosen backport. Other infrastructure providers which host Next.js applications are not impacted by this, as it is specific to Vercel's implementation of recursion protection.

Link to headingRemediation

This advisory was published in alignment with our new internal process for disclosure of vulnerabilities within OSS packages, based on our postmorten of CVE-2025-29927. We’ve patched 15.x, and offered backports for versions 12.x through 14.x, making an exception to our newly published LTS policy.

We’ve also worked proactively with new partners to Next.js for early disclosure. If you are an infrastructure provider and want to work with us, please email partners@nextjs.org.

Link to headingCredit

Thank you to Jinseo Kim (kjsman) and ryotak for the responsible disclosure. These researchers were awarded as part of our bug bounty program.