Next.js may leak x-middleware-subrequest-id to external hosts
## Summary In the process of remediating [CVE-2025-29927](https://github.com/advisories/GHSA-f82v-jwr5-mffw), we looked at other possible exploits of Middleware. We independently verified this low severity vulnerability in parallel with two reports from independent researchers. Learn more [here](https://vercel.com/changelog/cve-2025-30218-5DREmEH765PoeAsrNNQj3O). ## Credit Thank you to Jinseo Kim [kjsman](https://hackerone.com/kjsman?type=user) and [RyotaK](https://hackerone.com/ryotak?type=user) (GMO Flatt Security Inc.) with [takumi-san.ai](https://takumi-san.ai) for the responsible disclosure. These researchers were awarded as part of our bug bounty program.
describes (1)
Related CVEs.
| ID | severity | exploitable | ||
|---|---|---|---|---|
| CVE-2025-30218US | medium5.9 | — | ||
Next.js is a React framework for building full-stack web applications. To mitigate CVE-2025-29927, Next.js validated the x-middleware-subrequest-id which persisted across multiple… | ||||
references
External citations — patches, advisories, analysis.
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