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CVE-2025-30218: Next.js may leak x-middleware-subrequest-id to external hosts

• Severity: medium (CVSS 5.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-04-02 · updated 2026-06-17
• Weakness: CWE-200

Description

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 incoming requests. However, this subrequest ID is sent to all requests, even if the destination is not the same host as the Next.js application. Initiating a fetch request to a third-party within Middleware will send the x-middleware-subrequest-id to that third party. This vulnerability is fixed in 12.3.6, 13.5.10, 14.2.26, and 15.2.4.

Affected products

• Next.js: 12.3.5, 13.5.9, 14.2.25, 15.2.3

References

• https://github.com/vercel/next.js/security/advisories/GHSA-223j-4rm8-mrmf
• https://vercel.com/changelog/cve-2025-30218-5DREmEH765PoeAsrNNQj3O

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