Vulnerability Description
Impact: @fastify/http-proxy versions from 9.4.0 up to and including 11.5.0 fail to validate the resolved WebSocket destination path against the configured rewrite prefix. The WebSocket routing path in WebSocketProxy.findUpstream resolves the destination via the WHATWG URL constructor, which collapses dot segments, so a crafted upgrade request with path traversal sequences can escape the rewrite prefix and reach upstream endpoints that were not meant to be exposed by the proxy. This is a variant of CVE-2021-21322 in a code path that never went through the HTTP fix in fastify/reply-from. Exploitation requires a non-normalizing WebSocket client, since browsers and the ws package normalize the request path before sending, but raw HTTP clients or downstream proxies that forward the request target unchanged make the attack reachable in production topologies. Patches: upgrade to @fastify/http-proxy 11.6.0. Workarounds: none.
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Fastify | Fastify\/Http-Proxy | >= 9.4.0, < 11.6.0 |
Related Weaknesses (CWE)
References
- https://cna.openjsf.org/security-advisories.htmlVendor Advisory
- https://github.com/fastify/fastify-http-proxy/security/advisories/GHSA-7hrw-592wVendor Advisory
FAQ
What is CVE-2026-15631?
CVE-2026-15631 is a vulnerability with a CVSS score of 8.7 (HIGH). Impact: @fastify/http-proxy versions from 9.4.0 up to and including 11.5.0 fail to validate the resolved WebSocket destination path against the configured rewrite prefix. The WebSocket routing path in...
How severe is CVE-2026-15631?
CVE-2026-15631 has been rated HIGH with a CVSS base score of 8.7/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-15631?
Check the references section above for vendor advisories and patch information. Affected products include: Fastify Fastify\/Http-Proxy.