Vulnerability · Published Jul 18, 2026 · Assigned by openjs
CVE-2026-15631@fastify/http-proxy vulnerable to prefix escape via WebSocket path traversal
High 8.7
In plain English
A high-severity path traversal in @fastify/http-proxy from @fastify/http-proxy. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 43% of all known vulnerabilities).
Official 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.
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| @fastify/http-proxy | @fastify/http-proxy | 9.4.0 – before 11.6.0 |
Weakness: CWE-22 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:N