CVE-2026-15631 @fastify/http-proxy vulnerable to prefix escape via WebSocket path traversal
Attackers can bypass proxy restrictions to access internal upstream endpoints via WebSocket path traversal in @fastify/http-proxy.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-15631?
CVE-2026-15631 is a high-severity path traversal vulnerability in @fastify/http-proxy, published on 18 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-22
Which versions are affected by CVE-2026-15631?
It affects 1 product.
Products include Fastify Fastify\/Http-Proxy.
How do I fix CVE-2026-15631?
No fix has been published for CVE-2026-15631 yet. Limit exposure of the affected software until one is available.
Upgrade, and resolve every user-supplied path against an allowed base folder before opening it. More about Path traversal.
Is CVE-2026-15631 being exploited?
There is no known exploitation of CVE-2026-15631 in the wild and no public exploit code.
Other vulnerabilities in @fastify/http-proxy
Sources
Also known as GHSA-7hrw-592w-9wh2.