CVE-2026-15631: @fastify/http-proxy vulnerable to prefix escape via WebSocket path traversal
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 Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
@fastify/http-proxyto a version that resolves this vulnerability.Fixed in 11.6.0
Event History
Frequently Asked Questions
What is the severity of CVE-2026-15631?
CVE-2026-15631 has a high severity score of 8.7.
How do I fix CVE-2026-15631?
To fix CVE-2026-15631, upgrade @fastify/http-proxy to version 11.6.0 or later.
What is the impact of CVE-2026-15631?
The impact of CVE-2026-15631 includes the potential for prefix escape via WebSocket path traversal, allowing unauthorized access to resources.
Which versions are affected by CVE-2026-15631?
CVE-2026-15631 affects @fastify/http-proxy versions from 9.4.0 up to and including 11.5.0.
What type of vulnerability is CVE-2026-15631 classified as?
CVE-2026-15631 is classified as a Path Traversal vulnerability.