CVE-2026-15631: Fastify HTTP Proxy allows attackers to bypass security settings

Summary

The Fastify HTTP Proxy software has a security flaw that allows attackers to access sensitive information by manipulating URLs. This issue affects versions 9.4.0 to 11.5.0 and can be exploited by certain types of clients. To fix this issue, update to version 11.6.0.

What to do

No fix is available yet. Check with your software vendor for updates.

Affected software

Vendor	Product	Affected versions
@fastify/http-proxy	@fastify/http-proxy	< 11.6.0

Original title

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...

Original 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.

nvd CVSS3.1 8.7

Vulnerability type

CWE-22 Path Traversal