CVE-2026-15631
Published:July 18, 2026
Updated:August 06, 2026
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 Packages
@fastify/http-proxy (NPM):
Affected version(s) >=9.4.0 <11.6.0Fix Suggestion:
Update to version 11.6.0Related Resources (4)
Do you need more information?
Contact UsCVSS v4
Base Score:
9.4
Attack Vector
NETWORK
Attack Complexity
HIGH
Attack Requirements
NONE
Privileges Required
NONE
User Interaction
NONE
Vulnerable System Confidentiality
HIGH
Vulnerable System Integrity
HIGH
Vulnerable System Availability
NONE
Subsequent System Confidentiality
HIGH
Subsequent System Integrity
HIGH
Subsequent System Availability
NONE
CVSS v3
Base Score:
8.7
Attack Vector
NETWORK
Attack Complexity
HIGH
Privileges Required
NONE
User Interaction
NONE
Scope
CHANGED
Confidentiality
HIGH
Integrity
HIGH
Availability
NONE
Weakness Type (CWE)
Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
EPSS
Base Score:
0.33