fastify http-proxy up to 11.5.0 WebSocket Routing findUpstream path traversal

Summaryinfo

A vulnerability marked as critical has been reported in fastify http-proxy up to 11.5.0. Impacted is the function findUpstream of the component WebSocket Routing. Performing a manipulation results in path traversal. This vulnerability is known as CVE-2026-15631. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability, which was classified as critical, was found in fastify http-proxy up to 11.5.0. This affects the function findUpstream of the component WebSocket Routing. The manipulation with an unknown input leads to a path traversal vulnerability. CWE is classifying the issue as CWE-22. The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-15631 since 07/13/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1006 according to MITRE ATT&CK.

Upgrading to version 11.6.0 eliminates this vulnerability.

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Productinfo

Type

• Firewall Software

Vendor

• fastify

Name

• http-proxy

Version

• 11.0
• 11.1
• 11.2
• 11.3
• 11.4
• 11.5.0

CPE 2.3info

• 🔒
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CPE 2.2info

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• 🔒
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