CVE-2026-16117: Fastify HTTP Proxy versions up to 11.5.0 expose internal paths

Summary

An attacker can access internal or administrative paths on a server protected by a Fastify HTTP proxy if the proxy is configured to rewrite a URL prefix that contains encoded characters. To fix this, update the Fastify HTTP Proxy to version 11.6.0. There is no temporary workaround available.

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 up to and including 11.5.0 fail to rewrite the request prefix when the prefix segment is URL-encoded. Fastify's router URL-decodes paths for route matching, but...

Original description

Impact: @fastify/http-proxy versions up to and including 11.5.0 fail to rewrite the request prefix when the prefix segment is URL-encoded. Fastify's router URL-decodes paths for route matching, but request.url retains the original encoded form, and the prefix-rewrite step uses a literal string replace against the decoded prefix. A request that encodes one or more characters of the configured prefix therefore matches the route but skips the rewrite, so the raw encoded path is forwarded to the upstream unchanged. The upstream then decodes the path and serves it, letting an attacker reach upstream paths that the proxy was configured to hide via rewritePrefix, including internal or administrative endpoints.

Patches: upgrade to @fastify/http-proxy 11.6.0.

Workarounds: none.

nvd CVSS3.1 10.0

Vulnerability type

CWE-20 Improper Input Validation