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CVE-2026-16117 is a critical access-control bypass vulnerability in @fastify/http-proxy affecting versions up to and including 11.5.0. The flaw arises from inconsistent handling of URL-encoded path segments during proxy routing and prefix rewriting. Fastify route matching operates on a URL-decoded path, allowing requests with encoded characters in the configured proxy prefix to match the intended route. However, the proxy rewrite logic uses a literal string replacement against the decoded prefix while the request URL retains its original encoded form. As a result, a crafted request that URL-encodes one or more characters of the configured prefix can match the proxy route but evade the rewritePrefix transformation. The unchanged encoded path is then forwarded upstream, where it may be decoded and processed as a different effective path than the proxy intended to expose. This can allow access to upstream paths that were meant to be hidden behind prefix rewriting, including internal or administrative endpoints.
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