CVE-2026-16117: @fastify/http-proxy vulnerable to prefix escape via URL-encoded characters

Severity: critical (CVSS 10.0 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-18 · updated 2026-07-28
Weakness: CWE-20

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.

Affected products

@fastify/http-proxy: before 11.6.0 — fixed in 11.6.0

How to fix

Upgrade to a fixed version: @fastify/http-proxy 11.6.0. Check the vendor advisory for mitigations.

References

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