CVE-2026-16117: fastify fastify\/http-proxy Improper Input Validation

Vulnerability summary

CVE-2026-16117 is a critical (CVSS 10.0) Improper Input Validation vulnerability in fastify fastify\/http-proxy. EPSS estimates a 0.44% chance of exploitation in the next 30 days, higher than 37.2% of scored vulnerabilities.

Timeline

1. 18-07-2026
   PublishedVulnerability first disclosed.
2. 20-07-2026
   Last modifiedVulnerability information updated.

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.

• CWE-20Improper Input ValidationThe product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.