Is CVE-2026-16117 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-16117 being exploited in the wild.

How severe is CVE-2026-16117?

Critical severity, with a CVSS score of 10, published 18 July 2026.

Is there a fix for CVE-2026-16117?

A patch was published on 2026-07-18. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-16117 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-16117?

Check whether you run Fastify Http-Proxy at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-16117: Fastify @fastify/http-proxy prefix rewrite bypass via URL-encoded characters

CVE-2026-16117 · Severity: critical · CVSS 10 · Published 2026-07-18

@fastify/http-proxy is a tool used to forward web requests from one server to another, often used to hide internal systems or simplify web addresses. A security flaw allows attackers to bypass path restrictions by using special URL encoding (like using %61 instead of 'a'). This could allow an unauthorized user to access sensitive internal or administrative parts of the backend system that were supposed to be hidden.

Technologies: Fastify Http-Proxy. Vendors: Fastify.

Quick answers

Is CVE-2026-16117 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-16117 being exploited in the wild.
How severe is CVE-2026-16117?: Critical severity, with a CVSS score of 10, published 18 July 2026.
Which products and versions are affected?: Fastify Http-Proxy <= 11.5.0.
Is there a fix for CVE-2026-16117?: A patch was published on 2026-07-18. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-16117 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-16117?: Check whether you run Fastify Http-Proxy at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The vulnerability exists because Fastify's router URL-decodes paths for route matching, but the @fastify/http-proxy plugin uses the original encoded request.url for prefix rewriting. When a request uses percent-encoding (e.g., /%61pi/ instead of /api/), the router matches the route, but the literal string replacement for the prefix fails. Consequently, the raw encoded path is forwarded to the upstream server unchanged. If the upstream server then decodes the path, it may serve internal or administrative endpoints that the proxy was configured to hide via the rewritePrefix option. This is fixed in version 11.6.0.

Affected products

Fastify @fastify/http-proxy <= 11.5.0

Timeline

2026-07-18: disclosed
2026-07-18: advisory
2026-07-18: patched

References

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