Unintended Proxy or Intermediary in Fastify-reply-from - CVE-2026-16158
Published: July 20, 2026
Vulnerability identifier: #VU138405
CSH Severity: High
CVSS v4: 9.1 [CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-16158
CWE-ID: CWE-441
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to access and modify data across security-separated upstreams.
The vulnerability exists due to an unintended proxying behavior in the URL cache key handling in the @fastify/reply-from getUpstream() routing logic when handling requests that influence the source path or upstream selection. A remote attacker can send crafted requests that cause a URL cache key collision to access and modify data across security-separated upstreams.
Only applications that route between security-separated backends using the getUpstream() option with the default URL cache enabled are affected.
Affected software
Fastify-reply-from
How to mitigate CVE-2026-16158
Install security update from vendor's website.
Fastify-reply-from - update to 12.6.4