#VU125979 Use of Less Trusted Source in Fastify - CVE-2026-3635
Published: April 14, 2026
Vulnerability identifier: #VU125979
Vulnerability risk: Medium
CVSSv4.0: CVSS:4.0/AV:A/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: CVE-2026-3635
CWE-ID: CWE-348
Exploitation vector: Adjecent network
Exploit availability:
No public exploit available
Vulnerable software:
Fastify
Fastify
Software vendor:
fastify.io
fastify.io
Description
The vulnerability allows a remote attacker to spoof the protocol and host seen by the application.
The vulnerability exists due to use of a less trusted source in request.protocol and request.host getters when processing X-Forwarded-Proto and X-Forwarded-Host headers from direct untrusted connections. A remote attacker can send specially crafted X-Forwarded-Proto and X-Forwarded-Host headers to spoof the protocol and host seen by the application.
The issue only manifests when trustProxy is configured with a restrictive trust function, such as a specific IP, a subnet, a hop count, or a custom function.
Remediation
Install security update from vendor's website.