PT-2026-27132 · Fastify · Fastify

Kaka + 3 · Published 2026-03-23 · Updated 2026-03-23 · CVE-2026-3635

Mitre

CVSS v3.1

6.1

Medium

AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N

Summary When trustProxy is configured with a restrictive trust function (e.g., a specific IP like trustProxy: '10.0.0.1', a subnet, a hop count, or a custom function), the request.protocol and request.host getters read X-Forwarded-Proto and X-Forwarded-Host headers from any connection — including connections from untrusted IPs. This allows an attacker connecting directly to Fastify (bypassing the proxy) to spoof both the protocol and host seen by the application.

Affected Versions fastify <= 5.8.2

Impact Applications using request.protocol or request.host for security decisions (HTTPS enforcement, secure cookie flags, CSRF origin checks, URL construction, host-based routing) are affected when trustProxy is configured with a restrictive trust function.

When trustProxy: true (trust everything), both host and protocol trust all forwarded headers — this is expected behavior. The vulnerability only manifests with restrictive trust configurations.

Fix

Weakness Enumeration

CWE-348

Related Identifiers

CVE-2026-3635

Affected Products

Fastify

References · 4

• https://nvd.nist.gov/vuln/detail/CVE-2026-3635 · Security Note
• https://cve.org/CVERecord?id=CVE-2026-3635 · Security Note
• https://cna.openjsf.org/security-advisories.html · Note
• https://github.com/fastify/fastify/security/advisories/GHSA-444r-cwp2-x5xf · Note