CVE-2026-3635 | Fastify request.protocol and request.host spoofable via X-Forwarded-Proto/Host from untrusted connections when trustProxy uses restrictive trust function

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.

Published: 2026-03-23 Last update: 2026-04-16 Assigner: ce714d77-add3-4f53-aff5-83d477b104bb Source: ce714d77-add3-4f53-aff5-83d477b104bb
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-3635 is rated Low Risk (24.5/100): CVSS Medium severity, with low exploitation likelihood (EPSS 0.01%). Monitor for updates and reassess as exploit intelligence or EPSS changes.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-3635

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-03-24 — 0.01% —

Full EPSS history (1 record total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-3635

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
6.1 3.1 MEDIUM
CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N Click to expand
Attack vector (AV:A)
Attacker has to be nearby on the network—same office, same link, that vibe—not the whole wide internet.
Attack complexity (AC:H)
Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:N)
Service keeps running; no real outage angle.
 1.6 4.0 ce714d77-add3-4f53-aff5-83d477b104bb

Weakness enumeration for CVE-2026-3635

GitHub Security Advisory for CVE-2026-3635

GHSA-444r-cwp2-x5xf · Severity: medium · Ecosystem: npm — fastify: request.protocol and request.host Spoofable via X-Forwarded-Proto/Host from Untrusted Connections

OS Trackers for CVE-2026-3635

vendor priority summary link
redhat medium — https://access.redhat.com/security/cve/CVE-2026-3635

Affected software / configurations for CVE-2026-3635

Vendor Product Version Raw CPE
fastify fastify < 5.8.3 cpe:2.3:a:fastify:fastify:*:*:*:*:*:node.js:*:*

References for CVE-2026-3635

cvelogic Threat Intelligence