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Fencer Intel
medium 6.1

CVE-2026-3635 json

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

published 7mo ago modified 4mo ago also GHSA-444r-cwp2-x5xf , EUVD-2026-14431

cvss
6.1
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 4%
public exploits
0
none known

Summary

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.

Patch / fix

  • npm fastify 5.8.3
  • fastify fastify 5.8.3

Is CVE-2026-3635 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-3635 updated cvss 6.1

fastify: request.protocol and request.host spoofable via X-Forwarded-Proto/Host from untrusted connections when trustProxy uses restrictive trust function

  • Red Hat Enterprise Linux AI (RHEL AI) 3 rhelai3/bootc-cuda-rhel9, rhelai3/bootc-rocm-rhel9, rhelai3/disk-image-cuda-rhel9 Red Hat OpenShift AI (RHOAI) rhoai/odh-dashboard-rhel8, rhoai/odh-dashboard-rhel9, rhoai/odh-mod-arch-maas-rhel9, rhoai/odh-mod-arch-gen-ai-rhel9, rhoai/odh-mod-arch-model-registry-rhel9 Red Hat OpenShift Dev Spaces devspaces/dashboard-rhel9
    affected: workaround, no fix available “Fix deferred”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
7mo ago

Exploit prediction

0.15% chance of exploitation in the next 30 days, higher than 4% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
CNA 3.1 6.1 CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N
GitHub 3.1 6.1 CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N

Affected packages

ecosystem package affected fixed in
npm fastify
<5.8.3
5.8.3

Affected products

  • fastify fastify

Classification

Weakness
  • CWE-348 Use of Less Trusted Source

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.