Medium

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 Mar 23, 2026 · Updated Oct 13, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: openjs

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

EPSS 0.2% (30-day exploitation probability)+1
Technical severity (medium)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

0.17%

ahead of 6% of scored CVEs · Oct 14, 2026

Affects

Fastify

CVSS base

6.1

Medium · 2 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation

None

none · proof-of-concept · active

Automatable

can an attacker script the kill-chain steps

Technical impact

Partial

partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Fastify Fastify).
No fix documented in the sources: reduce exposure and monitor the vendor advisory.
Search available logs for exploit probes, errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Fastify	Fastify	< 5.8.3	Vulnerable	NIST National Vulnerability Database
Fastify	Fastify	≤ 5.8.2	Vulnerable	CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Adjacent

Attack complexity

High

Privileges required

None

User interaction

None

Scope

Changed

Confidentiality

High

Integrity

None

Availability

None

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
6.1	CVSS 3.1	Medium	1.6	4.0	ce714d77-add3-4f53-aff5-83d477b104bb via NIST National Vulnerability Database
6.1	CVSS 3.1	Medium	—	—	openjs via CVE Program (cve.org) e CISA ADP

CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N (score shown at the top: ce714d77-add3-4f53-aff5-83d477b104bb)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-348

Other CVEs with this weakness →

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.17%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Not enough vector or weakness data: use the affected application’s logs as a starting point.

application logs

Timeline

What happened to this CVE, newest first.

2026
Affected products added · fastify/fastify
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
SSVC assessment updated
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 13, 2026
CVSS changed · 6.1
Oct 13, 2026 · NIST National Vulnerability Database
Source status changed · → Analyzed
Oct 13, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 13, 2026 · NIST National Vulnerability Database
CVE published
Mar 23, 2026 · openjs

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

3 on the record

github.com/fastify/fastify/security/advisories/GHSA-444r-cwp2-x5xf
Vendor Advisory, NIST National Vulnerability Database
cna.openjsf.org/security-advisories.html
Third Party Advisory, NIST National Vulnerability Database
www.cve.org/CVERecord
Third Party Advisory, NIST National Vulnerability Database

Elsewhere on this site

Fastify
every CVE for this vendor

CWE-348
other pages naming this weakness

Related CVEs (same products)

Low CVE-2026-107832 fastify vulnerable to header validation bypass via case-sensitive propertyNames 2d
High CVE-2026-84428 fastify versions before 5.12.2 implement the case-insensitive nature of HTTP header… Sep 4, 2026
High CVE-2026-84469 Improper Input Validation in Fastify Sep 4, 2026
High CVE-2026-84504 Improper Input Validation in Fastify Sep 4, 2026
High CVE-2026-76169 Authentication Bypass Using an Alternate Path or Channel in Fastify Sep 4, 2026
Medium CVE-2026-16732 fastify is a fast and low overhead web framework for Node.js. Impact: the fix for… Aug 18, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No fixed version, patch or mitigation documented.
No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-3635 is "Fastify request.protocol and request.host spoofable via X-Forwarded-Proto/Host from untrusted connections when trustProxy uses restrictive trust function", rated Medium. It affects Fastify (Fastify); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 6.1 describes technical severity. EPSS 0.17% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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