NVD / CVE Program· CNA openjs

CVE-2026-16732: fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count

Noroxi assessment

Priority: Monitor (action score 24/100)

The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
Probability of exploitation in the next 30 days: 0.15% (FIRST EPSS).
Someone on the same network segment can trigger it. No account or password is required. No user action is required. Special conditions such as timing or configuration are required.
Fixed version: fastify fastify 5.12.1.
Mentioned 1 time in Turkish national cyber security advisories (siberguvenlik.gov.tr).

Low priority for now. Upgrade to fastify fastify 5.12.1 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

fastify is a fast and low overhead web framework for Node.js. Impact: the fix for CVE-2026-3635 added a guard on the forwarded-header reads used to derive the request host, protocol, hostname, ip, and ips values, checking the connecting address. That guard closes the IP, CIDR, and custom-function forms of trustProxy correctly, because those forms compile to predicates that inspect the connecting address. The hop-count form, where trustProxy is set to a number, compiles to a predicate that structurally ignores the address, so the guard is always satisfied for any hop count of one or more. Applications configured with a numeric trustProxy value, such as trustProxy set to 1 for a single reverse proxy, remain vulnerable: an attacker who can reach the Fastify origin directly, bypassing the front-facing proxy, can spoof the forwarded request fields exactly as in the unpatched version. The impact class matches the parent CVE-2026-3635, including host injection in generated URLs, HTTPS-enforcement bypass, secure-cookie and CSRF-origin bypass, and host-based routing and cache poisoning. Affected versions are fastify from 5.8.3 up to but not including 5.12.1. Patches: patched in fastify 5.12.1, where the numeric form of trustProxy is disabled at runtime and removed from the TypeScript type union. Workarounds: migrate to an IP, CIDR, or custom-function trustProxy value that validates the connecting address, and ensure the Fastify origin is only reachable through the trusted proxy chain.

MediumCVSS 6.1 · v3.1—No exploit Fix available

Pre-auth

GHSA-3m5p-2c4r-xxw2

Published: Aug 18, 2026
Updated: Sep 2, 2026
EPSS: 0.1% · 4th percentile
CWE: CWE-348

Report tools

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Action score

Monitor

Low priority for now.

CVSS: 24 / 40 · 6.1 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.1%

CISA SSVC decision

Exploitation: none
Automatable: no
Technical impact: partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD: —
CNA · openjs: 6.1; NVD has not scored this yet; the score shown is the CNA’s.

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

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Not in KEV: no record of exploitation in the wild
No public exploit code on record
A fix has been released

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Noroxi analysis

What does it mean in your environment?

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Affected systems

Is your version affected?

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Vendor	Product	CPE
fastify	fastify	cpe:2.3:a:fastify:fastify

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

fastify fastify5.8.3 and later · before 5.12.1

Versions reported by the vendor

Affected version ranges reported by the assigning authority (openjs). Independent of NVD's CPE analysis and usually ahead of it.

fastify fastify
- 5.12.1not affected
- 5.8.3 and later · before 5.12.1affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Ecosystem	Package	Affected range	Fix
npm	fastify	from 5.8.3 · before 5.12.1	5.12.1

Same product

fastify: all records

Other highest-scoring records for the same primary product.

CVE-2022-39288Denial of service in Fastify via Content-Type header
48Plan
CVE-2022-41919Fastify vulnerable to Cross-Site Request Forgery (CSRF) attack via incorrect content type
35Monitor
CVE-2026-84504fastify vulnerable to request body replacement via an async validation result collision
32Monitor

Published before and after this one in the same product

← EarlierCVE-2026-3635Mar 23, 2026 · Fastify request.protocol and request.host spoofable via X-Forwarded-Proto/Host from untrusted connections when trustProxy uses restrictive trust function
24Monitor
← EarlierCVE-2026-33806Apr 15, 2026 · fastify vulnerable to Body Schema Validation Bypass via Leading Space in Content-Type Header
30Monitor
Later →CVE-2026-18504Aug 18, 2026 · fastify vulnerable to schema validation bypass via root primitive coercion mismatch
21Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / package	Fixed version	Source
fastify fastify	5.12.1	Vendor (CNA)
npm:fastify	5.12.1	Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

alimonyreporter
mcollinaremediation developer
climba03003remediation reviewer
UlisesGasconremediation reviewer

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

CVE-2026-3635148 days apartFastify request.protocol and request.host spoofable via X-Forwarded-Proto/Host from untrusted connections when trustProxy uses restrictive trust function
24Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).