NVD / CVE Program· CNA openjs

CVE-2026-18504: fastify vulnerable to schema validation bypass via root primitive coercion mismatch

Noroxi assessment

Priority: Monitor (action score 21/100)

Public proof-of-concept code exists (GitHub); there is no record of exploitation in the wild.
Probability of exploitation in the next 30 days: 0.28% (FIRST EPSS).
Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
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. Versions of fastify before 5.12.1 are affected by a schema validation bypass when a request body schema targets a root primitive value. When the schema validates a top-level primitive such as an integer, Ajv can coerce a JSON string into the expected type during validation, but Fastify does not replace the root request body with the coerced value, so the route handler receives the original unvalidated string. As a result, a request that should have failed validation can reach application logic with a value that does not satisfy the schema, which can undermine integrity and access-control checks that rely on the validated type. Users should upgrade to fastify 5.12.1, which fixes the mismatch. No known workarounds are available.

MediumCVSS 5.3 · v3.1—Proof of concept Fix available

Pre-auth

GHSA-w2qp-rph6-63g4

Published: Aug 18, 2026
Updated: Sep 2, 2026
EPSS: 0.2% · 20th percentile
CWE: CWE-20

Report tools

JSON

Action score

Monitor

Low priority for now.

CVSS: 21 / 40 · 5.3 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.2%

CISA SSVC decision

Exploitation: none
Automatable: no
Technical impact: partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD: 5.3
CNA · openjs: 5.4; 0.1 point gap

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N

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

Get told when this record changes

Not in KEV: no record of exploitation in the wild
Proof of concept exists, no weaponized module
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?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

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 fastifybefore 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
- 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	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-2026-33806Apr 15, 2026 · fastify vulnerable to Body Schema Validation Bypass via Leading Space in Content-Type Header
30Monitor
← EarlierCVE-2026-16732Aug 18, 2026 · fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count
24Monitor
Later →CVE-2026-76169Sep 4, 2026 · fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers
30Monitor

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

Proof of concept available

A public proof of concept or detection signature exists. Exploit knowledge is out there; don't delay patching.

Evidence

Proof-of-concept repository on GitHub ×1

Noroxi does not host or link to exploit code. Metasploit module names and Nuclei template ids are shown (names, not code); Exploit-DB entries and PoC repositories are given as counts only.

Public exploit evidence
Source	Kind	Date
Proof-of-concept repository on GitHub	stars:0	2026-08-19

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.

PublishedAug 18, 2026
First PoCAug 19, 2026same day
TodayNov 9, 2026+83 d

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.

velgusgus599reporter
mcollinaremediation developer
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-2026-8450417 days apartfastify vulnerable to request body replacement via an async validation result collision
32Monitor
CVE-2026-8446917 days apartfastify vulnerable to request validation bypass via skipped boolean false schemas
30Monitor

Chain candidates

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

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Bug bounty scope

No known public program.

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