NVD / CVE Program· CNA openjs

CVE-2026-84504: fastify vulnerable to request body replacement via an async validation result collision

Noroxi assessment

Priority: Monitor (action score 32/100)

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: 0.42% (FIRST EPSS).
Anyone who can reach it over the internet can trigger it. A low-privileged account is enough. No user action is required. No special conditions are required; it is repeatable.
Fixed version: fastify fastify 5.12.2.
Mentioned 1 time in Turkish national cyber security advisories (siberguvenlik.gov.tr).

Low priority for now. Upgrade to fastify fastify 5.12.2 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 versions before 5.12.2 treat the object resolved by a successful Ajv async validator as the value result protocol used by custom validator compilers. If a request that passes its route schema contains a property named value at the root, fastify replaces the entire request body with that property's value before the handler runs, so the handler receives a different object than the one that satisfied the schema. An authenticated low-privilege caller can use this to make nested data replace the validated body and trigger an operation the route schema did not authorize, leading to unauthorized state changes and data disclosure. Users should upgrade to fastify 5.12.2 or later.

HighCVSS 8.1 · v3.1—No exploit Fix available

Published: Sep 4, 2026
Updated: Sep 15, 2026
EPSS: 0.4% · 37th percentile
CWE: CWE-20

Report tools

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Action score

Monitor

Low priority for now.

CVSS: 32 / 40 · 8.1 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.4%

CISA SSVC decision

Exploitation: none
Automatable: no
Technical impact: total

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD: —
CNA · openjs: 8.1; 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?

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 5.12.2

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
- 5.12.2not affected
- before 5.12.2affected · 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 5.12.2	5.12.2

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-2018-3711Fastify node module before 0.38.0 is vulnerable to a denial-of-service attack by sending a request with "Content-Type: application/json" and
31Monitor

Published before and after this one in the same product

← EarlierCVE-2026-76169Sep 4, 2026 · fastify vulnerable to authentication bypass via malformed URLs reaching encapsulated not-found handlers
30Monitor
← EarlierCVE-2026-84469Sep 4, 2026 · fastify vulnerable to request validation bypass via skipped boolean false schemas
30Monitor
Later →CVE-2026-84428Sep 4, 2026 · fastify vulnerable to header validation bypass via incomplete schema case normalization
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	5.12.2	Vendor (CNA)
npm:fastify	5.12.2	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.

velgusgus599reporter
UlisesGasconremediation developer
climba03003remediation reviewer
mcollinaremediation reviewer
Eommremediation reviewer

Variant candidates

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

CVE-2026-844690 days apartfastify vulnerable to request validation bypass via skipped boolean false schemas
30Monitor
CVE-2026-1850417 days apartfastify vulnerable to schema validation bypass via root primitive coercion mismatch
21Monitor

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).