CVE-2026-84504 — fastify versions before 5.12.2 treat the object resolved by a successful Ajv async…¶

Key facts

CVE-2026-84504 is a High (8.1) vulnerability: fastify versions before 5.12.2 treat the object resolved by a successful Ajv async…. This entry ships a detection-only PoC and a Docker test lab.


CVE	CVE-2026-84504
Severity	High (8.1)
Detection / prevention	—
PoC	poc.py
Source	2026/CVE-2026-84504/

Details¶

English | 한국어

Description¶

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.

Vulnerability Details¶

Property	Value
CVE ID	CVE-2026-84504
CVSS Score	8.1

Proof of Concept¶

Download¶

# Download PoC script
curl -O https://raw.githubusercontent.com/fankh/vulnerability-poc/main/2026/CVE-2026-84504/poc.py

Usage¶

# Run PoC against target
python3 poc.py --target <TARGET_URL>

# With authentication
python3 poc.py --target <TARGET_URL> --username <USER> --password <PASS>

⚠️ Warning: This PoC is for authorized security testing only. It uses detection-only techniques and does not exploit or damage target systems.

Test Lab¶

This repository includes a Docker-based test environment with both vulnerable and patched applications.

Quick Start¶

# Clone and navigate to CVE directory
cd CVE-2026-84504

# Start both applications
docker-compose up -d

# Run tests
./run-tests.sh    # Linux/Mac
./run-tests.ps1   # Windows PowerShell

Application Endpoints¶

Application	URL	Status
Vulnerable App	http://localhost:8080	Shows vulnerability
Patched App	http://localhost:8081	Demonstrates fix

Directory Structure¶

├── README.md           # English documentation
├── README_KO.md        # Korean documentation
├── poc.py              # PoC detection script
├── docker-compose.yml  # Container orchestration
├── run-tests.sh        # Linux/Mac test runner
├── run-tests.ps1       # Windows test runner
├── vulnerable-app/     # Vulnerable version
│   ├── app.py
│   ├── Dockerfile
│   └── requirements.txt
└── patched-app/        # Fixed version
    ├── app.py
    ├── Dockerfile
    └── requirements.txt

References¶

https://cna.openjsf.org/security-advisories.html
https://github.com/fastify/fastify/security/advisories/GHSA-667r-xxjv-c9mm

Generated by PoC Generator on 2026-09-04T20:16:02.056042896

Frequently asked questions¶

What is CVE-2026-84504?¶

CVE-2026-84504 is a High (8.1) vulnerability: fastify versions before 5.12.2 treat the object resolved by a successful Ajv async…. This entry ships a detection-only PoC and a Docker test lab.

How do I detect CVE-2026-84504?¶

Run the detection-only PoC included with this entry against your Docker test lab to confirm whether a target is affected; pair it with your own IDS logging.

How do I block or mitigate CVE-2026-84504?¶

Upgrade to a fixed version per the vendor advisory; the NVD record lists the affected versions. Use the included PoC to verify your remediation.

For authorized security testing and research only. See the full source and NVD record.