CVE-2026-18500 — @fastify/jwt is a JSON Web Token plugin for Fastify.¶

Key facts

CVE-2026-18500 is a High (8.1) vulnerability: @fastify/jwt is a JSON Web Token plugin for Fastify.. This entry ships a detection-only PoC and a Docker test lab.


CVE	CVE-2026-18500
Severity	High (8.1)
Detection / prevention	—
PoC	poc.py
Source	2026/CVE-2026-18500/

Details¶

English | 한국어

Description¶

@fastify/jwt is a JSON Web Token plugin for Fastify. In versions before 10.2.2, a per-request verification key passed to request.jwtVerify({ key }) is silently overridden by the plugin's globally configured secret, because the option merge applies the global key last. Applications that use different keys for different authorization domains, for example separate user and admin keys, therefore accept a token signed with the global key on a route that explicitly requires another key. This lets an ordinary authenticated user cross a key-based trust boundary without knowing either secret. The issue is fixed in @fastify/jwt 10.2.2, where an explicit per-call key takes precedence over the global secret. Users should upgrade to 10.2.2.

Vulnerability Details¶

Property	Value
CVE ID	CVE-2026-18500
CVSS Score	8.1

Proof of Concept¶

Download¶

# Download PoC script
curl -O https://raw.githubusercontent.com/fankh/vulnerability-poc/main/2026/CVE-2026-18500/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-2026-18500

# 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-jwt/security/advisories/GHSA-j4cx-787j-xjqg

Generated by PoC Generator on 2026-08-16T00:18:24.541734005

Frequently asked questions¶

What is CVE-2026-18500?¶

CVE-2026-18500 is a High (8.1) vulnerability: @fastify/jwt is a JSON Web Token plugin for Fastify.. This entry ships a detection-only PoC and a Docker test lab.

How do I detect CVE-2026-18500?¶

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-2026-18500?¶

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.