Free JWT Header 'kid' Injection Vulnerability Tester

Application Overview

The JWT Header 'kid' Injection Vulnerability Tester is a client-side, educational tool designed to help developers visualize and simulate how the 'kid' parameter in a JWT header can be exploited if improperly handled. The application provides a clean, professional interface for constructing JWT headers, injecting malicious 'kid' values, and viewing the resulting token structure.

Technical Constraints & Architecture

• Single File: The entire application must be contained in one HTML file, using CDN links for Tailwind CSS and any required utility libraries.
• Stateless: ZERO use of localStorage, sessionStorage, or cookies. The state must be handled purely in memory (JavaScript variables).
• Sandbox-Ready: No alerts, confirms, or prompts. All interactions must be handled via custom DOM elements (modals/tooltips).
• No Branding: The UI must be purely functional and descriptive.

Feature List

1. Live JWT Constructor: A two-pane input system (Header Editor, Payload Editor) that generates a live JWT string.
2. 'kid' Injection Field: A dedicated input field specifically for the 'kid' parameter, allowing users to inject characters like ../, SQL snippets, or other payloads.
3. Signature Simulation Panel: A visual breakdown that shows how the 'kid' parameter is processed by a hypothetical server logic.
4. Educational Context Tooltips: Hoverable explanations for what each part of the JWT header represents and why the 'kid' field is a common attack vector.
5. Security Best Practices Overlay: A non-intrusive guide that appears when specific "dangerous" characters are detected, explaining how to sanitize these inputs in backend code.

UI Layout

• Header: A simple, high-contrast bar with the tool title and a "Reset" button.
• Main Content: A responsive flexbox grid.
  • Left Column (The Workshop): Input fields for Header (JSON textarea), Payload (JSON textarea), and Secret/Key field. A dedicated input row for the kid parameter.
  • Right Column (The Output): A live-updated view of the JWT encoded string and a "Verification Simulation" panel that breaks down how the hypothetical server reads the header.
• Footer: None.

Design Specifications

• Color Palette:
  • Primary: #2563EB (Professional Blue for actions).
  • Background: #F8FAFC (Clean, light grey/white).
  • Text: #1E293B (Dark slate for high readability).
  • Borders/Inputs: #E2E8F0 (Soft neutral grey).
• Typography: Sans-serif, system-ui (Inter or Arial/Helvetica).
• Animations:
  • Smooth transitions (duration: 200ms) for input focus states.
  • Subtle fade-in effects for the simulation output panel when the payload updates.

Developer Instructions

1. Validation: Implement a custom JSON validator function to catch syntax errors in the header/payload editors immediately, highlighting them in the UI rather than using alert().
2. Encoding: Use a lightweight, CDN-based base64 encoding library to ensure the JWT output is accurate.
3. Security: Ensure all user input is sanitized before rendering to the DOM to prevent XSS (even though it is a local tool, practice good habits). Use .textContent for rendering dynamic values.
4. Responsiveness: Ensure that on smaller mobile screens, the left and right columns stack vertically with ample whitespace for readability.