OpenCode Security Vulnerability Demo

This proof-of-concept demonstrates a critical security vulnerability in OpenCode's server architecture.

The Vulnerability

OpenCode's local server has two critical security issues:

1. Open CORS Policy: Uses cors() with default settings, allowing Access-Control-Allow-Origin: *
2. No Authentication: All API endpoints are accessible without any authentication

This means any website you visit can:

• Execute shell commands on your machine
• Read files from your filesystem
• Manipulate AI agents
• Exfiltrate sensitive data

Running the Demo

Prerequisites

1. OpenCode must be installed and running. Any of these are vulnerable:

   # Production binary (installed via npm/bun)
   opencode serve
   
   # Or just running opencode (TUI also starts the server)
   opencode
   
   # Or development mode
   cd /path/to/opencode
   bun run dev

   Note: The vulnerability exists in BOTH development AND production builds. The same cors() middleware with default settings (allowing all origins) is compiled into the production binary.

2. Python 3 for the demo server (or any HTTP server)

Start the Demo

cd opencode-vuln-demo
python3 serve.py

Then open http://localhost:8080 in your browser.

Alternative: Using any HTTP server

# Using Node.js
npx serve .

# Using PHP
php -S localhost:8080

Attack Vectors Demonstrated

Attack	API Endpoint	Risk Level
Server Info Leak	GET /global/health, GET /path	Medium
Session Enumeration	GET /session	Medium
File Reading	GET /file/content?path=...	Critical
Directory Listing	GET /file?path=...	High
Config Extraction	GET /config	Critical
PTY Shell Creation	POST /pty	Critical
Shell Command Exec	WebSocket /pty/:id/connect	Critical
AI Agent Manipulation	POST /session/:id/message	Critical
Permission Bypass	POST /session/:id/permissions/:id	Critical

Why This Works

Even though OpenCode binds to 127.0.0.1 (localhost), browsers allow JavaScript from any origin to make requests to localhost. The open CORS policy (Access-Control-Allow-Origin: *) explicitly permits these cross-origin requests.

Mitigation Recommendations

1. Token-based authentication for all API endpoints
2. Strict CORS policy - only allow requests from trusted origins
3. Origin header validation - reject requests from web pages
4. Rate limiting on sensitive endpoints
5. Permission system for API access - not just for tool execution

Responsible Disclosure

This demo is for educational and security research purposes only. Please report security vulnerabilities to the OpenCode maintainers responsibly.

Files

• index.html - Main demo interface
• serve.py - Simple Python HTTP server
• README.md - This file