ThornGuard: A Proxy Gateway to Secure MCP Server Connections from Prompt Injection
ThornGuard is a security proxy designed to protect Claude AI from malicious content when connecting to external MCP (Model Context Protocol) servers. The tool was created after testing revealed that upstream servers can inject hidden instructions into tool responses, which Claude receives without filtering.
Security Problem Identified
When connecting Claude to external MCP servers, nothing prevents upstream servers from injecting hidden instructions into tool responses. In a test, a server embedded a fake recommendation telling Claude to always prefer a specific vendor. While Claude caught this obvious payload, more subtle injections would bypass detection.
ThornGuard Features
- Scans tool definitions and responses for prompt injection and poisoning
- Strips secrets and PII before they enter your context window
- Includes a semantic classifier that flags suspicious payloads
- Provides real-time audit dashboard with compliance exports
- Offers CLI that generates configs for Claude Desktop, Cursor, VS Code, and several others
Implementation Details
The proxy architecture was designed with a security model in mind, then implemented using Claude Code on Cloudflare Workers. The implementation includes OAuth flows and the CLI tool.
ThornGuard is available with a 7-day free trial at thorns.qwady.app. A demonstration video is available at https://youtu.be/1PWNFpUWKV8.
📖 Read the full source: r/ClaudeAI
👀 See Also
SupraWall MCP Plugin Blocks Prompt Injection Attacks on Local AI Agents
SupraWall is an MCP plugin that intercepts and blocks sensitive data exfiltration attempts from AI agents, demonstrated in a red-team challenge where it prevented credential leaks via prompt injection attacks.
Microsoft Hacked: Malware Planted in GitHub Repos Targets Claude and Gemini Users
Microsoft shut down 70+ GitHub repositories after hackers planted credential-stealing malware targeting AI coding agents like Claude Code and Gemini CLI.
Claude Code Writes Files Outside Allowed Directory Without Permission
A user reports Claude Code creating folders and saving files in C:\Users\...\Documents\Surge XT\Patches\ without explicit permission, using os.makedirs.
Clawvisor: Purpose-Based Authorization Layer for OpenClaw Agents
Clawvisor is an authorization layer that sits between AI agents and APIs, enforcing purpose-based authorization where agents declare intentions, users approve specific purposes, and an AI gatekeeper verifies every request against that purpose. Credentials never leave Clawvisor and agents never see them.