I noticed the growing security concerns around MCP (
https://news.ycombinator.com/item?id=43600192) and built an open source tool that can detect several patterns of tool poisoning attacks, exfiltration channels and cross-origin manipulations.
MCP-Shield scans your installed servers (Cursor, Claude Desktop, etc.) and shows what each tool is trying to do at the instruction level, beyond just the API surface. It catches hidden instructions that try to read sensitive files, shadow other tools' behavior, or exfiltrate data.
Example of what it detects:
- Hidden instructions attempting to access ~/.ssh/id_rsa
- Cross-origin manipulations between server that can redirect WhatsApp messages
- Tool shadowing that overrides behavior of other MCP tools
- Potential exfiltration channels through optional parameters
I've included clear examples of detection outputs in the README and multiple example vulnerabilities in the repo so you can see the kinds of things it catches.
This is an early version, but I'd appreciate feedback from the community, especially around detection patterns and false positives.
I expect a lot of people to refine the tool as they use it; one big challenge in maintaining the project is going to be incorporating pull requests that improve the prompt in different directions.
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