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The 10 Most Common MCP Server Vulnerabilities (With Code Examples)

#security #mcp #typescript #claudecode

The 10 Most Common MCP Server Vulnerabilities (With Code Examples)

After auditing dozens of open-source MCP servers, I've identified the vulnerabilities that appear most frequently. Here's the complete list with real code patterns and fixes.

1. Path Traversal

Frequency: ~65% of file-handling servers

// Vulnerable
async function readFile(path: string) {
  return fs.readFileSync(path, 'utf8'); // reads ~/.ssh/id_rsa if asked
}

// Fixed
async function readFile(relativePath: string) {
  const base = path.resolve('./allowed');
  const target = path.resolve(base, relativePath);
  if (!target.startsWith(base + path.sep)) throw new Error('Access denied');
  return fs.readFileSync(target, 'utf8');
}
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2. Command Injection

Frequency: ~43% of servers that execute shell commands

// Vulnerable — shell interpolation
const result = await exec(`git log --oneline ${userInput}`);
// Input: 'main; cat ~/.aws/credentials' → exfiltrates credentials

// Fixed — execFile with arg array
const result = await execFile('git', ['log', '--oneline', userInput]);
// execFile doesn't invoke a shell — no injection possible
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3. Prompt Injection via Tool Responses

Frequency: ~38% of servers that fetch external content

// Vulnerable — returns raw external content
async function fetchPage(url: string) {
  const html = await fetch(url).then(r => r.text());
  return { content: html }; // external HTML can contain instructions to Claude
}

// Fixed — extract only structured data
async function fetchPage(url: string) {
  const html = await fetch(url).then(r => r.text());
  return {
    title: extractTitle(html),    // string
    links: extractLinks(html),    // string[]
    wordCount: countWords(html),  // number
    // Never return raw HTML
  };
}
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4. Hardcoded Credentials

Frequency: ~27% of servers

// Vulnerable
const client = new APIClient({ apiKey: 'sk-abc123...' });

// Fixed
const apiKey = process.env.API_KEY;
if (!apiKey) throw new Error('API_KEY is required');
const client = new APIClient({ apiKey });
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5. Credentials in Error Messages

Frequency: ~31% of servers

// Vulnerable
} catch (e) {
  return { error: `Failed with key ${process.env.API_KEY}: ${e.message}` };
}

// Fixed
} catch (e) {
  console.error('Auth failed:', e.message); // log locally only
  return { error: 'Authentication failed', isError: true };
}
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6. Missing Input Validation

Frequency: ~61% of servers (most common)

// Vulnerable — no validation
server.setRequestHandler(CallToolRequestSchema, async (req) => {
  const { ticker } = req.params.arguments;
  return await getPrice(ticker); // what if ticker is undefined or 10,000 chars?
});

// Fixed — Zod validation
const TickerSchema = z.string().min(1).max(10).regex(/^[A-Z]+$/);

server.setRequestHandler(CallToolRequestSchema, async (req) => {
  const parsed = TickerSchema.safeParse(req.params.arguments.ticker);
  if (!parsed.success) return { error: 'Invalid ticker', isError: true };
  return await getPrice(parsed.data);
});
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7. SSRF (Server-Side Request Forgery)

Frequency: ~18% of servers

// Vulnerable — fetches any URL the user provides
async function fetchData(url: string) {
  return await fetch(url); // can fetch http://169.254.169.254/metadata (AWS metadata)
}

// Fixed — validate URL before fetching
const ALLOWED_SCHEMES = ['https:'];
const BLOCKED_RANGES = ['127.', '10.', '192.168.', '169.254.'];

function validateUrl(url: string): URL {
  const parsed = new URL(url); // throws on invalid URL
  if (!ALLOWED_SCHEMES.includes(parsed.protocol)) throw new Error('HTTPS only');
  if (BLOCKED_RANGES.some(r => parsed.hostname.startsWith(r))) {
    throw new Error('Internal addresses not allowed');
  }
  return parsed;
}
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8. Dependency Confusion / Typosquatting

Frequency: Discovered in 12% of audited servers

Dependencies with typosquatted names or compromised maintainer accounts. Not detectable from source code alone — requires dependency audit.

npm audit
npx better-npm-audit audit
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9. Unscoped File System Write

Frequency: ~22% of file-writing servers

// Vulnerable
async function writeFile(path: string, content: string) {
  fs.writeFileSync(path, content); // can overwrite ~/.ssh/authorized_keys
}

// Fixed — same pattern as read: resolve and scope
async function writeFile(relativePath: string, content: string) {
  const base = path.resolve('./workspace');
  const target = path.resolve(base, relativePath);
  if (!target.startsWith(base + path.sep)) throw new Error('Access denied');
  fs.writeFileSync(target, content);
}
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10. No Caller Authentication on HTTP Transport

Frequency: ~45% of HTTP-transport servers

// Vulnerable — anyone can call the server
const httpServer = createServer(async (req, res) => {
  const transport = new StreamableHTTPServerTransport(...);
  await server.connect(transport);
});

// Fixed — verify API key before processing
const httpServer = createServer(async (req, res) => {
  const key = req.headers['x-api-key'];
  if (!key || key !== process.env.MCP_API_KEY) {
    res.writeHead(401);
    res.end();
    return;
  }
  const transport = new StreamableHTTPServerTransport(...);
  await server.connect(transport);
});
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Severity-rated output with file paths, line numbers, and fix recommendations.

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Top comments (1)

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mickyarun profile image
arun rajkumar •

The prompt injection via tool responses (#3) is the one that keeps me up at night in a fintech context. If your MCP server fetches external content and returns it raw, you're essentially letting any webpage author give instructions to your AI agent. For payments infrastructure, that's not a theoretical risk — it's the kind of thing that shows up in FCA audit findings.

The Zod validation fix in #6 is exactly the pattern we use across our services too, though I'd push it further: validate the output of tool calls with Zod, not just the input. If an MCP tool is expected to return a structured payment record, parse it against a schema before the agent acts on it. Garbage in from an external API shouldn't become garbage that drives a financial decision.

Good list. The credentials-in-error-messages one (#5) is embarrassingly common and embarrassingly easy to fix.