mcp-flashcards-cloud

A hosted, multi-user flashcards MCP server. Each person signs in with OAuth and sees only their own deck. Built on the official MCP Python SDK 2.x.

Status: work in progress. Step 1 of 4 is done: the server, per-user isolation, abuse limits, and an automated test suite that proves one user can't read or change another user's cards. Still to come: real sign-in with an identity provider, deployment, and a full security write-up. See TEST-LOG.md for what has been verified.

How isolation works

• Identity comes only from the verified access token. The server takes the token's subject (sub) after the SDK has checked the token, and keys all data by issuer plus subject. The token's own issuer must match the configured one, so the same sub from a different identity provider is a different person. Tool arguments, request metadata and headers are never used as identity, because a client controls all of them.

• Isolation is enforced in code, not in prompts. Every database query filters on the owner. There is no function that reads or changes a card without naming its owner.

• Nothing leaks across users. Card numbers are per user, so they reveal nothing about other decks. A card that belongs to someone else returns exactly the same "No card with id N" error as a card that doesn't exist.

• Tokens for other servers are rejected. The server only accepts tokens issued for its own URL (RFC 8707 audience check).

Related MCP server: mymlh-mcp-server

Limits

This is meant to run on free-tier hosting with open sign-up, so one account must not be able to exhaust memory, storage, or the server's attention:

Tools

Run the tests

Requires Python 3.12+.

python -m venv .venv
# Windows: .venv\Scripts\activate    macOS/Linux: source .venv/bin/activate
pip install -r requirements-dev.txt
python -m pytest
python tests/run_mutations.py   # proves the tests catch deliberately broken code

License

MIT - see LICENSE.