mcp-auth-middleware 0.1.3

JWE authentication middleware for MCP/Starlette applications

mcp-auth-middleware

JWE authentication middleware for HTTP MCP servers. Encrypt user data end-to-end so that only your MCP server can read it.

What it does

mcp-auth-middleware gives your MCP server two things:

1. A middleware that intercepts incoming requests, decrypts a JWE Bearer token, and makes the authenticated user's claims available via get_user().
2. A CLI (mcp-auth-middleware) that generates RSA key pairs in JWKS format, outputs Kubernetes Secret YAML, and securely deletes local keys when you're done.

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Installation

pip install mcp-auth-middleware

This installs the library and the mcp-auth-middleware CLI.

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Quick start (local development)

1. Generate keys

mcp-auth-middleware generate

Output:

Keys generated (JWKS format):
  Private: .keys/mcp-private.json
  Public:  .keys/mcp-public.json

Add .keys/ to your .gitignore immediately.

2. Set the environment variable

Create a .env file (or export directly):

MCP_KEY_FILE_PATH=.keys/mcp-private.json

3. Add the middleware to your MCP server

Works with any Starlette-based MCP server:

from fastmcp import FastMCP
from mcp_auth_middleware import JWKSAuthMiddleware, get_user
import uvicorn

mcp = FastMCP("My Server")

# Register your tools on `mcp` as usual …

app = mcp.http_app()
app.add_middleware(JWKSAuthMiddleware)

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8000)

Or with the official MCP Python SDK directly:

from mcp.server.fastmcp import FastMCP
from mcp_auth_middleware import JWKSAuthMiddleware, get_user
import uvicorn

mcp = FastMCP("My Server")

app = mcp.http_app()
app.add_middleware(JWKSAuthMiddleware)

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8000)

4. Access user claims inside any tool

@mcp.tool()
def whoami() -> str:
    user = get_user()
    return f"Hello, {user.name or 'anonymous'}!"

get_user() returns an AuthUser dict. Access claims as attributes — missing keys return None instead of raising.

---

Kubernetes deployment

1. Generate keys and pipe straight into kubectl

mcp-auth-middleware k8s | kubectl apply -f -

This creates a Kubernetes Secret named mcp-server-keys in the default namespace. Customise with flags:

mcp-auth-middleware k8s --namespace my-ns --secret-name my-mcp-keys | kubectl apply -f -

2. Clean up local key material

mcp-auth-middleware clean

Uses shred (Linux) for secure deletion when available; falls back to overwrite-then-delete.

3. Mount the secret in your deployment

Add the following snippets to your existing Deployment YAML.

Volume definition (under spec.template.spec.volumes):

- name: mcp-secret-volume
  secret:
    secretName: mcp-server-keys   # must match --secret-name
    defaultMode: 0400
    items:
      - key: mcp_jwks
        path: key.json

Volume mount (under your container's volumeMounts):

- mountPath: /etc/mcp/secrets
  name: mcp-secret-volume
  readOnly: true

Environment variable (under env):

- name: MCP_KEY_FILE_PATH
  value: "/etc/mcp/secrets/key.json"

A full example Deployment + Service is in examples/k8s-deployment.yaml.

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CLI reference

All commands accept -o / --output <dir> to change the key directory (default: .keys).

Command	Description
mcp-auth-middleware generate	Generate an RSA-4096 JWKS key pair
mcp-auth-middleware k8s	Generate keys and print a Kubernetes Secret YAML to stdout
mcp-auth-middleware clean	Securely delete keys from the output directory

mcp-auth-middleware k8s flags

Flag	Default	Description
-n, --namespace	default	Kubernetes namespace
-s, --secret-name	mcp-server-keys	Name of the K8s Secret

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API reference

JWKSAuthMiddleware

Starlette middleware. Attach it to any HTTP-based MCP server app:

app.add_middleware(
    JWKSAuthMiddleware,
    verifier=None,        # optional: provide your own JWETokenVerifier
    jwks_path="/.well-known/jwks.json",  # public-key endpoint path
)

The middleware automatically serves your server's public JWKS at the configured path so that token producers can discover your encryption key.

get_user() -> AuthUser

Returns the authenticated user's claims for the current request. Safe to call from any async context (tools, routes, dependencies) during request handling.

user = get_user()
user.email   # claim value or None
user["role"]  # also works as a regular dict

JWETokenVerifier

Lower-level class if you need to verify tokens outside the middleware:

from mcp_auth_middleware import JWETokenVerifier

verifier = JWETokenVerifier()          # reads MCP_KEY_FILE_PATH
claims = await verifier.verify_token(token_string)
public_jwks = verifier.get_jwks()       # for publishing

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Environment variables

Variable	Required	Description
MCP_KEY_FILE_PATH	Yes	Path to the private JWKS JSON file

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How the token flow works

Producer (client/gateway)             MCP Server
――――――――――――――――――――――――――             ――――――――――
                                      GET /.well-known/jwks.json
                              ◄────   { public RSA key }
Encrypt claims with public key
POST /mcp/tool  ──────────────────────►
  Authorization: Bearer <JWE>
                                      Middleware decrypts JWE
                                      with private key
                                      ─► get_user() returns claims

1. The token producer fetches the server's public key from /.well-known/jwks.json.
2. It encrypts a JSON payload (user claims) into a JWE token using that public key.
3. The token is sent as a Bearer token in the Authorization header.
4. The middleware decrypts it with the private key and exposes claims via get_user().

---

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Accessing the JWKS endpoint from a browser

If your token producer is a browser-based application that needs to fetch the public key from /.well-known/jwks.json, you must add CORS middleware after JWKSAuthMiddleware. Starlette applies middleware in reverse registration order, so the last one added runs first on incoming requests — this ensures CORS headers are set before auth is checked.

from mcp_auth_middleware import JWKSAuthMiddleware
from starlette.middleware.cors import CORSMiddleware

app = mcp.http_app()
app.add_middleware(JWKSAuthMiddleware)
app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],  # restrict to your domain(s) in production 
)

Without this, browsers will block the cross-origin request to the JWKS endpoint due to the same-origin policy.

Project structure

mcp-auth-middleware/
├── pyproject.toml
├── README.md
├── LICENSE
├── CHANGELOG.md
├── requirements.txt
├── requirements-dev.txt
├── MANIFEST.in
├── .gitignore
├── mcp_auth_middleware/
│   ├── __init__.py
│   ├── py.typed
│   ├── cli.py
│   ├── middleware.py
│   └── verifier.py
├── tests/
└── examples/
    ├── server.py
    └── k8s-deployment.yaml

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License

MIT