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NextMCP Authentication Architecture

This document explains how NextMCP's authentication system works internally, how the components fit together, and the design decisions behind it.

Table of Contents

  1. System Overview
  2. Component Architecture
  3. Request Flow
  4. Data Flow
  5. Design Decisions
  6. Security Considerations
  7. Performance Characteristics

System Overview

NextMCP's auth system consists of three main layers:

┌──────────────────────────────────────────────────────────────┐
│                      MCP Client/Host                         │
│  (Claude Desktop, Cursor, etc.)                              │
└────────────────────┬─────────────────────────────────────────┘
                     │
                     │ 1. Reads auth metadata
                     │ 2. Initiates OAuth flow
                     │ 3. Sends requests with tokens
                     │
┌────────────────────▼─────────────────────────────────────────┐
│                  Auth Metadata Protocol                      │
│  • Announces auth requirements                               │
│  • Lists providers, scopes, permissions                      │
│  • JSON schema for validation                                │
└────────────────────┬─────────────────────────────────────────┘
                     │
┌────────────────────▼─────────────────────────────────────────┐
│              Request Enforcement Middleware                  │
│  • Validates every request                                   │
│  • Checks auth credentials                                   │
│  • Enforces scopes/permissions                               │
│  • Manages sessions                                          │
└────────────────────┬─────────────────────────────────────────┘
                     │
         ┌───────────┴──────────┐
         │                      │
┌────────▼──────────┐  ┌────────▼──────────┐
│  OAuth Providers  │  │  Session Store    │
│  • GitHub         │  │  • Memory         │
│  • Google         │  │  • File           │
│  • Custom         │  │  • Redis (future) │
└───────────────────┘  └───────────────────┘

Component Architecture

1. Auth Metadata Protocol

Location: nextmcp/protocol/auth_metadata.py

Purpose: Allows servers to announce their auth requirements so hosts can discover them.

Key Classes:

class AuthMetadata:
    """Top-level auth requirements."""
    requirement: AuthRequirement  # REQUIRED, OPTIONAL, NONE
    providers: list[AuthProviderMetadata]
    required_scopes: list[str]
    optional_scopes: list[str]
    permissions: list[str]
    supports_multi_user: bool
    token_refresh_enabled: bool

class AuthProviderMetadata:
    """Single OAuth provider info."""
    name: str  # "google", "github"
    type: str  # "oauth2"
    flows: list[AuthFlowType]  # [OAUTH2_PKCE]
    authorization_url: str
    token_url: str
    scopes: list[str]
    supports_refresh: bool

Usage:

# Server creates metadata
metadata = AuthMetadata(requirement=AuthRequirement.REQUIRED)
metadata.add_provider(
    name="google",
    type="oauth2",
    flows=[AuthFlowType.OAUTH2_PKCE],
    ...
)

# Serialize for transmission
json_data = metadata.to_dict()

# Host reads and understands auth requirements
metadata = AuthMetadata.from_dict(json_data)

Why it exists: Before this, hosts had no way to know what auth a server needed. Now they can:

  • Show "Connect Google Account" UI
  • Request appropriate OAuth scopes
  • Handle auth failures gracefully

2. Session Store

Location: nextmcp/session/session_store.py

Purpose: Persistent storage for OAuth tokens and user sessions

Interface:

class SessionStore(ABC):
    """Abstract interface for session storage."""

    def save(self, session: SessionData) -> None
    def load(self, user_id: str) -> SessionData | None
    def delete(self, user_id: str) -> bool
    def exists(self, user_id: str) -> bool
    def list_users(self) -> list[str]
    def clear_all(self) -> int
    def update_tokens(...) -> None

Implementations:

MemorySessionStore:

class MemorySessionStore(SessionStore):
    """In-memory, lost on restart."""
    _sessions: dict[str, SessionData]  # user_id -> session
    _lock: threading.RLock  # Thread-safe
  • Pros: Fast (O(1) lookup), simple
  • Cons: Lost on restart, single-process only
  • Use case: Development, testing

FileSessionStore:

class FileSessionStore(SessionStore):
    """JSON files on disk."""
    directory: Path  # .sessions/
    # Each user = one JSON file: session_user123.json
  • Pros: Persists across restarts
  • Cons: Single-server only, file I/O overhead
  • Use case: Production (single server)

Future: RedisSessionStore:

class RedisSessionStore(SessionStore):
    """Distributed, scalable."""
    # redis.set(f"session:{user_id}", json.dumps(session))
  • Pros: Distributed, scalable, TTL support
  • Cons: Requires Redis
  • Use case: Production (multi-server)

SessionData Model:

@dataclass
class SessionData:
    user_id: str
    access_token: str | None
    refresh_token: str | None
    expires_at: float | None  # Unix timestamp
    scopes: list[str]
    user_info: dict  # Name, email, etc.
    provider: str  # "google", "github"
    created_at: float
    updated_at: float
    metadata: dict  # Custom app data

    def is_expired(self) -> bool
    def needs_refresh(self, buffer_seconds=300) -> bool

3. Request Enforcement Middleware

Location: nextmcp/auth/request_middleware.py

Purpose: Intercept every MCP request and enforce auth automatically

How it Works:

Request arrives
     │
     ▼
┌─────────────────────────────────────┐
│ Is auth required?                   │
│ (Check metadata.requirement)        │
└────┬────────────────────────────┬───┘
     │                            │
     │ YES                        │ NO
     ▼                            │
┌─────────────────────────────────┐   │
│ Extract credentials from request│   │
│ request.get("auth")             │   │
└────┬────────────────────────────┘   │
     │                                │
     ▼                                │
┌─────────────────────────────────┐   │
│ Check session store              │   │
│ Does user have existing session? │   │
└────┬────────────────────────┬───┘   │
     │                        │       │
     │ YES                    │ NO    │
     ▼                        ▼       │
┌────────────────┐   ┌──────────────┐│
│ Reuse session  │   │ Authenticate ││
│ Check expired  │   │ with provider││
└────┬───────────┘   └──────┬───────┘│
     │                      │        │
     ▼                      ▼        │
┌──────────────────────────────────┐ │
│ Token expired?                   │ │
│ Auto-refresh if enabled          │ │
└────┬─────────────────────────────┘ │
     │                                │
     ▼                                │
┌──────────────────────────────────┐ │
│ Check scopes & permissions       │ │
│ (required_scopes, manifest)      │ │
└────┬─────────────────────────────┘ │
     │                                │
     │ All checks passed              │
     ▼                                ▼
┌──────────────────────────────────────┐
│ Inject auth_context into request    │
│ request["_auth_context"] = context  │
└────┬─────────────────────────────────┘
     │
     ▼
┌──────────────────────────────────────┐
│ Call next handler                    │
│ Tool function executes               │
└──────────────────────────────────────┘

Code Structure:

class AuthEnforcementMiddleware:
    def __init__(
        self,
        provider: AuthProvider,
        session_store: SessionStore | None,
        metadata: AuthMetadata | None,
        manifest: PermissionManifest | None,
        auto_refresh_tokens: bool = True,
    ):
        ...

    async def __call__(self, request: dict, handler: Callable):
        # 1. Check if auth required
        if self.metadata.requirement == AuthRequirement.NONE:
            return await handler(request)

        # 2. Extract credentials
        credentials = request.get("auth", {})

        # 3. Authenticate
        auth_result = await self._authenticate(credentials, request)

        # 4. Check authorization
        self._check_authorization(auth_result.context, request)

        # 5. Inject context
        request["_auth_context"] = auth_result.context

        # 6. Call handler
        return await handler(request)

Key Methods:

async def _authenticate(self, credentials, request):
    """Validate credentials, check session, refresh if needed."""
    # 1. Check session store first (reuse)
    # 2. If not found, call provider.authenticate()
    # 3. Save new session
    # 4. Auto-refresh if expiring
    # 5. Return AuthResult

def _check_authorization(self, auth_context, request):
    """Check scopes and permissions."""
    # 1. Check required_scopes
    # 2. Check manifest (if provided)
    # 3. Raise AuthorizationError if denied

4. OAuth Providers

Location: nextmcp/auth/oauth.py, nextmcp/auth/oauth_providers.py

Base Class:

class OAuthProvider(AuthProvider, ABC):
    """Base OAuth 2.0 provider with PKCE."""

    def generate_authorization_url(self, state=None) -> dict:
        """Create auth URL with PKCE."""
        pkce = PKCEChallenge.generate()
        url = f"{self.config.authorization_url}?..."
        return {"url": url, "state": state, "verifier": pkce.verifier}

    async def exchange_code_for_token(self, code, state, verifier):
        """Exchange code for access token."""
        # POST to token_url with code + verifier
        # Handle both JSON and form-encoded responses
        return token_data

    async def refresh_access_token(self, refresh_token):
        """Refresh expired token."""
        # POST to token_url with refresh_token
        return new_token_data

    @abstractmethod
    async def get_user_info(self, access_token):
        """Provider-specific user info endpoint."""
        pass

Built-in Providers:

class GitHubOAuthProvider(OAuthProvider):
    """GitHub OAuth with PKCE."""
    # authorization_url: github.com/login/oauth/authorize
    # token_url: github.com/login/oauth/access_token
    # Returns form-encoded tokens

class GoogleOAuthProvider(OAuthProvider):
    """Google OAuth with PKCE."""
    # authorization_url: accounts.google.com/o/oauth2/v2/auth
    # token_url: oauth2.googleapis.com/token
    # Returns JSON tokens
    # Supports refresh tokens with access_type=offline

PKCE Flow:

1. Server generates PKCE challenge
   verifier = random_43_chars()
   challenge = SHA256(verifier)

2. Server sends challenge in auth URL
   redirect to: auth_url?code_challenge=...

3. User authorizes, provider sends code

4. Server exchanges code + verifier for token
   POST token_url with code + code_verifier

5. Provider validates: SHA256(verifier) == challenge
   Returns access_token + refresh_token

Why PKCE: Secure for public clients (no client secret exposed)

Request Flow

Complete Request Lifecycle:

1. Client sends request
   ┌────────────────────────────────────┐
   │ {                                  │
   │   "method": "tools/call",          │
   │   "params": {"name": "get_data"},  │
   │   "auth": {                        │
   │     "access_token": "ya29.a0..."   │
   │   }                                │
   │ }                                  │
   └────────────────────────────────────┘
                 │
                 ▼
2. Middleware intercepts
   ┌────────────────────────────────────┐
   │ AuthEnforcementMiddleware.__call__ │
   └────────────────────────────────────┘
                 │
                 ▼
3. Check auth requirement
   if metadata.requirement == NONE:
       skip auth
                 │
                 ▼
4. Extract credentials
   access_token = request["auth"]["access_token"]
                 │
                 ▼
5. Check session store
   session = session_store.load_by_token(access_token)
   if session:
       if session.is_expired():
           reject
       if session.needs_refresh():
           auto_refresh
       use session
                 │
                 ▼
6. Or authenticate with provider
   result = await provider.authenticate({
       "access_token": access_token
   })
                 │
                 ▼
7. Save new session
   session_store.save(SessionData(...))
                 │
                 ▼
8. Check authorization
   • Check required_scopes
   • Check manifest permissions
                 │
                 ▼
9. Inject auth context
   request["_auth_context"] = AuthContext(...)
                 │
                 ▼
10. Call tool handler
   result = await tool_function()
                 │
                 ▼
11. Return result to client
   ┌────────────────────────────────────┐
   │ {                                  │
   │   "result": {...}                  │
   │ }                                  │
   └────────────────────────────────────┘

Data Flow

OAuth Token Acquisition:

┌──────────┐                    ┌──────────────┐
│  Client  │                    │ OAuth Server │
│  (Host)  │                    │ (Google)     │
└────┬─────┘                    └──────┬───────┘
     │                                 │
     │ 1. GET /auth/metadata          │
     ├──────────────────────────────►  │
     │ Returns auth requirements       │
     │ {providers: [google], ...}      │
     │◄────────────────────────────────┤
     │                                 │
     │ 2. Generate auth URL            │
     │ provider.generate_authorization_url()
     │ Returns: {url, state, verifier} │
     │                                 │
     │ 3. Open browser to auth URL     │
     ├─────────────────────────────────►
     │                                 │
     │ 4. User authorizes              │
     │                        ┌────────┤
     │                        │Authorize│
     │                        └────────┤
     │                                 │
     │ 5. Redirect to callback with code
     │◄─────────────────────────────────┤
     │ http://localhost:8080/callback? │
     │ code=abc123&state=xyz           │
     │                                 │
     │ 6. Exchange code for token      │
     │ POST /token                     │
     │ code=abc123                     │
     │ code_verifier=...               │
     ├─────────────────────────────────►
     │                                 │
     │ 7. Returns tokens               │
     │ {access_token, refresh_token}   │
     │◄─────────────────────────────────┤
     │                                 │
     │ 8. Save session                 │
     │ session_store.save(...)         │
     │                                 │
     │ 9. Make authenticated requests  │
     │ {auth: {access_token: ...}}     │
     └──────────────────────────────────

Design Decisions

Why Middleware Instead of Decorators?

Decorators (old way):

@requires_auth_async(provider)
@requires_scope_async("read:data")
async def tool1():
    pass

@requires_auth_async(provider)
@requires_scope_async("read:data")
async def tool2():
    pass

Middleware (new way):

server.use(create_auth_middleware(
    provider=provider,
    required_scopes=["read:data"]
))

# All tools automatically protected

Advantages:

  1. DRY: Don't repeat decorators on every tool
  2. Centralized: One place to configure auth
  3. Automatic: Impossible to forget to add auth
  4. Flexible: Can still use decorators for tool-specific requirements

Why Session Store?

Without session store:

  • Must re-authenticate every request
  • Can't refresh tokens
  • Can't support multiple users
  • No session persistence

With session store:

  • ✅ Authenticate once, reuse session
  • ✅ Automatic token refresh
  • ✅ Multi-user support
  • ✅ Persists across restarts

Why Auth Metadata Protocol?

Before: Hosts had to guess or hardcode server auth requirements

After: Hosts can discover auth requirements dynamically

Benefits:

  • Standardized auth discovery
  • Better UX (show "Connect Google" UI)
  • Future-proof (new auth methods just work)

Security Considerations

Token Storage

Tokens in memory: MemorySessionStore

  • ✅ Fast
  • ⚠️ Lost on crash
  • ⚠️ Vulnerable to memory dumps

Tokens on disk: FileSessionStore

  • ✅ Persists
  • ⚠️ Vulnerable to file system access
  • Mitigation: Proper file permissions (chmod 600)

Tokens in Redis: RedisSessionStore (future)

  • ✅ Distributed
  • ✅ TTL support
  • Security: Encrypt Redis connection, use ACLs

PKCE

Why: Prevents authorization code interception attacks

How: Verifier proves client initiated the auth flow

Without PKCE: Attacker could intercept code and use it

With PKCE: Attacker can't use code without verifier

Token Refresh

Automatic refresh:

  • ✅ Good UX (no expiration errors)
  • ⚠️ Longer-lived access

Manual refresh:

  • ⚠️ Poor UX (users see errors)
  • ✅ Shorter-lived access

Recommendation: Use automatic refresh with short-lived access tokens (1 hour)

Scope Validation

Always validate scopes:

middleware = create_auth_middleware(
    required_scopes=["profile", "email"],
    # Don't trust client claims!
)

Why: Client could send fake scopes

How: Middleware validates against actual OAuth token scopes

Performance Characteristics

Session Store Performance:

Operation Memory File Redis
Save O(1) O(1) O(1)
Load O(1) O(1) O(1)
List O(n) O(n) O(n)
Delete O(1) O(1) O(1)

Middleware Overhead:

Per request:

  1. Session lookup: O(1) - ~0.1ms
  2. Token validation: O(1) - ~0.5ms (if cached)
  3. Scope check: O(m) where m = number of scopes - ~0.01ms
  4. Total overhead: ~1ms per request

With auto-refresh:

  • Check if token expiring: O(1) - ~0.01ms
  • Refresh if needed: ~500ms (network call, rare)

Scalability:

Single server:

  • MemorySessionStore: 10,000+ concurrent users
  • FileSessionStore: 100,000+ users

Distributed:

  • RedisSessionStore: Millions of users

Extension Points

Custom Session Store:

class DatabaseSessionStore(SessionStore):
    """Store sessions in PostgreSQL."""

    def __init__(self, db_url):
        self.engine = create_engine(db_url)

    def save(self, session):
        with self.engine.connect() as conn:
            conn.execute(
                "INSERT INTO sessions (...) VALUES (...)"
            )

Custom OAuth Provider:

class CustomOAuthProvider(OAuthProvider):
    """Your company's OAuth."""

    async def get_user_info(self, access_token):
        # Call your user info endpoint
        return user_data

    def extract_user_id(self, user_info):
        return user_info["id"]

Custom Middleware:

class AuditMiddleware:
    """Log all auth events."""

    async def __call__(self, request, handler):
        auth_context = request.get("_auth_context")
        if auth_context:
            log_auth_event(auth_context.user_id, request)
        return await handler(request)

Summary

NextMCP's auth architecture provides:

  1. Discovery - Servers announce requirements (Auth Metadata)
  2. Enforcement - Every request is validated (Request Middleware)
  3. Persistence - Sessions survive restarts (Session Store)
  4. Flexibility - Pluggable providers and stores
  5. Security - PKCE, scope validation, token refresh
  6. Performance - <1ms overhead per request

The three-layer design (metadata + middleware + storage) creates a complete, production-ready auth system for MCP servers.