python-sdk/src/mcp/client/auth/oauth2.py

"""
OAuth2 Authentication implementation for HTTPX.

Implements authorization code flow with PKCE and automatic token refresh.
"""

import base64
import hashlib
import logging
import re
import secrets
import string
import time
from collections.abc import AsyncGenerator, Awaitable, Callable
from dataclasses import dataclass, field
from typing import Any, Protocol
from urllib.parse import urlencode, urljoin, urlparse

import anyio
import httpx
from pydantic import BaseModel, Field, ValidationError

from mcp.client.streamable_http import MCP_PROTOCOL_VERSION
from mcp.shared.auth import (
    OAuthClientInformationFull,
    OAuthClientMetadata,
    OAuthMetadata,
    OAuthToken,
    ProtectedResourceMetadata,
)
from mcp.shared.auth_utils import check_resource_allowed, resource_url_from_server_url
from mcp.types import LATEST_PROTOCOL_VERSION

logger = logging.getLogger(__name__)


class OAuthFlowError(Exception):
    """Base exception for OAuth flow errors."""


class OAuthTokenError(OAuthFlowError):
    """Raised when token operations fail."""


class OAuthRegistrationError(OAuthFlowError):
    """Raised when client registration fails."""


class PKCEParameters(BaseModel):
    """PKCE (Proof Key for Code Exchange) parameters."""

    code_verifier: str = Field(..., min_length=43, max_length=128)
    code_challenge: str = Field(..., min_length=43, max_length=128)

    @classmethod
    def generate(cls) -> "PKCEParameters":
        """Generate new PKCE parameters."""
        code_verifier = "".join(secrets.choice(string.ascii_letters + string.digits + "-._~") for _ in range(128))
        digest = hashlib.sha256(code_verifier.encode()).digest()
        code_challenge = base64.urlsafe_b64encode(digest).decode().rstrip("=")
        return cls(code_verifier=code_verifier, code_challenge=code_challenge)


class TokenStorage(Protocol):
    """Protocol for token storage implementations."""

    async def get_tokens(self) -> OAuthToken | None:
        """Get stored tokens."""
        ...

    async def set_tokens(self, tokens: OAuthToken) -> None:
        """Store tokens."""
        ...

    async def get_client_info(self) -> OAuthClientInformationFull | None:
        """Get stored client information."""
        ...

    async def set_client_info(self, client_info: OAuthClientInformationFull) -> None:
        """Store client information."""
        ...


@dataclass
class OAuthContext:
    """OAuth flow context."""

    server_url: str
    client_metadata: OAuthClientMetadata
    storage: TokenStorage
    redirect_handler: Callable[[str], Awaitable[None]] | None
    callback_handler: Callable[[], Awaitable[tuple[str, str | None]]] | None
    timeout: float = 300.0

    # Discovered metadata
    protected_resource_metadata: ProtectedResourceMetadata | None = None
    oauth_metadata: OAuthMetadata | None = None
    auth_server_url: str | None = None
    protocol_version: str | None = None

    # Client registration
    client_info: OAuthClientInformationFull | None = None

    # Token management
    current_tokens: OAuthToken | None = None
    token_expiry_time: float | None = None

    # State
    lock: anyio.Lock = field(default_factory=anyio.Lock)

    def get_authorization_base_url(self, server_url: str) -> str:
        """Extract base URL by removing path component."""
        parsed = urlparse(server_url)
        return f"{parsed.scheme}://{parsed.netloc}"

    def update_token_expiry(self, token: OAuthToken) -> None:
        """Update token expiry time."""
        if token.expires_in:
            self.token_expiry_time = time.time() + token.expires_in
        else:
            self.token_expiry_time = None

    def is_token_valid(self) -> bool:
        """Check if current token is valid."""
        return bool(
            self.current_tokens
            and self.current_tokens.access_token
            and (not self.token_expiry_time or time.time() <= self.token_expiry_time)
        )

    def can_refresh_token(self) -> bool:
        """Check if token can be refreshed."""
        return bool(self.current_tokens and self.current_tokens.refresh_token and self.client_info)

    def clear_tokens(self) -> None:
        """Clear current tokens."""
        self.current_tokens = None
        self.token_expiry_time = None

    def get_resource_url(self) -> str:
        """Get resource URL for RFC 8707.

        Uses PRM resource if it's a valid parent, otherwise uses canonical server URL.
        """
        resource = resource_url_from_server_url(self.server_url)

        # If PRM provides a resource that's a valid parent, use it
        if self.protected_resource_metadata and self.protected_resource_metadata.resource:
            prm_resource = str(self.protected_resource_metadata.resource)
            if check_resource_allowed(requested_resource=resource, configured_resource=prm_resource):
                resource = prm_resource

        return resource

    def should_include_resource_param(self, protocol_version: str | None = None) -> bool:
        """Determine if the resource parameter should be included in OAuth requests.

        Returns True if:
        - Protected resource metadata is available, OR
        - MCP-Protocol-Version header is 2025-06-18 or later
        """
        # If we have protected resource metadata, include the resource param
        if self.protected_resource_metadata is not None:
            return True

        # If no protocol version provided, don't include resource param
        if not protocol_version:
            return False

        # Check if protocol version is 2025-06-18 or later
        # Version format is YYYY-MM-DD, so string comparison works
        return protocol_version >= "2025-06-18"


class OAuthClientProvider(httpx.Auth):
    """
    OAuth2 authentication for httpx.
    Handles OAuth flow with automatic client registration and token storage.
    """

    requires_response_body = True

    def __init__(
        self,
        server_url: str,
        client_metadata: OAuthClientMetadata,
        storage: TokenStorage,
        redirect_handler: Callable[[str], Awaitable[None]] | None = None,
        callback_handler: Callable[[], Awaitable[tuple[str, str | None]]] | None = None,
        timeout: float = 300.0,
    ):
        """Initialize OAuth2 authentication."""
        self.context = OAuthContext(
            server_url=server_url,
            client_metadata=client_metadata,
            storage=storage,
            redirect_handler=redirect_handler,
            callback_handler=callback_handler,
            timeout=timeout,
        )
        self._initialized = False

    def _build_protected_resource_discovery_urls(self, init_response: httpx.Response) -> list[str]:
        """
        Build ordered list of URLs to try for protected resource metadata discovery.

        Per SEP-985, the client MUST:
        1. Try resource_metadata from WWW-Authenticate header (if present)
        2. Fall back to path-based well-known URI: /.well-known/oauth-protected-resource/{path}
        3. Fall back to root-based well-known URI: /.well-known/oauth-protected-resource

        Args:
            init_response: The initial 401 response from the server

        Returns:
            Ordered list of URLs to try for discovery
        """
        urls: list[str] = []

        # Priority 1: WWW-Authenticate header with resource_metadata parameter
        www_auth_url = self._extract_resource_metadata_from_www_auth(init_response)
        if www_auth_url:
            urls.append(www_auth_url)

        # Priority 2-3: Well-known URIs (RFC 9728)
        parsed = urlparse(self.context.server_url)
        base_url = f"{parsed.scheme}://{parsed.netloc}"

        # Priority 2: Path-based well-known URI (if server has a path component)
        if parsed.path and parsed.path != "/":
            path_based_url = urljoin(base_url, f"/.well-known/oauth-protected-resource{parsed.path}")
            urls.append(path_based_url)

        # Priority 3: Root-based well-known URI
        root_based_url = urljoin(base_url, "/.well-known/oauth-protected-resource")
        urls.append(root_based_url)

        return urls

    def _extract_field_from_www_auth(self, init_response: httpx.Response, field_name: str) -> str | None:
        """
        Extract field from WWW-Authenticate header.

        Returns:
            Field value if found in WWW-Authenticate header, None otherwise
        """
        www_auth_header = init_response.headers.get("WWW-Authenticate")
        if not www_auth_header:
            return None

        # Pattern matches: field_name="value" or field_name=value (unquoted)
        pattern = rf'{field_name}=(?:"([^"]+)"|([^\s,]+))'
        match = re.search(pattern, www_auth_header)

        if match:
            # Return quoted value if present, otherwise unquoted value
            return match.group(1) or match.group(2)

        return None

    def _extract_resource_metadata_from_www_auth(self, init_response: httpx.Response) -> str | None:
        """
        Extract protected resource metadata URL from WWW-Authenticate header as per RFC9728.

        Returns:
            Resource metadata URL if found in WWW-Authenticate header, None otherwise
        """
        if not init_response or init_response.status_code != 401:
            return None

        return self._extract_field_from_www_auth(init_response, "resource_metadata")

    def _extract_scope_from_www_auth(self, init_response: httpx.Response) -> str | None:
        """
        Extract scope parameter from WWW-Authenticate header as per RFC6750.

        Returns:
            Scope string if found in WWW-Authenticate header, None otherwise
        """
        return self._extract_field_from_www_auth(init_response, "scope")

    async def _handle_protected_resource_response(self, response: httpx.Response) -> bool:
        """
        Handle protected resource metadata discovery response.

        Per SEP-985, supports fallback when discovery fails at one URL.

        Returns:
            True if metadata was successfully discovered, False if we should try next URL
        """
        if response.status_code == 200:
            try:
                content = await response.aread()
                metadata = ProtectedResourceMetadata.model_validate_json(content)
                self.context.protected_resource_metadata = metadata
                if metadata.authorization_servers:
                    self.context.auth_server_url = str(metadata.authorization_servers[0])
                return True

            except ValidationError:
                # Invalid metadata - try next URL
                logger.warning(f"Invalid protected resource metadata at {response.request.url}")
                return False
        elif response.status_code == 404:
            # Not found - try next URL in fallback chain
            logger.debug(f"Protected resource metadata not found at {response.request.url}, trying next URL")
            return False
        else:
            # Other error - fail immediately
            raise OAuthFlowError(f"Protected Resource Metadata request failed: {response.status_code}")

    def _select_scopes(self, init_response: httpx.Response) -> None:
        """Select scopes as outlined in the 'Scope Selection Strategy in the MCP spec."""
        # Per MCP spec, scope selection priority order:
        # 1. Use scope from WWW-Authenticate header (if provided)
        # 2. Use all scopes from PRM scopes_supported (if available)
        # 3. Omit scope parameter if neither is available
        #
        www_authenticate_scope = self._extract_scope_from_www_auth(init_response)
        if www_authenticate_scope is not None:
            # Priority 1: WWW-Authenticate header scope
            self.context.client_metadata.scope = www_authenticate_scope
        elif (
            self.context.protected_resource_metadata is not None
            and self.context.protected_resource_metadata.scopes_supported is not None
        ):
            # Priority 2: PRM scopes_supported
            self.context.client_metadata.scope = " ".join(self.context.protected_resource_metadata.scopes_supported)
        else:
            # Priority 3: Omit scope parameter
            self.context.client_metadata.scope = None

    def _get_discovery_urls(self) -> list[str]:
        """Generate ordered list of (url, type) tuples for discovery attempts."""
        urls: list[str] = []
        auth_server_url = self.context.auth_server_url or self.context.server_url
        parsed = urlparse(auth_server_url)
        base_url = f"{parsed.scheme}://{parsed.netloc}"

        # RFC 8414: Path-aware OAuth discovery
        if parsed.path and parsed.path != "/":
            oauth_path = f"/.well-known/oauth-authorization-server{parsed.path.rstrip('/')}"
            urls.append(urljoin(base_url, oauth_path))

        # OAuth root fallback
        urls.append(urljoin(base_url, "/.well-known/oauth-authorization-server"))

        # RFC 8414 section 5: Path-aware OIDC discovery
        # See https://www.rfc-editor.org/rfc/rfc8414.html#section-5
        if parsed.path and parsed.path != "/":
            oidc_path = f"/.well-known/openid-configuration{parsed.path.rstrip('/')}"
            urls.append(urljoin(base_url, oidc_path))

        # OIDC 1.0 fallback (appends to full URL per OIDC spec)
        oidc_fallback = f"{auth_server_url.rstrip('/')}/.well-known/openid-configuration"
        urls.append(oidc_fallback)

        return urls

    async def _register_client(self) -> httpx.Request | None:
        """Build registration request or skip if already registered."""
        if self.context.client_info:
            return None

        if self.context.oauth_metadata and self.context.oauth_metadata.registration_endpoint:
            registration_url = str(self.context.oauth_metadata.registration_endpoint)
        else:
            auth_base_url = self.context.get_authorization_base_url(self.context.server_url)
            registration_url = urljoin(auth_base_url, "/register")

        registration_data = self.context.client_metadata.model_dump(by_alias=True, mode="json", exclude_none=True)

        return httpx.Request(
            "POST", registration_url, json=registration_data, headers={"Content-Type": "application/json"}
        )

    async def _handle_registration_response(self, response: httpx.Response) -> None:
        """Handle registration response."""
        if response.status_code not in (200, 201):
            await response.aread()
            raise OAuthRegistrationError(f"Registration failed: {response.status_code} {response.text}")

        try:
            content = await response.aread()
            client_info = OAuthClientInformationFull.model_validate_json(content)
            self.context.client_info = client_info
            await self.context.storage.set_client_info(client_info)
        except ValidationError as e:
            raise OAuthRegistrationError(f"Invalid registration response: {e}")

    async def _perform_authorization(self) -> httpx.Request:
        """Perform the authorization flow."""
        auth_code, code_verifier = await self._perform_authorization_code_grant()
        token_request = await self._exchange_token_authorization_code(auth_code, code_verifier)
        return token_request

    async def _perform_authorization_code_grant(self) -> tuple[str, str]:
        """Perform the authorization redirect and get auth code."""
        if self.context.client_metadata.redirect_uris is None:
            raise OAuthFlowError("No redirect URIs provided for authorization code grant")
        if not self.context.redirect_handler:
            raise OAuthFlowError("No redirect handler provided for authorization code grant")
        if not self.context.callback_handler:
            raise OAuthFlowError("No callback handler provided for authorization code grant")

        if self.context.oauth_metadata and self.context.oauth_metadata.authorization_endpoint:
            auth_endpoint = str(self.context.oauth_metadata.authorization_endpoint)
        else:
            auth_base_url = self.context.get_authorization_base_url(self.context.server_url)
            auth_endpoint = urljoin(auth_base_url, "/authorize")

        if not self.context.client_info:
            raise OAuthFlowError("No client info available for authorization")

        # Generate PKCE parameters
        pkce_params = PKCEParameters.generate()
        state = secrets.token_urlsafe(32)

        auth_params = {
            "response_type": "code",
            "client_id": self.context.client_info.client_id,
            "redirect_uri": str(self.context.client_metadata.redirect_uris[0]),
            "state": state,
            "code_challenge": pkce_params.code_challenge,
            "code_challenge_method": "S256",
        }

        # Only include resource param if conditions are met
        if self.context.should_include_resource_param(self.context.protocol_version):
            auth_params["resource"] = self.context.get_resource_url()  # RFC 8707

        if self.context.client_metadata.scope:
            auth_params["scope"] = self.context.client_metadata.scope

        authorization_url = f"{auth_endpoint}?{urlencode(auth_params)}"
        await self.context.redirect_handler(authorization_url)

        # Wait for callback
        auth_code, returned_state = await self.context.callback_handler()

        if returned_state is None or not secrets.compare_digest(returned_state, state):
            raise OAuthFlowError(f"State parameter mismatch: {returned_state} != {state}")

        if not auth_code:
            raise OAuthFlowError("No authorization code received")

        # Return auth code and code verifier for token exchange
        return auth_code, pkce_params.code_verifier

    def _get_token_endpoint(self) -> str:
        if self.context.oauth_metadata and self.context.oauth_metadata.token_endpoint:
            token_url = str(self.context.oauth_metadata.token_endpoint)
        else:
            auth_base_url = self.context.get_authorization_base_url(self.context.server_url)
            token_url = urljoin(auth_base_url, "/token")
        return token_url

    async def _exchange_token_authorization_code(
        self, auth_code: str, code_verifier: str, *, token_data: dict[str, Any] | None = {}
    ) -> httpx.Request:
        """Build token exchange request for authorization_code flow."""
        if self.context.client_metadata.redirect_uris is None:
            raise OAuthFlowError("No redirect URIs provided for authorization code grant")
        if not self.context.client_info:
            raise OAuthFlowError("Missing client info")

        token_url = self._get_token_endpoint()
        token_data = token_data or {}
        token_data.update(
            {
                "grant_type": "authorization_code",
                "code": auth_code,
                "redirect_uri": str(self.context.client_metadata.redirect_uris[0]),
                "client_id": self.context.client_info.client_id,
                "code_verifier": code_verifier,
            }
        )

        # Only include resource param if conditions are met
        if self.context.should_include_resource_param(self.context.protocol_version):
            token_data["resource"] = self.context.get_resource_url()  # RFC 8707

        if self.context.client_info.client_secret:
            token_data["client_secret"] = self.context.client_info.client_secret

        return httpx.Request(
            "POST", token_url, data=token_data, headers={"Content-Type": "application/x-www-form-urlencoded"}
        )

    async def _handle_token_response(self, response: httpx.Response) -> None:
        """Handle token exchange response."""
        if response.status_code != 200:
            body = await response.aread()
            body = body.decode("utf-8")
            raise OAuthTokenError(f"Token exchange failed ({response.status_code}): {body}")

        try:
            content = await response.aread()
            token_response = OAuthToken.model_validate_json(content)

            # Validate scopes
            if token_response.scope and self.context.client_metadata.scope:
                requested_scopes = set(self.context.client_metadata.scope.split())
                returned_scopes = set(token_response.scope.split())
                unauthorized_scopes = returned_scopes - requested_scopes
                if unauthorized_scopes:
                    raise OAuthTokenError(f"Server granted unauthorized scopes: {unauthorized_scopes}")

            self.context.current_tokens = token_response
            self.context.update_token_expiry(token_response)
            await self.context.storage.set_tokens(token_response)
        except ValidationError as e:
            raise OAuthTokenError(f"Invalid token response: {e}")

    async def _refresh_token(self) -> httpx.Request:
        """Build token refresh request."""
        if not self.context.current_tokens or not self.context.current_tokens.refresh_token:
            raise OAuthTokenError("No refresh token available")

        if not self.context.client_info:
            raise OAuthTokenError("No client info available")

        if self.context.oauth_metadata and self.context.oauth_metadata.token_endpoint:
            token_url = str(self.context.oauth_metadata.token_endpoint)
        else:
            auth_base_url = self.context.get_authorization_base_url(self.context.server_url)
            token_url = urljoin(auth_base_url, "/token")

        refresh_data = {
            "grant_type": "refresh_token",
            "refresh_token": self.context.current_tokens.refresh_token,
            "client_id": self.context.client_info.client_id,
        }

        # Only include resource param if conditions are met
        if self.context.should_include_resource_param(self.context.protocol_version):
            refresh_data["resource"] = self.context.get_resource_url()  # RFC 8707

        if self.context.client_info.client_secret:
            refresh_data["client_secret"] = self.context.client_info.client_secret

        return httpx.Request(
            "POST", token_url, data=refresh_data, headers={"Content-Type": "application/x-www-form-urlencoded"}
        )

    async def _handle_refresh_response(self, response: httpx.Response) -> bool:
        """Handle token refresh response. Returns True if successful."""
        if response.status_code != 200:
            logger.warning(f"Token refresh failed: {response.status_code}")
            self.context.clear_tokens()
            return False

        try:
            content = await response.aread()
            token_response = OAuthToken.model_validate_json(content)

            self.context.current_tokens = token_response
            self.context.update_token_expiry(token_response)
            await self.context.storage.set_tokens(token_response)

            return True
        except ValidationError:
            logger.exception("Invalid refresh response")
            self.context.clear_tokens()
            return False

    async def _initialize(self) -> None:
        """Load stored tokens and client info."""
        self.context.current_tokens = await self.context.storage.get_tokens()
        self.context.client_info = await self.context.storage.get_client_info()
        self._initialized = True

    def _add_auth_header(self, request: httpx.Request) -> None:
        """Add authorization header to request if we have valid tokens."""
        if self.context.current_tokens and self.context.current_tokens.access_token:
            request.headers["Authorization"] = f"Bearer {self.context.current_tokens.access_token}"

    def _create_oauth_metadata_request(self, url: str) -> httpx.Request:
        return httpx.Request("GET", url, headers={MCP_PROTOCOL_VERSION: LATEST_PROTOCOL_VERSION})

    async def _handle_oauth_metadata_response(self, response: httpx.Response) -> None:
        content = await response.aread()
        metadata = OAuthMetadata.model_validate_json(content)
        self.context.oauth_metadata = metadata

    async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.Request, httpx.Response]:
        """HTTPX auth flow integration."""
        async with self.context.lock:
            if not self._initialized:
                await self._initialize()

            # Capture protocol version from request headers
            self.context.protocol_version = request.headers.get(MCP_PROTOCOL_VERSION)

            if not self.context.is_token_valid() and self.context.can_refresh_token():
                # Try to refresh token
                refresh_request = await self._refresh_token()
                refresh_response = yield refresh_request

                if not await self._handle_refresh_response(refresh_response):
                    # Refresh failed, need full re-authentication
                    self._initialized = False

            if self.context.is_token_valid():
                self._add_auth_header(request)

            response = yield request

            if response.status_code == 401:
                # Perform full OAuth flow
                try:
                    # OAuth flow must be inline due to generator constraints
                    # Step 1: Discover protected resource metadata (SEP-985 with fallback support)
                    discovery_urls = self._build_protected_resource_discovery_urls(response)
                    discovery_success = False
                    for url in discovery_urls:
                        discovery_request = httpx.Request(
                            "GET", url, headers={MCP_PROTOCOL_VERSION: LATEST_PROTOCOL_VERSION}
                        )
                        discovery_response = yield discovery_request
                        discovery_success = await self._handle_protected_resource_response(discovery_response)
                        if discovery_success:
                            break

                    if not discovery_success:
                        raise OAuthFlowError("Protected resource metadata discovery failed: no valid metadata found")

                    # Step 2: Apply scope selection strategy
                    self._select_scopes(response)

                    # Step 3: Discover OAuth metadata (with fallback for legacy servers)
                    discovery_urls = self._get_discovery_urls()
                    for url in discovery_urls:
                        oauth_metadata_request = self._create_oauth_metadata_request(url)
                        oauth_metadata_response = yield oauth_metadata_request

                        if oauth_metadata_response.status_code == 200:
                            try:
                                await self._handle_oauth_metadata_response(oauth_metadata_response)
                                break
                            except ValidationError:
                                continue
                        elif oauth_metadata_response.status_code < 400 or oauth_metadata_response.status_code >= 500:
                            break  # Non-4XX error, stop trying

                    # Step 4: Register client if needed
                    registration_request = await self._register_client()
                    if registration_request:
                        registration_response = yield registration_request
                        await self._handle_registration_response(registration_response)

                    # Step 5: Perform authorization and complete token exchange
                    token_response = yield await self._perform_authorization()
                    await self._handle_token_response(token_response)
                except Exception:
                    logger.exception("OAuth flow error")
                    raise

                # Retry with new tokens
                self._add_auth_header(request)
                yield request
            elif response.status_code == 403:
                # Step 1: Extract error field from WWW-Authenticate header
                error = self._extract_field_from_www_auth(response, "error")

                # Step 2: Check if we need to step-up authorization
                if error == "insufficient_scope":
                    try:
                        # Step 2a: Update the required scopes
                        self._select_scopes(response)

                        # Step 2b: Perform (re-)authorization and token exchange
                        token_response = yield await self._perform_authorization()
                        await self._handle_token_response(token_response)
                    except Exception:
                        logger.exception("OAuth flow error")
                        raise

                # Retry with new tokens
                self._add_auth_header(request)
                yield request