Validate the authorization server metadata issuer on every discovery path (#3398)
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Max
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3 changed files with 760 additions and 135 deletions
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@ -9,6 +9,7 @@ Provides OAuth providers for machine-to-machine authentication flows:
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import time
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from collections.abc import Awaitable, Callable
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from typing import Any, Literal
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from urllib.parse import urlparse
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from uuid import uuid4
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import httpx2
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@ -16,14 +17,50 @@ import jwt
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from pydantic import BaseModel, Field
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from mcp.client.auth import OAuthClientProvider, OAuthFlowError, TokenStorage
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from mcp.client.auth.oauth2 import OAuthContext
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from mcp.client.auth.utils import issuers_match
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from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata
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def _checked_issuer(issuer: str | None) -> str | None:
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if issuer is not None and urlparse(issuer).scheme not in ("http", "https"):
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raise ValueError(f"issuer must be the authorization server's http(s) issuer URL, got {issuer!r}")
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return issuer
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def _preferred_authorization_server(advertised: list[str], issuer: str | None) -> str:
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"""The advertised server matching the configured issuer if there is one, else the first."""
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return next(
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(server for server in advertised if issuer is not None and issuers_match(server, issuer)), advertised[0]
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)
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def _require_metadata_for_configured_issuer(context: OAuthContext, issuer: str | None) -> None:
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"""With an issuer configured, a token request is only built from metadata discovered for that issuer.
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Anything else held is dropped along with the tokens, so the next request starts discovery afresh
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rather than refreshing against it.
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"""
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if issuer is None:
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return
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metadata = context.oauth_metadata
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if metadata is not None and issuers_match(str(metadata.issuer), issuer):
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return
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context.oauth_metadata = None
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context.clear_tokens()
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if metadata is None:
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raise OAuthFlowError(f"No authorization server metadata discovered for configured issuer {issuer}")
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raise OAuthFlowError(f"Authorization server metadata issuer mismatch: {metadata.issuer} != {issuer}")
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class ClientCredentialsOAuthProvider(OAuthClientProvider):
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"""OAuth provider for client_credentials grant with client_id + client_secret.
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This provider sets client_info directly, bypassing dynamic client registration.
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Use this when you already have client credentials (client_id and client_secret).
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Pass `issuer` to name the authorization server those credentials belong to: token
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requests are then only built from authorization server metadata for that issuer, and
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the flow stops if the MCP server leads anywhere else.
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Example:
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```python
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@ -32,6 +69,7 @@ class ClientCredentialsOAuthProvider(OAuthClientProvider):
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storage=my_token_storage,
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client_id="my-client-id",
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client_secret="my-client-secret",
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issuer="https://auth.example.com",
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)
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```
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"""
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@ -44,6 +82,7 @@ class ClientCredentialsOAuthProvider(OAuthClientProvider):
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client_secret: str,
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token_endpoint_auth_method: Literal["client_secret_basic", "client_secret_post"] = "client_secret_basic",
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scope: str | None = None,
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issuer: str | None = None,
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) -> None:
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"""Initialize client_credentials OAuth provider.
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@ -55,6 +94,11 @@ class ClientCredentialsOAuthProvider(OAuthClientProvider):
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token_endpoint_auth_method: Authentication method for token endpoint.
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Either "client_secret_basic" (default) or "client_secret_post".
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scope: Optional space-separated list of scopes to request.
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issuer: The issuer identifier of the authorization server that issued
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`client_id` and `client_secret`. When set, token requests are only built from
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discovered authorization server metadata whose `issuer` is exactly this string;
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otherwise the flow stops with `OAuthFlowError`. When omitted, whichever
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authorization server discovery yields is used.
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"""
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# Build minimal client_metadata for the base class
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client_metadata = OAuthClientMetadata(
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@ -64,6 +108,7 @@ class ClientCredentialsOAuthProvider(OAuthClientProvider):
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scope=scope,
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)
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super().__init__(server_url, client_metadata, storage, None, None)
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self._issuer = _checked_issuer(issuer)
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# Store client_info to be set during _initialize - no dynamic registration needed
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self._fixed_client_info = OAuthClientInformationFull(
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redirect_uris=None,
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@ -80,12 +125,17 @@ class ClientCredentialsOAuthProvider(OAuthClientProvider):
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self.context.client_info = self._fixed_client_info
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self._initialized = True
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def _select_authorization_server(self, advertised: list[str]) -> str:
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return _preferred_authorization_server(advertised, self._issuer)
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async def _perform_authorization(self) -> httpx2.Request:
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"""Perform client_credentials authorization."""
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return await self._exchange_token_client_credentials()
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async def _exchange_token_client_credentials(self) -> httpx2.Request:
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"""Build token exchange request for client_credentials grant."""
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_require_metadata_for_configured_issuer(self.context, self._issuer)
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token_data: dict[str, Any] = {
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"grant_type": "client_credentials",
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}
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@ -196,7 +246,10 @@ class PrivateKeyJWTOAuthProvider(OAuthClientProvider):
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The JWT assertion's audience MUST be the authorization server's issuer identifier
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(per RFC 7523bis security updates). The `assertion_provider` callback receives
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this audience value and must return a JWT with that audience.
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this audience value and must return a JWT with that audience. Pass `issuer` to name
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the authorization server this client is registered with: an assertion is then only
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minted once metadata for that issuer has been discovered, and token requests are only
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built from that metadata.
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**Option 1: Pre-built JWT via Workload Identity Federation**
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@ -256,6 +309,7 @@ class PrivateKeyJWTOAuthProvider(OAuthClientProvider):
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client_id: str,
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assertion_provider: Callable[[str], Awaitable[str]],
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scope: str | None = None,
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issuer: str | None = None,
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) -> None:
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"""Initialize private_key_jwt OAuth provider.
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@ -269,6 +323,11 @@ class PrivateKeyJWTOAuthProvider(OAuthClientProvider):
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`static_assertion_provider()` for pre-built JWTs, or provide your own
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callback for workload identity federation.
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scope: Optional space-separated list of scopes to request.
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issuer: The issuer identifier of the authorization server `client_id` is
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registered with. When set, an assertion is only minted, and token requests
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are only built, once authorization server metadata whose `issuer` is exactly this
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string has been discovered; otherwise the flow stops with `OAuthFlowError`.
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When omitted, whichever authorization server discovery yields is used.
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"""
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# Build minimal client_metadata for the base class
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client_metadata = OAuthClientMetadata(
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@ -279,6 +338,7 @@ class PrivateKeyJWTOAuthProvider(OAuthClientProvider):
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)
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super().__init__(server_url, client_metadata, storage, None, None)
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self._assertion_provider = assertion_provider
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self._issuer = _checked_issuer(issuer)
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# Store client_info to be set during _initialize - no dynamic registration needed
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self._fixed_client_info = OAuthClientInformationFull(
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redirect_uris=None,
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@ -294,6 +354,9 @@ class PrivateKeyJWTOAuthProvider(OAuthClientProvider):
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self.context.client_info = self._fixed_client_info
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self._initialized = True
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def _select_authorization_server(self, advertised: list[str]) -> str:
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return _preferred_authorization_server(advertised, self._issuer)
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async def _perform_authorization(self) -> httpx2.Request:
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"""Perform client_credentials authorization with private_key_jwt."""
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return await self._exchange_token_client_credentials()
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@ -314,6 +377,8 @@ class PrivateKeyJWTOAuthProvider(OAuthClientProvider):
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async def _exchange_token_client_credentials(self) -> httpx2.Request:
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"""Build token exchange request for client_credentials grant with private_key_jwt."""
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_require_metadata_for_configured_issuer(self.context, self._issuer)
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token_data: dict[str, Any] = {
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"grant_type": "client_credentials",
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}
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@ -17,7 +17,7 @@ from urllib.parse import quote, urlencode, urljoin, urlparse
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import anyio
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import httpx2
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from mcp_types.version import is_version_at_least
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from pydantic import BaseModel, Field, ValidationError
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from pydantic import AnyHttpUrl, BaseModel, ConfigDict, Field, TypeAdapter, ValidationError
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from mcp.client.auth.exceptions import OAuthFlowError, OAuthRegistrationError, OAuthTokenError
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from mcp.client.auth.utils import (
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@ -36,6 +36,7 @@ from mcp.client.auth.utils import (
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handle_registration_response,
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handle_token_response_scopes,
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is_valid_client_metadata_url,
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issuers_match,
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should_use_client_metadata_url,
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union_scopes,
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validate_authorization_response_iss,
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@ -276,6 +277,16 @@ class OAuthContext:
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return data, headers
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_ORIGIN_URL = TypeAdapter(AnyHttpUrl, config=ConfigDict(url_preserve_empty_path=True))
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def _origin_issuer(server_url: str) -> str:
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"""The resource server's origin as an issuer identifier: `scheme://authority`, rendered the way
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`OAuthMetadata.issuer` renders URLs (host case, default ports) so the two compare as strings."""
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parsed = urlparse(server_url)
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return str(_ORIGIN_URL.validate_python(f"{parsed.scheme}://{parsed.netloc}"))
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class OAuthClientProvider(httpx2.Auth):
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"""OAuth2 authentication for httpx2.
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@ -577,6 +588,16 @@ class OAuthClientProvider(httpx2.Auth):
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if not check_resource_allowed(requested_resource=default_resource, configured_resource=prm_resource):
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raise OAuthFlowError(f"Protected resource {prm_resource} does not match expected {default_resource}")
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def _select_authorization_server(self, advertised: list[str]) -> str:
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"""Which of the servers listed in protected resource metadata to use: the first (the list is never empty)."""
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return advertised[0]
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def _expected_issuer(self) -> str:
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"""The issuer that authorization server metadata and client credentials must belong to: the
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PRM-advertised server, or on the legacy no-PRM path the resource server's origin, which is what
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the 2025-03-26 well-known URL is built from (RFC 8414 §3.3)."""
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return self.context.auth_server_url or _origin_issuer(self.context.server_url)
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async def async_auth_flow(self, request: httpx2.Request) -> AsyncGenerator[httpx2.Request, httpx2.Response]:
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"""httpx2 auth flow integration."""
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async with self.context.lock:
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@ -600,107 +621,122 @@ class OAuthClientProvider(httpx2.Auth):
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response = yield request
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if response.status_code == 401:
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step_up = (
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response.status_code == 403 and extract_field_from_www_auth(response, "error") == "insufficient_scope"
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)
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if response.status_code == 401 or step_up:
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# Perform full OAuth flow
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try:
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# OAuth flow must be inline due to generator constraints
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www_auth_resource_metadata_url = extract_resource_metadata_from_www_auth(response)
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# Read before discovery, which may clear the tokens: on a restart the stored
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# token's scope is the only record of what was granted (see Step 3).
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granted_scope = self.context.current_tokens.scope if self.context.current_tokens else None
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# Step 1: Discover protected resource metadata (SEP-985 with fallback support)
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prm_discovery_urls = build_protected_resource_metadata_discovery_urls(
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www_auth_resource_metadata_url, self.context.server_url
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)
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# OAuth flow must be inline due to generator constraints.
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# Steps 1-2 run on every 401. A scope step-up reuses the metadata discovered earlier
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# in this process, and discovers it first when none is held yet (for example when
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# tokens were loaded from storage), so re-authorization targets the right server.
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if response.status_code == 401 or self.context.oauth_metadata is None:
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www_auth_resource_metadata_url = extract_resource_metadata_from_www_auth(response)
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for url in prm_discovery_urls: # pragma: no branch
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discovery_request = create_oauth_metadata_request(url)
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discovery_response = yield discovery_request # sending request
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prm = await handle_protected_resource_response(discovery_response)
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if prm:
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# Validate PRM resource matches server URL (RFC 8707)
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await self._validate_resource_match(prm)
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self.context.protected_resource_metadata = prm
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# todo: try all authorization_servers to find the OASM
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assert (
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len(prm.authorization_servers) > 0
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) # this is always true as authorization_servers has a min length of 1
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self.context.auth_server_url = str(prm.authorization_servers[0])
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break
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else:
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logger.debug(f"Protected resource metadata discovery failed: {url}")
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# SEP-2352: stored credentials are bound to the issuer that registered them.
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# If the authorization server changed, drop them (and the old tokens) so the
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# flow re-registers instead of presenting another server's credentials.
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if (
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self.context.client_info is not None
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and self.context.auth_server_url is not None
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and not credentials_match_issuer(
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self.context.client_info, self.context.auth_server_url, self.context.client_metadata_url
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# Step 1: Discover protected resource metadata (SEP-985 with fallback support)
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prm_discovery_urls = build_protected_resource_metadata_discovery_urls(
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www_auth_resource_metadata_url, self.context.server_url
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)
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):
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logger.debug("Authorization server changed; discarding bound credentials and re-registering")
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self.context.client_info = None
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self.context.clear_tokens()
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# Any cached AS metadata is for the old server; drop it so a failed
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# rediscovery cannot leak the old registration/token endpoints into Step 4.
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self.context.oauth_metadata = None
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asm_discovery_urls = build_oauth_authorization_server_metadata_discovery_urls(
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self.context.auth_server_url, self.context.server_url
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)
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prm_request_failed: int | None = None
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for url in prm_discovery_urls:
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discovery_request = create_oauth_metadata_request(url)
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# Step 2: Discover OAuth Authorization Server Metadata (OASM) (with fallback for legacy servers)
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for url in asm_discovery_urls: # pragma: no branch
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oauth_metadata_request = create_oauth_metadata_request(url)
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oauth_metadata_response = yield oauth_metadata_request
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discovery_response = yield discovery_request # sending request
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ok, asm = await handle_auth_metadata_response(oauth_metadata_response)
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if not ok:
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break
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if ok and asm:
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# SEP-2468: metadata issuer must match the discovery issuer
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if self.context.auth_server_url is not None:
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validate_metadata_issuer(asm, self.context.auth_server_url)
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self.context.oauth_metadata = asm
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break
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if discovery_response.status_code >= 500 or discovery_response.status_code == 429:
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prm_request_failed = discovery_response.status_code
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prm = await handle_protected_resource_response(discovery_response)
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if prm:
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# Validate PRM resource matches server URL (RFC 8707)
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await self._validate_resource_match(prm)
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self.context.protected_resource_metadata = prm
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self.context.auth_server_url = self._select_authorization_server(
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[str(url) for url in prm.authorization_servers]
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)
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break
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else:
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logger.debug(f"Protected resource metadata discovery failed: {url}")
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else:
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logger.debug(f"OAuth metadata discovery failed: {url}")
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if prm_request_failed is not None:
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# A server error says nothing about whether the resource publishes
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# metadata, so it must not send the flow down the legacy path.
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raise OAuthFlowError(
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f"Protected resource metadata request failed: HTTP {prm_request_failed}"
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)
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# SEP-2352: on the legacy no-PRM path the issuer is only known after ASM
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# discovery, so re-evaluate the binding here using the discovered metadata
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# issuer (mirroring the bound_issuer fallback in Step 4).
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if (
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self.context.client_info is not None
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and self.context.auth_server_url is None
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and self.context.oauth_metadata is not None
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and not credentials_match_issuer(
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self.context.client_info,
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str(self.context.oauth_metadata.issuer),
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self.context.client_metadata_url,
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expected_issuer = self._expected_issuer()
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# SEP-2352: stored credentials are bound to the issuer that registered them.
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# Decided before any metadata is fetched: if the expected issuer is a different
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# server, drop them (and the old tokens) so the flow re-registers instead of
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# presenting another server's credentials.
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if self.context.client_info is not None and not credentials_match_issuer(
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self.context.client_info, expected_issuer, self.context.client_metadata_url
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):
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logger.debug(
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"Authorization server changed; discarding bound credentials and re-registering"
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)
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self.context.client_info = None
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self.context.clear_tokens()
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# Any cached AS metadata is for the old server; drop it so a failed
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# rediscovery cannot leak the old registration/token endpoints into Step 4.
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self.context.oauth_metadata = None
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asm_discovery_urls = build_oauth_authorization_server_metadata_discovery_urls(
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self.context.auth_server_url, self.context.server_url
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)
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):
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logger.debug("Authorization server changed; discarding bound credentials and re-registering")
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self.context.client_info = None
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self.context.clear_tokens()
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# Step 2: Discover OAuth Authorization Server Metadata (OASM) (with fallback for legacy servers)
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for url in asm_discovery_urls: # pragma: no branch
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oauth_metadata_request = create_oauth_metadata_request(url)
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oauth_metadata_response = yield oauth_metadata_request
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ok, asm = await handle_auth_metadata_response(oauth_metadata_response)
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if not ok:
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break
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if ok and asm:
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# SEP-2468 / RFC 8414 §3.3: the metadata must name the expected issuer.
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# On the legacy path a root issuer rendered with its trailing slash
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# names the same origin.
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if self.context.auth_server_url is None and issuers_match(
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str(asm.issuer), expected_issuer
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):
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expected_issuer = str(asm.issuer)
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validate_metadata_issuer(asm, expected_issuer)
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self.context.oauth_metadata = asm
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break
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else:
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logger.debug(f"OAuth metadata discovery failed: {url}")
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# Step 3: Apply scope selection strategy
|
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self.context.client_metadata.scope = get_client_metadata_scopes(
|
||||
challenged_scope = get_client_metadata_scopes(
|
||||
extract_scope_from_www_auth(response),
|
||||
self.context.protected_resource_metadata,
|
||||
self.context.oauth_metadata,
|
||||
self.context.client_metadata.grant_types,
|
||||
)
|
||||
if step_up:
|
||||
# SEP-2350: union previously requested scopes with the newly challenged ones so
|
||||
# escalating one operation keeps the others' grants, folding in the granted
|
||||
# scope read above since client_metadata.scope is not reloaded on a restart.
|
||||
prior_scope = union_scopes(self.context.client_metadata.scope, granted_scope)
|
||||
self.context.client_metadata.scope = union_scopes(prior_scope, challenged_scope)
|
||||
else:
|
||||
self.context.client_metadata.scope = challenged_scope
|
||||
|
||||
# Step 4: Register client or use URL-based client ID (CIMD)
|
||||
if not self.context.client_info:
|
||||
# SEP-2352: the issuer to bind these credentials to, when known.
|
||||
discovered_issuer: str | None = None
|
||||
if self.context.oauth_metadata is not None:
|
||||
discovered_issuer = self.context.auth_server_url or str(self.context.oauth_metadata.issuer)
|
||||
# SEP-2352: the issuer to bind these credentials to, once metadata for it
|
||||
# was actually found.
|
||||
discovered_issuer = self._expected_issuer() if self.context.oauth_metadata is not None else None
|
||||
|
||||
if should_use_client_metadata_url(
|
||||
self.context.oauth_metadata, self.context.client_metadata_url
|
||||
|
|
@ -752,34 +788,3 @@ class OAuthClientProvider(httpx2.Auth):
|
|||
# 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 = extract_field_from_www_auth(response, "error")
|
||||
|
||||
# Step 2: Check if we need to step-up authorization
|
||||
if error == "insufficient_scope": # pragma: no branch
|
||||
try:
|
||||
# Step 2a: Union previously requested scopes with the newly challenged
|
||||
# scopes (SEP-2350) so escalating one operation keeps the others' grants.
|
||||
# Fold in the stored token's scope too: on a restart the token is reloaded
|
||||
# but client_metadata.scope is not, so it would otherwise be the only basis.
|
||||
challenged_scope = get_client_metadata_scopes(
|
||||
extract_scope_from_www_auth(response),
|
||||
self.context.protected_resource_metadata,
|
||||
self.context.oauth_metadata,
|
||||
self.context.client_metadata.grant_types,
|
||||
)
|
||||
granted_scope = self.context.current_tokens.scope if self.context.current_tokens else None
|
||||
prior_scope = union_scopes(self.context.client_metadata.scope, granted_scope)
|
||||
self.context.client_metadata.scope = union_scopes(prior_scope, challenged_scope)
|
||||
|
||||
# Step 2b: Perform (re-)authorization and token exchange
|
||||
token_response = yield await self._perform_authorization()
|
||||
await self._handle_token_response(token_response)
|
||||
except Exception: # pragma: no cover
|
||||
logger.exception("OAuth flow error")
|
||||
raise
|
||||
|
||||
# Retry with new tokens
|
||||
self._add_auth_header(request)
|
||||
yield request
|
||||
|
|
|
|||
|
|
@ -54,7 +54,7 @@ def extract_resource_metadata_from_www_auth(response: Response) -> str | None:
|
|||
Returns:
|
||||
Resource metadata URL if found in WWW-Authenticate header, None otherwise
|
||||
"""
|
||||
if not response or response.status_code != 401:
|
||||
if not response or response.status_code not in (401, 403):
|
||||
return None # pragma: no cover
|
||||
|
||||
return extract_field_from_www_auth(response, "resource_metadata")
|
||||
|
|
@ -343,7 +343,8 @@ def credentials_match_issuer(
|
|||
document is resolved by whichever server is in use — so it always matches; CIMD is identified
|
||||
by the client ID being the configured `client_metadata_url`, not by URL shape (a registration
|
||||
server may also issue URL-shaped IDs that are bound to it). Credentials with a recorded issuer
|
||||
match only when it equals `issuer` (simple string comparison). Credentials with no recorded
|
||||
match only when it equals `issuer` (simple string comparison; a root issuer with and without
|
||||
its trailing slash count as equal). Credentials with no recorded
|
||||
issuer (pre-registered, or stored before issuer binding existed) carry no binding to enforce
|
||||
and are left as-is.
|
||||
"""
|
||||
|
|
@ -351,7 +352,18 @@ def credentials_match_issuer(
|
|||
return True
|
||||
if client_info.issuer is None:
|
||||
return True
|
||||
return client_info.issuer == issuer
|
||||
return issuers_match(client_info.issuer, issuer)
|
||||
|
||||
|
||||
def issuers_match(a: str, b: str) -> bool:
|
||||
"""Simple string comparison of two issuer identifiers (RFC 8414 section 3.3), except that a root
|
||||
issuer with and without its trailing slash (`scheme://authority` and `scheme://authority/`) name
|
||||
the same server."""
|
||||
if a == b:
|
||||
return True
|
||||
shorter, longer = sorted((a, b), key=len)
|
||||
parsed = urlparse(shorter)
|
||||
return longer == f"{shorter}/" and shorter == f"{parsed.scheme}://{parsed.netloc}"
|
||||
|
||||
|
||||
def should_use_client_metadata_url(
|
||||
|
|
|
|||
Loading…
Reference in a new issue