# Licensed to the Apache Software Foundation (ASF) under one # or more contributor license agreements. See the NOTICE file # distributed with this work for additional information # regarding copyright ownership. The ASF licenses this file # to you under the Apache License, Version 2.0 (the # "License"); you may not use this file except in compliance # with the License. You may obtain a copy of the License at # # http://www.apache.org/licenses/LICENSE-2.0 # # Unless required by applicable law or agreed to in writing, # software distributed under the License is distributed on an # "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY # KIND, either express or implied. See the License for the # specific language governing permissions and limitations # under the License. """ Detailed JWT verification for the MCP service. Provides step-by-step JWT validation with tiered server-side logging: - WARNING level: generic failure categories only (e.g. "Issuer mismatch") - DEBUG level: detailed claim values and config for troubleshooting - Secrets (e.g. HS256 keys) are NEVER logged at any level HTTP responses always return generic errors per RFC 6750 Section 3.1. """ import base64 import logging import time from contextvars import ContextVar from typing import Any, cast from authlib.jose.errors import ( BadSignatureError, DecodeError, ExpiredTokenError, JoseError, ) from fastmcp.server.auth.auth import AccessToken from fastmcp.server.auth.providers.jwt import JWTVerifier from mcp.server.auth.middleware.auth_context import AuthContextMiddleware from mcp.server.auth.middleware.bearer_auth import BearerAuthBackend from starlette.authentication import AuthenticationError from starlette.middleware import Middleware from starlette.middleware.authentication import AuthenticationMiddleware from starlette.requests import HTTPConnection from starlette.responses import JSONResponse from superset.utils import json logger = logging.getLogger(__name__) # Thread-safe storage for the specific JWT failure reason. # Set by DetailedJWTVerifier.load_access_token() on failure, # read by DetailedBearerAuthBackend.authenticate() to raise # an AuthenticationError with the specific reason. # SECURITY: Must ALWAYS contain generic failure categories only. # Claim values and secrets must NEVER be stored here. _jwt_failure_reason: ContextVar[str | None] = ContextVar( "_jwt_failure_reason", default=None ) def _json_auth_error_handler( conn: HTTPConnection, exc: AuthenticationError ) -> JSONResponse: """JSON 401 error handler for authentication failures. Per RFC 6750 Section 3.1, error responses MUST NOT leak server configuration or token claim values. Only generic error codes are returned to clients. Detailed failure reasons are logged server-side only for debugging. References: - RFC 6750 Section 3.1: https://datatracker.ietf.org/doc/html/rfc6750#section-3.1 - CVE-2022-29266, CVE-2019-7644: verbose JWT errors led to exploits """ # Log detailed reason server-side only logger.warning("JWT authentication failed: %s", exc) return JSONResponse( status_code=401, content={ "error": "invalid_token", "error_description": "Authentication failed", }, headers={ "WWW-Authenticate": 'Bearer error="invalid_token"', }, ) class DetailedBearerAuthBackend(BearerAuthBackend): """ Bearer auth backend that raises AuthenticationError with specific JWT failure reasons instead of silently returning None. """ async def authenticate(self, conn: HTTPConnection) -> tuple[Any, Any] | None: result = await super().authenticate(conn) if result is not None: # Clear any stale failure reason on success _jwt_failure_reason.set(None) return result # Check if there's a Bearer token present - if so, there was a # validation failure we can report with a specific reason auth_header = next( ( conn.headers.get(key) for key in conn.headers if key.lower() == "authorization" ), None, ) if auth_header and auth_header.lower().startswith("bearer "): reason = _jwt_failure_reason.get() if reason: _jwt_failure_reason.set(None) raise AuthenticationError(reason) return None class DetailedJWTVerifier(JWTVerifier): """ JWT verifier with tiered server-side logging for each validation step. Logging tiers: - WARNING: generic failure categories only (via _jwt_failure_reason ContextVar and the error handler). No claim values, no server config. - DEBUG: detailed values (issuer, audience, client_id, scopes, exceptions) for operator troubleshooting. - Secrets (e.g. HS256 signing keys) are NEVER logged at any level. HTTP responses always return generic errors per RFC 6750 Section 3.1. Controlled by MCP_JWT_DEBUG_ERRORS config flag. """ async def load_access_token(self, token: str) -> AccessToken | None: # noqa: C901 """ Validate a JWT bearer token with detailed error reporting. Each validation step stores a specific failure reason in the _jwt_failure_reason ContextVar before returning None. """ # Reset any previous failure reason _jwt_failure_reason.set(None) try: # Step 1: Decode header and check algorithm try: header = self._decode_token_header(token) except (ValueError, DecodeError) as e: reason = "Malformed token header" _jwt_failure_reason.set(reason) logger.debug("Malformed token header: %s", e) return None token_alg = header.get("alg") if self.algorithm and token_alg != self.algorithm: reason = "Algorithm mismatch" _jwt_failure_reason.set(reason) logger.debug( "Algorithm mismatch: token uses '%s', expected '%s'", token_alg, self.algorithm, ) return None # Step 2: Get verification key (static or JWKS) try: verification_key = await self._get_verification_key(token) except ValueError as e: reason = "Failed to get verification key" _jwt_failure_reason.set(reason) logger.debug("Failed to get verification key: %s", e) return None # Step 3: Decode and verify signature try: claims = self.jwt.decode(token, verification_key) except BadSignatureError: reason = "Signature verification failed" _jwt_failure_reason.set(reason) return None except ExpiredTokenError: reason = "Token has expired (detected during decode)" _jwt_failure_reason.set(reason) return None except JoseError as e: reason = "Token decode failed" _jwt_failure_reason.set(reason) logger.debug("Token decode failed: %s", e) return None # Extract client ID for logging client_id = ( claims.get("client_id") or claims.get("azp") or claims.get("sub") or "unknown" ) # Step 4: Check expiration exp = claims.get("exp") if exp and exp < time.time(): reason = "Token expired" _jwt_failure_reason.set(reason) logger.debug("Token expired for client '%s'", client_id) return None # Step 5: Validate issuer if self.issuer: iss = claims.get("iss") if isinstance(self.issuer, list): issuer_valid = iss in self.issuer else: issuer_valid = iss == self.issuer if not issuer_valid: reason = "Issuer mismatch" _jwt_failure_reason.set(reason) logger.debug( "Issuer mismatch: token has '%s', expected '%s'", iss, self.issuer, ) return None # Step 6: Validate audience if self.audience: aud = claims.get("aud") if isinstance(self.audience, list): if isinstance(aud, list): audience_valid = any( expected in aud for expected in cast(list[str], self.audience) ) else: audience_valid = aud in cast(list[str], self.audience) else: if isinstance(aud, list): audience_valid = self.audience in aud else: audience_valid = aud == self.audience if not audience_valid: reason = "Audience mismatch" _jwt_failure_reason.set(reason) logger.debug( "Audience mismatch: token has '%s', expected '%s'", aud, self.audience, ) return None # Step 7: Check required scopes scopes = self._extract_scopes(claims) if self.required_scopes: token_scopes = set(scopes) required = set(self.required_scopes) if not required.issubset(token_scopes): missing = required - token_scopes reason = "Missing required scopes" _jwt_failure_reason.set(reason) logger.debug( "Missing required scopes: %s. Token has: %s", missing, token_scopes, ) return None # All validations passed return AccessToken( token=token, client_id=str(client_id), scopes=scopes, expires_at=int(exp) if exp else None, claims=dict(claims), ) except (ValueError, JoseError, KeyError, AttributeError, TypeError) as e: reason = "Token validation failed" _jwt_failure_reason.set(reason) logger.debug("Token validation failed: %s", e) return None def get_middleware(self) -> list[Any]: """ Get middleware with detailed server-side error logging. Uses DetailedBearerAuthBackend which raises AuthenticationError with specific reasons for server-side logging. The error handler always returns generic 401 responses per RFC 6750. """ return [ Middleware( AuthenticationMiddleware, backend=DetailedBearerAuthBackend(self), on_error=_json_auth_error_handler, ), Middleware(AuthContextMiddleware), ] @staticmethod def _decode_token_header(token: str) -> dict[str, Any]: """Decode the JWT header without verifying the signature.""" parts = token.split(".") if len(parts) != 3: raise ValueError( f"Token must have 3 parts (header.payload.signature), got {len(parts)}" ) header_b64 = parts[0] # Add padding only if needed header_b64 += "=" * (-len(header_b64) % 4) header_bytes = base64.urlsafe_b64decode(header_b64) return json.loads(header_bytes)