fix(auth): add token issuer validation per MCP spec (closes #1442)

Author: Ujjwal-Bajpayee

src/mcp/client/auth.py

@@ -111,6 +111,8 @@ class OAuthContext:
     # Discovery state for fallback support
     discovery_base_url: str | None = None
     discovery_pathname: str | None = None
+    # Optional expected issuer for access tokens (JWT iss claim)
+    expected_issuer: str | None = None
 
     def get_authorization_base_url(self, server_url: str) -> str:
         """Extract base URL by removing path component."""
@@ -126,12 +128,64 @@ def update_token_expiry(self, token: OAuthToken) -> None:
 
     def is_token_valid(self) -> bool:
         """Check if current token is valid."""
-        return bool(
+        # Basic existence and expiry checks
+        basic_valid = bool(
             self.current_tokens
             and self.current_tokens.access_token
             and (not self.token_expiry_time or time.time() <= self.token_expiry_time)
         )
 
+        if not basic_valid:
+            return False
+
+        # If no expected issuer is configured, behave as before
+        if not getattr(self, "expected_issuer", None):
+            return True
+
+        # If expected_issuer is set, ensure token issuer matches
+        try:
+            return self._token_issuer_matches(self.current_tokens.access_token)
+        except Exception:
+            # On any parsing issue, treat token as invalid
+            logger.exception("Failed to validate token issuer")
+            return False
+
+    def _token_issuer_matches(self, token: str) -> bool:
+        """Decode a JWT access token (no signature verification) and compare its 'iss' claim.
+
+        This performs a safe, minimal check: split the token, base64-decode the payload,
+        parse JSON, and compare the 'iss' field to self.expected_issuer. Returns False
+        if the token is malformed or the claim is missing/mismatched.
+        """
+        # JWTs are in the form header.payload.signature
+        parts = token.split(".")
+        if len(parts) < 2:
+            return False
+
+        payload_b64 = parts[1]
+
+        # Add padding for base64 if necessary
+        padding = "=" * (-len(payload_b64) % 4)
+        payload_b64 += padding
+
+        try:
+            payload_bytes = base64.urlsafe_b64decode(payload_b64.encode())
+        except Exception:
+            return False
+
+        try:
+            import json
+
+            payload = json.loads(payload_bytes)
+        except Exception:
+            return False
+
+        iss = payload.get("iss")
+        if not iss:
+            return False
+
+        return iss == self.expected_issuer
+
     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)