Validates registered redirect_uris for DCR are a secure schema with no fragments

Author: Varshith-Kali

src/mcp/server/auth/handlers/register.py

@@ -9,6 +9,7 @@
 from starlette.responses import Response
 
 from mcp.server.auth.errors import stringify_pydantic_error
+from mcp.server.auth.routes import validate_redirect_uri
 from mcp.server.auth.json_response import PydanticJSONResponse
 from mcp.server.auth.provider import OAuthAuthorizationServerProvider, RegistrationError, RegistrationErrorCode
 from mcp.server.auth.settings import ClientRegistrationOptions
@@ -35,6 +36,20 @@ async def handle(self, request: Request) -> Response:
             body = await request.body()
             client_metadata = OAuthClientMetadata.model_validate_json(body)
 
+            # Validate redirect_uris per RFC 7591 section 2
+            if client_metadata.redirect_uris:
+                for uri in client_metadata.redirect_uris:
+                    try:
+                        validate_redirect_uri(uri)
+                    except ValueError as e:
+                        return PydanticJSONResponse(
+                            content=RegistrationErrorResponse(
+                                error="invalid_redirect_uri",
+                                error_description=str(e),
+                            ),
+                            status_code=400,
+                        )
+
             # Scope validation is handled below
         except ValidationError as validation_error:
             return PydanticJSONResponse(

src/mcp/server/auth/routes.py

@@ -21,6 +21,32 @@
 from mcp.shared.inbound import MCP_PROTOCOL_VERSION_HEADER
 
 
+def validate_redirect_uri(url: AnyHttpUrl):
+    """Validate a registered redirect_uri for DCR.
+
+    RFC 9700 section 4.1.1 and RFC 7591 section 2 require HTTPS for
+    redirect_uris, with an HTTP loopback exception for local development.
+
+    Args:
+        url: The redirect URI to validate.
+
+    Raises:
+        ValueError: If the redirect URI uses an unsafe scheme or contains
+            a fragment.
+    """
+    if url.scheme != "https" and url.host not in (
+        "localhost",
+        "127.0.0.1",
+        "[::1]",
+    ):
+        raise ValueError(
+            "Redirect URI must use HTTPS (or HTTP loopback for local development)"
+        )
+
+    if url.fragment is not None:
+        raise ValueError("Redirect URI must not contain a fragment")
+
+
 def validate_issuer_url(url: AnyHttpUrl):
     """Validate that the issuer URL meets OAuth 2.0 requirements.
 

tests/server/auth/test_routes.py

@@ -1,7 +1,7 @@
 import pytest
 from pydantic import AnyHttpUrl
 
-from mcp.server.auth.routes import build_metadata, validate_issuer_url
+from mcp.server.auth.routes import build_metadata, validate_issuer_url, validate_redirect_uri
 from mcp.server.auth.settings import AuthSettings, ClientRegistrationOptions, RevocationOptions
 
 
@@ -70,3 +70,43 @@ def test_build_metadata_serves_issuer_without_trailing_slash():
     assert served["issuer"] == "https://as.example.com"
     assert served["authorization_endpoint"] == "https://as.example.com/authorize"
     assert served["token_endpoint"] == "https://as.example.com/token"
+
+def test_validate_redirect_uri_https_allowed():
+    validate_redirect_uri(AnyHttpUrl('https://example.com/cb'))
+
+
+def test_validate_redirect_uri_http_localhost_allowed():
+    validate_redirect_uri(AnyHttpUrl('http://localhost:3000/cb'))
+
+
+def test_validate_redirect_uri_http_127_0_0_1_allowed():
+    validate_redirect_uri(AnyHttpUrl('http://127.0.0.1:8080/cb'))
+
+
+def test_validate_redirect_uri_http_ipv6_loopback_allowed():
+    validate_redirect_uri(AnyHttpUrl('http://[::1]:9090/cb'))
+
+
+def test_validate_redirect_uri_javascript_scheme_rejected():
+    with pytest.raises(ValueError, match='Redirect URI must use HTTPS'):
+        validate_redirect_uri(AnyHttpUrl('javascript:alert(1)'))
+
+
+def test_validate_redirect_uri_file_scheme_rejected():
+    with pytest.raises(ValueError, match='Redirect URI must use HTTPS'):
+        validate_redirect_uri(AnyHttpUrl('file:///etc/passwd'))
+
+
+def test_validate_redirect_uri_http_non_loopback_rejected():
+    with pytest.raises(ValueError, match='Redirect URI must use HTTPS'):
+        validate_redirect_uri(AnyHttpUrl('http://evil.com/cb'))
+
+
+def test_validate_redirect_uri_fragment_rejected():
+    with pytest.raises(ValueError, match='Redirect URI must not contain a fragment'):
+        validate_redirect_uri(AnyHttpUrl('https://example.com/cb#frag'))
+
+
+def test_validate_redirect_uri_empty_fragment_rejected():
+    with pytest.raises(ValueError, match='Redirect URI must not contain a fragment'):
+        validate_redirect_uri(AnyHttpUrl('https://example.com/cb#'))