modelcontextprotocol · Varshith-Kali · Jul 28, 2026 · Jul 29, 2026 · Jul 29, 2026 · Jul 29, 2026
diff --git a/src/mcp/server/auth/handlers/register.py b/src/mcp/server/auth/handlers/register.py
@@ -1,17 +1,18 @@
 import secrets
 import time
 from dataclasses import dataclass
-from typing import Any
+from typing import Any, cast
 from uuid import uuid4
 
-from pydantic import BaseModel, ValidationError
+from pydantic import AnyUrl, BaseModel, ValidationError
 from starlette.requests import Request
 from starlette.responses import Response
 
 from mcp.server.auth.errors import stringify_pydantic_error
 from mcp.server.auth.json_response import PydanticJSONResponse
 from mcp.server.auth.provider import OAuthAuthorizationServerProvider, RegistrationError, RegistrationErrorCode
 from mcp.server.auth.settings import ClientRegistrationOptions
+from mcp.server.auth.url_validators import validate_redirect_uri
 from mcp.shared.auth import JWT_BEARER_GRANT_TYPE, OAuthClientInformationFull, OAuthClientMetadata
 
 # this alias is a no-op; it's just to separate out the types exposed to the
@@ -35,6 +36,21 @@ 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. The metadata
+            # model requires a non-empty list (min_length=1), so no presence
+            # guard is needed; cast narrows the optional field for pyright.
+            for uri in cast(list[AnyUrl], 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(

diff --git a/src/mcp/server/auth/url_validators.py b/src/mcp/server/auth/url_validators.py
@@ -0,0 +1,28 @@
+"""OAuth 2.0 URL validation helpers for MCP authorization servers.
+
+RFC 9700 4.1.1 and RFC 7591 2 require HTTPS for authorization endpoint URLs
+and registered redirect_uris, with an HTTP loopback exception for local
+development.
+"""
+
+from pydantic import AnyUrl
+
+
+def validate_redirect_uri(url: AnyUrl):
+    """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 not in ("http", "https"):
+        raise ValueError("Redirect URI must use an HTTP(S) scheme")
+
+    if url.fragment is not None:
+        raise ValueError("Redirect URI must not contain a fragment")
diff --git a/tests/server/auth/test_error_handling.py b/tests/server/auth/test_error_handling.py
@@ -288,3 +288,39 @@ async def test_token_error_handling_refresh_token(
         data = refresh_response.json()
         assert data["error"] == "invalid_scope"
         assert data["error_description"] == "The requested scope is invalid"
+
+
+@pytest.mark.anyio
+async def test_registration_rejects_redirect_uri_with_fragment(client: httpx2.AsyncClient):
+    client_data = {
+        "redirect_uris": ["https://client.example.com/callback#frag"],
+        "token_endpoint_auth_method": "client_secret_post",
+        "grant_types": ["authorization_code", "refresh_token"],
+        "response_types": ["code"],
+        "client_name": "Test Client",
+    }
+
+    response = await client.post("/register", json=client_data)
+
+    assert response.status_code == 400, response.content
+    data = response.json()
+    assert data["error"] == "invalid_redirect_uri"
+    assert data["error_description"] == "Redirect URI must not contain a fragment"
+
+
+@pytest.mark.anyio
+async def test_registration_rejects_non_http_redirect_uri_scheme(client: httpx2.AsyncClient):
+    client_data = {
+        "redirect_uris": ["javascript:alert(1)"],
+        "token_endpoint_auth_method": "client_secret_post",
+        "grant_types": ["authorization_code", "refresh_token"],
+        "response_types": ["code"],
+        "client_name": "Test Client",
+    }
+
+    response = await client.post("/register", json=client_data)
+
+    assert response.status_code == 400, response.content
+    data = response.json()
+    assert data["error"] == "invalid_redirect_uri"
+    assert data["error_description"] == "Redirect URI must use an HTTP(S) scheme"
diff --git a/tests/server/auth/test_routes.py b/tests/server/auth/test_routes.py
@@ -1,8 +1,9 @@
 import pytest
-from pydantic import AnyHttpUrl
+from pydantic import AnyHttpUrl, AnyUrl
 
 from mcp.server.auth.routes import build_metadata, validate_issuer_url
 from mcp.server.auth.settings import AuthSettings, ClientRegistrationOptions, RevocationOptions
+from mcp.server.auth.url_validators import validate_redirect_uri
 
 
 def test_validate_issuer_url_https_allowed():
@@ -70,3 +71,43 @@ 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 an HTTP"):
+        validate_redirect_uri(AnyUrl("javascript:alert(1)"))
+
+
+def test_validate_redirect_uri_file_scheme_rejected():
+    with pytest.raises(ValueError, match="Redirect URI must use an HTTP"):
+        validate_redirect_uri(AnyUrl("file:///etc/passwd"))
+
+
+def test_validate_redirect_uri_http_non_loopback_allowed():
+    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#"))