Allow browser-based MCP clients via CORS and cross-origin bypass

Author: RossTarrant

pkg/http/handler.go

@@ -223,10 +223,16 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Bypass cross-origin protection: this server uses bearer tokens (not
+	// cookies), so Sec-Fetch-Site CSRF checks are unnecessary. See PR #2359.
+	crossOriginProtection := http.NewCrossOriginProtection()
+	crossOriginProtection.AddInsecureBypassPattern("/")
+
 	mcpHandler := mcp.NewStreamableHTTPHandler(func(_ *http.Request) *mcp.Server {
 		return ghServer
 	}, &mcp.StreamableHTTPOptions{
-		Stateless: true,
+		Stateless:             true,
+		CrossOriginProtection: crossOriginProtection,
 	})
 
 	mcpHandler.ServeHTTP(w, r)

pkg/http/handler_test.go

@@ -756,3 +756,72 @@ func buildStaticInventoryFromTools(cfg *ServerConfig, tools []inventory.ServerTo
 	ctx := context.Background()
 	return inv.AvailableTools(ctx), inv.AvailableResourceTemplates(ctx), inv.AvailablePrompts(ctx)
 }
+
+func TestCrossOriginProtection(t *testing.T) {
+	jsonRPCBody := `{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-03-26","capabilities":{},"clientInfo":{"name":"test","version":"0.1"}}}`
+
+	apiHost, err := utils.NewAPIHost("https://api.githubcopilot.com")
+	require.NoError(t, err)
+
+	handler := NewHTTPMcpHandler(
+		context.Background(),
+		&ServerConfig{
+			Version: "test",
+		},
+		nil,
+		translations.NullTranslationHelper,
+		slog.Default(),
+		apiHost,
+		WithInventoryFactory(func(_ *http.Request) (*inventory.Inventory, error) {
+			return inventory.NewBuilder().Build()
+		}),
+		WithGitHubMCPServerFactory(func(_ *http.Request, _ github.ToolDependencies, _ *inventory.Inventory, _ *github.MCPServerConfig) (*mcp.Server, error) {
+			return mcp.NewServer(&mcp.Implementation{Name: "test", Version: "0.0.1"}, nil), nil
+		}),
+		WithScopeFetcher(allScopesFetcher{}),
+	)
+
+	r := chi.NewRouter()
+	handler.RegisterMiddleware(r)
+	handler.RegisterRoutes(r)
+
+	tests := []struct {
+		name         string
+		secFetchSite string
+		origin       string
+	}{
+		{
+			name:         "cross-site request with bearer token succeeds",
+			secFetchSite: "cross-site",
+			origin:       "https://example.com",
+		},
+		{
+			name:         "same-origin request succeeds",
+			secFetchSite: "same-origin",
+		},
+		{
+			name:         "native client without Sec-Fetch-Site succeeds",
+			secFetchSite: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req := httptest.NewRequest(http.MethodPost, "/", strings.NewReader(jsonRPCBody))
+			req.Header.Set("Content-Type", "application/json")
+			req.Header.Set("Accept", "application/json, text/event-stream")
+			req.Header.Set(headers.AuthorizationHeader, "Bearer github_pat_xyz")
+			if tt.secFetchSite != "" {
+				req.Header.Set("Sec-Fetch-Site", tt.secFetchSite)
+			}
+			if tt.origin != "" {
+				req.Header.Set("Origin", tt.origin)
+			}
+
+			rr := httptest.NewRecorder()
+			r.ServeHTTP(rr, req)
+
+			assert.Equal(t, http.StatusOK, rr.Code, "unexpected status code; body: %s", rr.Body.String())
+		})
+	}
+}

pkg/http/middleware/cors.go

@@ -0,0 +1,43 @@
+package middleware
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/github/github-mcp-server/pkg/http/headers"
+)
+
+// SetCorsHeaders is middleware that sets CORS headers to allow browser-based
+// MCP clients to connect from any origin. This is safe because the server
+// authenticates via bearer tokens (not cookies), so cross-origin requests
+// cannot exploit ambient credentials.
+func SetCorsHeaders(h http.Handler) http.Handler {
+	allowHeaders := strings.Join([]string{
+		"Content-Type",
+		"Mcp-Session-Id",
+		"Mcp-Protocol-Version",
+		"Last-Event-ID",
+		headers.AuthorizationHeader,
+		headers.MCPReadOnlyHeader,
+		headers.MCPToolsetsHeader,
+		headers.MCPToolsHeader,
+		headers.MCPExcludeToolsHeader,
+		headers.MCPFeaturesHeader,
+		headers.MCPLockdownHeader,
+		headers.MCPInsidersHeader,
+	}, ", ")
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Access-Control-Allow-Origin", "*")
+		w.Header().Set("Access-Control-Allow-Methods", "GET, POST, DELETE, OPTIONS")
+		w.Header().Set("Access-Control-Max-Age", "86400")
+		w.Header().Set("Access-Control-Expose-Headers", "Mcp-Session-Id, WWW-Authenticate")
+		w.Header().Set("Access-Control-Allow-Headers", allowHeaders)
+
+		if r.Method == http.MethodOptions {
+			w.WriteHeader(http.StatusOK)
+			return
+		}
+		h.ServeHTTP(w, r)
+	})
+}

pkg/http/middleware/cors_test.go

@@ -0,0 +1,45 @@
+package middleware_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/github/github-mcp-server/pkg/http/middleware"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSetCorsHeaders(t *testing.T) {
+	inner := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
+	handler := middleware.SetCorsHeaders(inner)
+
+	t.Run("OPTIONS preflight returns 200 with CORS headers", func(t *testing.T) {
+		req := httptest.NewRequest(http.MethodOptions, "/", nil)
+		req.Header.Set("Origin", "http://localhost:6274")
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+
+		assert.Equal(t, http.StatusOK, rr.Code)
+		assert.Equal(t, "*", rr.Header().Get("Access-Control-Allow-Origin"))
+		assert.Contains(t, rr.Header().Get("Access-Control-Allow-Methods"), "POST")
+		assert.Contains(t, rr.Header().Get("Access-Control-Allow-Headers"), "Authorization")
+		assert.Contains(t, rr.Header().Get("Access-Control-Allow-Headers"), "Content-Type")
+		assert.Contains(t, rr.Header().Get("Access-Control-Allow-Headers"), "Mcp-Session-Id")
+		assert.Contains(t, rr.Header().Get("Access-Control-Allow-Headers"), "X-MCP-Lockdown")
+		assert.Contains(t, rr.Header().Get("Access-Control-Allow-Headers"), "X-MCP-Insiders")
+		assert.Contains(t, rr.Header().Get("Access-Control-Expose-Headers"), "Mcp-Session-Id")
+		assert.Contains(t, rr.Header().Get("Access-Control-Expose-Headers"), "WWW-Authenticate")
+	})
+
+	t.Run("POST request includes CORS headers", func(t *testing.T) {
+		req := httptest.NewRequest(http.MethodPost, "/", nil)
+		req.Header.Set("Origin", "http://localhost:6274")
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+
+		assert.Equal(t, http.StatusOK, rr.Code)
+		assert.Equal(t, "*", rr.Header().Get("Access-Control-Allow-Origin"))
+	})
+}

pkg/http/server.go

@@ -13,6 +13,7 @@ import (
 
 	ghcontext "github.com/github/github-mcp-server/pkg/context"
 	"github.com/github/github-mcp-server/pkg/github"
+	"github.com/github/github-mcp-server/pkg/http/middleware"
 	"github.com/github/github-mcp-server/pkg/http/oauth"
 	"github.com/github/github-mcp-server/pkg/inventory"
 	"github.com/github/github-mcp-server/pkg/lockdown"
@@ -167,6 +168,8 @@ func RunHTTPServer(cfg ServerConfig) error {
 	}
 
 	r.Group(func(r chi.Router) {
+		r.Use(middleware.SetCorsHeaders)
+
 		// Register Middleware First, needs to be before route registration
 		handler.RegisterMiddleware(r)