Fixing unhandled NUL Bytes in API Requests

Author: Avish Porwal

internal/api/handlers/v0/servers.go

@@ -42,6 +42,8 @@ type ServerVersionsInput struct {
 }
 
 // RegisterServersEndpoints registers all server-related endpoints with a custom path prefix
+//
+//nolint:cyclop
 func RegisterServersEndpoints(api huma.API, pathPrefix string, registry service.RegistryService) {
 	// List servers endpoint
 	huma.Register(api, huma.Operation{
@@ -52,6 +54,10 @@ func RegisterServersEndpoints(api huma.API, pathPrefix string, registry service.
 		Description: "Get a paginated list of MCP servers from the registry",
 		Tags:        []string{"servers"},
 	}, func(ctx context.Context, input *ListServersInput) (*Response[apiv0.ServerListResponse], error) {
+		if containsNULByte(input.Cursor) {
+			return nil, huma.Error400BadRequest("Invalid cursor: NUL byte not allowed")
+		}
+
 		// Build filter from input parameters
 		filter := &database.ServerFilter{}
 
@@ -119,12 +125,18 @@ func RegisterServersEndpoints(api huma.API, pathPrefix string, registry service.
 		if err != nil {
 			return nil, huma.Error400BadRequest("Invalid server name encoding", err)
 		}
+		if containsNULByte(serverName) {
+			return nil, huma.Error400BadRequest("Invalid server name: NUL byte not allowed")
+		}
 
 		// URL-decode the version
 		version, err := url.PathUnescape(input.Version)
 		if err != nil {
 			return nil, huma.Error400BadRequest("Invalid version encoding", err)
 		}
+		if containsNULByte(version) {
+			return nil, huma.Error400BadRequest("Invalid version: NUL byte not allowed")
+		}
 
 		var serverResponse *apiv0.ServerResponse
 		// Handle "latest" as a special version
@@ -160,6 +172,9 @@ func RegisterServersEndpoints(api huma.API, pathPrefix string, registry service.
 		if err != nil {
 			return nil, huma.Error400BadRequest("Invalid server name encoding", err)
 		}
+		if containsNULByte(serverName) {
+			return nil, huma.Error400BadRequest("Invalid server name: NUL byte not allowed")
+		}
 
 		// Get all versions for this server
 		servers, err := registry.GetAllVersionsByServerName(ctx, serverName)
@@ -186,3 +201,7 @@ func RegisterServersEndpoints(api huma.API, pathPrefix string, registry service.
 		}, nil
 	})
 }
+
+func containsNULByte(s string) bool {
+	return strings.IndexByte(s, 0) >= 0
+}

internal/api/handlers/v0/servers_test.go

@@ -461,6 +461,8 @@ func TestServersEndpointEdgeCases(t *testing.T) {
 			{"limit too high", "?limit=1000", http.StatusUnprocessableEntity, "validation failed"},
 			{"negative limit", "?limit=-1", http.StatusUnprocessableEntity, "validation failed"},
 			{"invalid updated_since format", "?updated_since=invalid", http.StatusBadRequest, "Invalid updated_since format"},
+			{"cursor contains NUL", "?cursor=%00", http.StatusBadRequest, "Invalid cursor"},
+			{"cursor contains non NUL", "?cursor=server", http.StatusOK, ""},
 			{"future updated_since", "?updated_since=2030-01-01T00:00:00Z", http.StatusOK, ""},
 			{"very old updated_since", "?updated_since=1990-01-01T00:00:00Z", http.StatusOK, ""},
 			{"empty search parameter", "?search=", http.StatusOK, ""},
@@ -489,6 +491,16 @@ func TestServersEndpointEdgeCases(t *testing.T) {
 		}
 	})
 
+	t.Run("path parameter NUL byte rejected", func(t *testing.T) {
+		req := httptest.NewRequest(http.MethodGet, "/v0/servers/%00/versions", nil)
+		w := httptest.NewRecorder()
+
+		mux.ServeHTTP(w, req)
+
+		assert.Equal(t, http.StatusBadRequest, w.Code)
+		assert.Contains(t, w.Body.String(), "Invalid server name")
+	})
+
 	t.Run("response structure validation", func(t *testing.T) {
 		req := httptest.NewRequest(http.MethodGet, "/v0/servers", nil)
 		w := httptest.NewRecorder()