code: // Licensed to the Software Freedom Conservancy (SFC) under one
  // or more contributor license agreements.  See the NOTICE file
  // distributed with this work for additional information
  // regarding copyright ownership.  The SFC licenses this file
  // to you under the Apache License, Version 2.0 (the
  // "License"); you may not use this file except in compliance
  // with the License.  You may obtain a copy of the License at
  //
  //   http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing,
  // software distributed under the License is distributed on an
  // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  // KIND, either express or implied.  See the License for the
  // specific language governing permissions and limitations
  // under the License.
  
  package org.openqa.selenium.grid.web;
  
  import static java.net.HttpURLConnection.HTTP_INTERNAL_ERROR;
  import static org.openqa.selenium.json.Json.JSON_UTF_8;
  
  import com.google.common.collect.ImmutableMap;
  import com.google.common.collect.ImmutableSet;
  import java.util.Collection;
  import java.util.Set;
  import org.openqa.selenium.internal.Require;
  import org.openqa.selenium.remote.http.Contents;
  import org.openqa.selenium.remote.http.Filter;
  import org.openqa.selenium.remote.http.HttpHandler;
  import org.openqa.selenium.remote.http.HttpResponse;
  
  public class CheckOriginHeader implements Filter {
  
    private final Set<String> allowedHosts;
    private final Set<String> skipChecksOn;
  
    public CheckOriginHeader(Collection<String> allowedOriginHosts, Set<String> skipChecksOn) {
      Require.nonNull("Allowed origins list", allowedOriginHosts);
      allowedHosts = ImmutableSet.copyOf(allowedOriginHosts);
      this.skipChecksOn =
          ImmutableSet.copyOf(Require.nonNull("URLs where checks are skipped", skipChecksOn));
    }
  
    @Override
    public HttpHandler apply(HttpHandler httpHandler) {
      Require.nonNull("Next handler", httpHandler);
  
      return req -> {
        if (skipChecksOn.contains(req.getUri())) {
          return httpHandler.execute(req);
        }
  
        String origin = req.getHeader("Origin");
        if (origin != null && !allowedHosts.contains(origin)) {
          return new HttpResponse()
              .setStatus(HTTP_INTERNAL_ERROR)
              .addHeader("Content-Type", JSON_UTF_8)
              .setContent(
                  Contents.asJson(
                      ImmutableMap.of(
                          "value",
                          ImmutableMap.of(
                              "error", "unknown error",
                              "message", "Origin not allowed: " + origin,
                              "stacktrace", ""))));
        }
  
        return httpHandler.execute(req);
      };
    }
  }
  