Implement CSP for the registrar console (#3129)

Implement a hybrid Content Security Policy (CSP) for the Registrar Console
to protect against XSS

- The CspFilter injects the proper headers on the Java backend endpoints
- Uinsg Jetty's HeaderFilter to inject the header for statically-served
  frontend assets

We need to add the ee10-servlets.ini file for Jetty to have acess to the
HeaderFilter class

core/src/main/java/google/registry/ui/server/CspFilter.java

@@ -0,0 +1,41 @@
+// Copyright 2026 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed 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 google.registry.ui.server;
+
+import jakarta.servlet.Filter;
+import jakarta.servlet.FilterChain;
+import jakarta.servlet.ServletException;
+import jakarta.servlet.ServletRequest;
+import jakarta.servlet.ServletResponse;
+import jakarta.servlet.http.HttpServletResponse;
+import java.io.IOException;
+
+/** Filter to inject security headers, including CSP, for defense-in-depth. */
+public class CspFilter implements Filter {
+
+  private static final String CSP_POLICY =
+      "default-src 'self'; object-src 'none'; base-uri 'self';";
+
+  @Override
+  public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
+      throws IOException, ServletException {
+    if (response instanceof HttpServletResponse httpResponse) {
+      httpResponse.setHeader("Content-Security-Policy", CSP_POLICY);
+      httpResponse.setHeader("X-Content-Type-Options", "nosniff");
+      httpResponse.setHeader("X-Frame-Options", "DENY");
+    }
+    chain.doFilter(request, response);
+  }
+}

core/src/test/java/google/registry/ui/server/CspFilterTest.java

@@ -0,0 +1,44 @@
+// Copyright 2026 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed 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 google.registry.ui.server;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+
+import jakarta.servlet.FilterChain;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+import org.junit.jupiter.api.Test;
+
+/** Unit tests for {@link CspFilter}. */
+class CspFilterTest {
+
+  @Test
+  void testDoFilter_setsHeaders() throws Exception {
+    CspFilter filter = new CspFilter();
+    HttpServletRequest request = mock(HttpServletRequest.class);
+    HttpServletResponse response = mock(HttpServletResponse.class);
+    FilterChain chain = mock(FilterChain.class);
+
+    filter.doFilter(request, response, chain);
+
+    verify(response)
+        .setHeader(
+            "Content-Security-Policy", "default-src 'self'; object-src 'none'; base-uri 'self';");
+    verify(response).setHeader("X-Content-Type-Options", "nosniff");
+    verify(response).setHeader("X-Frame-Options", "DENY");
+    verify(chain).doFilter(request, response);
+  }
+}