diff --git a/flow-server/src/main/java/com/vaadin/flow/internal/DevModeHandler.java b/flow-server/src/main/java/com/vaadin/flow/internal/DevModeHandler.java
index a31edf60c92..9af6e27e4d3 100644
--- a/flow-server/src/main/java/com/vaadin/flow/internal/DevModeHandler.java
+++ b/flow-server/src/main/java/com/vaadin/flow/internal/DevModeHandler.java
@@ -35,7 +35,7 @@ public interface DevModeHandler extends RequestHandler {
      * Prepare a HTTP connection against webpack-dev-server.
      *
      * @param path
-     *            the file to request
+     *            the file to request, needs to be safe
      * @param method
      *            the http method to use
      * @return the connection
diff --git a/flow-server/src/main/java/com/vaadin/flow/server/HandlerHelper.java b/flow-server/src/main/java/com/vaadin/flow/server/HandlerHelper.java
index b7784a2d472..5edfe83647c 100644
--- a/flow-server/src/main/java/com/vaadin/flow/server/HandlerHelper.java
+++ b/flow-server/src/main/java/com/vaadin/flow/server/HandlerHelper.java
@@ -15,6 +15,7 @@
  */
 package com.vaadin.flow.server;
 
+import javax.servlet.http.HttpServletRequest;
 import java.io.Serializable;
 import java.io.UnsupportedEncodingException;
 import java.net.URLDecoder;
@@ -28,8 +29,6 @@
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 
-import javax.servlet.http.HttpServletRequest;
-
 import com.vaadin.flow.component.UI;
 import com.vaadin.flow.server.communication.PwaHandler;
 import com.vaadin.flow.server.frontend.FrontendUtils;
diff --git a/vaadin-dev-server/src/main/java/com/vaadin/base/devserver/DevModeHandlerImpl.java b/vaadin-dev-server/src/main/java/com/vaadin/base/devserver/DevModeHandlerImpl.java
index 2f699647aaa..bd6dd6855b2 100644
--- a/vaadin-dev-server/src/main/java/com/vaadin/base/devserver/DevModeHandlerImpl.java
+++ b/vaadin-dev-server/src/main/java/com/vaadin/base/devserver/DevModeHandlerImpl.java
@@ -18,7 +18,6 @@
 import javax.servlet.ServletOutputStream;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
-
 import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
@@ -48,8 +47,8 @@
 import com.vaadin.flow.di.Lookup;
 import com.vaadin.flow.internal.BrowserLiveReload;
 import com.vaadin.flow.internal.BrowserLiveReloadAccessor;
-import com.vaadin.flow.internal.Pair;
 import com.vaadin.flow.internal.DevModeHandler;
+import com.vaadin.flow.internal.Pair;
 import com.vaadin.flow.server.ExecutionFailedException;
 import com.vaadin.flow.server.HandlerHelper;
 import com.vaadin.flow.server.InitParameters;
@@ -99,6 +98,10 @@ public final class DevModeHandlerImpl
 
     private static final AtomicReference<DevModeHandlerImpl> atomicHandler = new AtomicReference<>();
 
+    // webpack dev-server allows " character if passed through, need to
+    // explicitly check requests for it
+    private static final Pattern WEBPACK_ILLEGAL_CHAR_PATTERN = Pattern
+            .compile("\"|%22");
     // It's not possible to know whether webpack is ready unless reading output
     // messages. When webpack finishes, it writes either a `Compiled` or a
     // `Failed` in the last line
@@ -316,7 +319,9 @@ public boolean serveDevModeRequest(HttpServletRequest request,
         // a valid request for webpack-dev-server should start with '/VAADIN/'
         String requestFilename = request.getPathInfo();
 
-        if (HandlerHelper.isPathUnsafe(requestFilename)) {
+        if (HandlerHelper.isPathUnsafe(requestFilename)
+                || WEBPACK_ILLEGAL_CHAR_PATTERN.matcher(requestFilename)
+                        .find()) {
             getLogger().info("Blocked attempt to access file: {}",
                     requestFilename);
             response.setStatus(HttpServletResponse.SC_FORBIDDEN);
@@ -393,6 +398,7 @@ private boolean checkWebpackConnection() {
     @Override
     public HttpURLConnection prepareConnection(String path, String method)
             throws IOException {
+        // path should have been checked at this point for any outside requests
         URL uri = new URL(WEBPACK_HOST + ":" + getPort() + path);
         HttpURLConnection connection = (HttpURLConnection) uri.openConnection();
         connection.setRequestMethod(method);
diff --git a/vaadin-dev-server/src/test/java/com/vaadin/base/devserver/DevModeHandlerImplTest.java b/vaadin-dev-server/src/test/java/com/vaadin/base/devserver/DevModeHandlerImplTest.java
index 9b93a0a1507..5e81ea91eef 100644
--- a/vaadin-dev-server/src/test/java/com/vaadin/base/devserver/DevModeHandlerImplTest.java
+++ b/vaadin-dev-server/src/test/java/com/vaadin/base/devserver/DevModeHandlerImplTest.java
@@ -21,12 +21,12 @@
 import javax.servlet.ServletOutputStream;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
-
 import java.io.ByteArrayOutputStream;
 import java.io.File;
 import java.io.IOException;
 import java.lang.reflect.Field;
 import java.net.ConnectException;
+import java.net.HttpURLConnection;
 import java.net.InetSocketAddress;
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
@@ -640,6 +640,46 @@ public void devModeNotReady_handleRequest_returnsHtml() throws Exception {
         Mockito.verify(response).setContentType("text/html;charset=utf-8");
     }
 
+    @Test
+    public void serveDevModeRequest_uriForDevmodeGizmo_goesToWebpack()
+            throws Exception {
+        HttpServletRequest request = prepareRequest(
+                "/VAADIN/build/vaadin-devmodeGizmo-f679dbf313191ec3d018.cache.js");
+        HttpServletResponse response = prepareResponse();
+
+        final String manifestJsonResponse = "{ \"sw.js\": "
+                + "\"sw.js\", \"index.html\": \"index.html\" }";
+        int port = prepareHttpServer(0, HTTP_OK, manifestJsonResponse);
+
+        DevModeHandlerImpl devModeHandler = DevModeHandlerImpl.start(port,
+                createDevModeLookup(), npmFolder,
+                CompletableFuture.completedFuture(null));
+        devModeHandler.join();
+
+        assertTrue(devModeHandler.serveDevModeRequest(request, response));
+        assertEquals(HTTP_OK, responseStatus);
+    }
+
+    @Test
+    public void serveDevModeRequest_uriWithScriptInjected_returnsImmediatelyAndSetsForbiddenStatus()
+            throws Exception {
+        HttpServletRequest request = prepareRequest(
+                "/VAADIN/build/vaadin-devmodeGizmo-f679dbf313191ec3d018.cache%3f%22onload=%22alert(1)");
+        HttpServletResponse response = prepareResponse();
+
+        final String manifestJsonResponse = "{ \"sw.js\": "
+                + "\"sw.js\", \"index.html\": \"index.html\" }";
+        int port = prepareHttpServer(0, HTTP_OK, manifestJsonResponse);
+
+        DevModeHandlerImpl devModeHandler = DevModeHandlerImpl.start(port,
+                createDevModeLookup(), npmFolder,
+                CompletableFuture.completedFuture(null));
+        devModeHandler.join();
+
+        assertTrue(devModeHandler.serveDevModeRequest(request, response));
+        assertEquals(HTTP_FORBIDDEN, responseStatus);
+    }
+
     @Test
     public void serveDevModeRequest_uriWithDirectoryChangeWithSlash_returnsImmediatelyAndSetsForbiddenStatus()
             throws IOException {