diff --git a/lib/Server.js b/lib/Server.js
index 5538f6f454..5cdeccffaf 100644
--- a/lib/Server.js
+++ b/lib/Server.js
@@ -685,7 +685,14 @@ class Server {
         return;
       }
 
-      if (headers && (!this.checkHost(headers) || !this.checkOrigin(headers))) {
+      if (!headers) {
+        this.log.warn(
+          'serverMode implementation must pass headers to the callback of onConnection(f) ' +
+            'via f(connection, headers) in order for clients to pass a headers security check'
+        );
+      }
+
+      if (!headers || !this.checkHost(headers) || !this.checkOrigin(headers)) {
         this.sockWrite([connection], 'error', 'Invalid Host/Origin header');
 
         this.socketServer.close(connection);
diff --git a/lib/servers/SockJSServer.js b/lib/servers/SockJSServer.js
index 49c42538d2..464bc3cbd1 100644
--- a/lib/servers/SockJSServer.js
+++ b/lib/servers/SockJSServer.js
@@ -57,7 +57,7 @@ module.exports = class SockJSServer extends BaseServer {
     connection.close();
   }
 
-  // f should return the resulting connection and, optionally, the connection headers
+  // f should be passed the resulting connection and the connection headers
   onConnection(f) {
     this.socket.on('connection', (connection) => {
       f(connection, connection.headers);
diff --git a/lib/servers/WebsocketServer.js b/lib/servers/WebsocketServer.js
index 03c5a56e46..2e237d2856 100644
--- a/lib/servers/WebsocketServer.js
+++ b/lib/servers/WebsocketServer.js
@@ -27,7 +27,7 @@ module.exports = class WebsocketServer extends BaseServer {
     connection.close();
   }
 
-  // f should return the resulting connection
+  // f should be passed the resulting connection and the connection headers
   onConnection(f) {
     this.wsServer.on('connection', (connection, req) => {
       f(connection, req.headers);
diff --git a/test/server/__snapshots__/serverMode-option.test.js.snap b/test/server/__snapshots__/serverMode-option.test.js.snap
index a7ad43ca4e..ef6ab793a5 100644
--- a/test/server/__snapshots__/serverMode-option.test.js.snap
+++ b/test/server/__snapshots__/serverMode-option.test.js.snap
@@ -7,3 +7,11 @@ Array [
   "close",
 ]
 `;
+
+exports[`serverMode option without a header results in an error 1`] = `
+Array [
+  "open",
+  "{\\"type\\":\\"error\\",\\"data\\":\\"Invalid Host/Origin header\\"}",
+  "close",
+]
+`;
diff --git a/test/server/serverMode-option.test.js b/test/server/serverMode-option.test.js
index 8a660ddab6..6f64331b2b 100644
--- a/test/server/serverMode-option.test.js
+++ b/test/server/serverMode-option.test.js
@@ -223,4 +223,95 @@ describe('serverMode option', () => {
       }, 5000);
     });
   });
+
+  describe('without a header', () => {
+    let mockWarn;
+    beforeAll((done) => {
+      server = testServer.start(
+        config,
+        {
+          port,
+          serverMode: class MySockJSServer extends BaseServer {
+            constructor(serv) {
+              super(serv);
+              this.socket = sockjs.createServer({
+                // Use provided up-to-date sockjs-client
+                sockjs_url: '/__webpack_dev_server__/sockjs.bundle.js',
+                // Limit useless logs
+                log: (severity, line) => {
+                  if (severity === 'error') {
+                    this.server.log.error(line);
+                  } else {
+                    this.server.log.debug(line);
+                  }
+                },
+              });
+
+              this.socket.installHandlers(this.server.listeningApp, {
+                prefix: this.server.sockPath,
+              });
+            }
+
+            send(connection, message) {
+              connection.write(message);
+            }
+
+            close(connection) {
+              connection.close();
+            }
+
+            onConnection(f) {
+              this.socket.on('connection', (connection) => {
+                f(connection);
+              });
+            }
+
+            onConnectionClose(connection, f) {
+              connection.on('close', f);
+            }
+          },
+        },
+        done
+      );
+
+      mockWarn = jest.spyOn(server.log, 'warn').mockImplementation(() => {});
+    });
+
+    it('results in an error', (done) => {
+      const data = [];
+      const client = new SockJS(`http://localhost:${port}/sockjs-node`);
+
+      client.onopen = () => {
+        data.push('open');
+      };
+
+      client.onmessage = (e) => {
+        data.push(e.data);
+      };
+
+      client.onclose = () => {
+        data.push('close');
+      };
+
+      setTimeout(() => {
+        expect(data).toMatchSnapshot();
+        const calls = mockWarn.mock.calls;
+        mockWarn.mockRestore();
+
+        let foundWarning = false;
+        const regExp = /serverMode implementation must pass headers to the callback of onConnection\(f\)/;
+        calls.every((call) => {
+          if (regExp.test(call)) {
+            foundWarning = true;
+            return false;
+          }
+          return true;
+        });
+
+        expect(foundWarning).toBeTruthy();
+
+        done();
+      }, 5000);
+    });
+  });
 });