@@ -92,7 +92,7 @@ func (cli *DockerCli) CmdExec(args ...string) error {

 		}

 	}

 	errCh = promise.Go(func() error {

-		return cli.hijack("POST", "/exec/"+execID+"/start", execConfig.Tty, in, out, stderr, hijacked, execConfig)

+		return cli.hijackWithContentType("POST", "/exec/"+execID+"/start", "application/json", execConfig.Tty, in, out, stderr, hijacked, execConfig)

 	})

 	// Acknowledge the hijack before starting

@@ -127,6 +127,10 @@ func (cli *DockerCli) dial() (net.Conn, error) {

 }

 func (cli *DockerCli) hijack(method, path string, setRawTerminal bool, in io.ReadCloser, stdout, stderr io.Writer, started chan io.Closer, data interface{}) error {

+	return cli.hijackWithContentType(method, path, "text/plain", setRawTerminal, in, stdout, stderr, started, data)

+}

+

+func (cli *DockerCli) hijackWithContentType(method, path, contentType string, setRawTerminal bool, in io.ReadCloser, stdout, stderr io.Writer, started chan io.Closer, data interface{}) error {

 	defer func() {

 		if started != nil {

 			close(started)

@@ -149,7 +153,7 @@ func (cli *DockerCli) hijack(method, path string, setRawTerminal bool, in io.Rea

 	}

 	req.Header.Set("User-Agent", "Docker-Client/"+dockerversion.VERSION+" ("+runtime.GOOS+")")

-	req.Header.Set("Content-Type", "text/plain")

+	req.Header.Set("Content-Type", contentType)

 	req.Header.Set("Connection", "Upgrade")

 	req.Header.Set("Upgrade", "tcp")

 	req.Host = cli.addr

@@ -60,6 +60,9 @@ func (s *router) postContainerExecStart(ctx context.Context, w http.ResponseWrit

 	if err := httputils.ParseForm(r); err != nil {

 		return err

 	}

+	if err := httputils.CheckForJSON(r); err != nil {

+		return err

+	}

 	var (

 		execName                  = vars["name"]

 		stdin, inStream           io.ReadCloser