How to make a html5 multiplayer game more secure?

I am using node.js and socket.io for websockets. My code looks like this:

client:

players = [], // contains all connected players
localPlayer; // contains 1 local player
function update() {
    if (localPlayer.update(keys)) {
        socket.emit("movePlayer", {x: localPlayer.getX(), y: localPlayer.getY()});
    };
};
function draw() {
    localPlayer.draw(context); // draw local player
    var i;
    for (i = 0; i < players.length; i++) {
        if (players[i].getNick() != localPlayer.getNick()) {
            players[i].draw(context); // draw all players except local player
        }

    };
function animate() {
    update();
    draw(); // canvas
    window.requestAnimFrame(animate);
};

server:

socket.on('movePlayer', function(data) {
    var movePlayer = playerById(socket.id);
    movePlayer.setX(data.x); // update new position
    movePlayer.setY(data.y); // update new position
    io.sockets.emit("onMovePlayer", {id: movePlayer.id, x: movePlayer.getX(), y: movePlayer.getY()}); // send new position of player to all players
});

Everything works great, it's not lagging and it's fast. But problem is that I think this is a very hackable solution. Can you help me with that? (something with more security)