What good ways are there to prevent cheating in JavaScript multiplayer games?

Question

Imagine a space shooter with a scrolling level. What methods are there for preventing a malicious player from modifying the game to their benefit? Things he could do that are hard to limit server-side is auto-aiming, peeking outside the visible area, speed hacking and other things.

What ways are there of preventing this? Assume that the server is any language and that the clients are connected via WebSocket.

Always assume that the code is 100% hackable. Think of ways to prevent a client completely rewritten (for the purposes of cheating) from cheating. These can be things such as methods for writing a secure game protocol, server-side detection, etc.

Illuminism

Whatever you do, one can always execute what he wants as he actually owns the computer which executes the JavaScript. – Culpable

I suspect this will be hard, maybe impossible to prevent given the current Javascript environments in browsers, but great food for thought. – Signature

Answer 1

The server is king. Clients are hackable.

What you want to do is two things with your websocket.

Send game actions to the server and receive game state from the server.

You render the game state. and you send input to the server.

• auto aiming - this one is hard to solve. You have to go for realism. If a user hits 10 headshots in 10ms then you kick him. Write a clever cheat detection algorithm.
• peeking outside the visibile area - solved by only sending the visible area to each client
• speeding hacking - solved by handling input correctly. You receive an event that user a moved forward and you control how fast he goes.

You can NOT solve these problems by minifying code. Code on the client is ONLY there to handle input and display output. ALL logic has to be done on the server.

You simply need to write server side validation . The only thing is that a game input is significantly harder to validate then form input due to complexity. It's the exact same thing you would do to make forms secure.

You need to be really careful with your "input is valid" detection though. You do not want to kick/ban highly skilled players from your game. It's very hard to hit the balance of too lax on bot detection and too strict on bot detection. The whole realm of bot detection is very hard overall. For example Quake had an auto aim detection that kicked legitedly skilled players back in the day.

As for stopping a bots from connecting to your websocket directly set up a seperate HTTP or HTTPS verification channel on your multiplayer game for added security. Use multiple Http/https/ws channels to validate a client as being "official", acting as some form of handshake. This will make connecting to the ws directly harder.

Example:

Think of a simple multiplayer game. A 2D room based racing game. Upto n users go on a flat 2D platformer map and race to get from A to B.

Let's say for arguments sake that you have a foolsafe system where there's a complex authetication going over a HTTPS channel so that users can not access your websocket channel directly and are forced to go through the browser. You might have a chrome extension that deals with the authentication and you force users to use that. This reduces the problem domain.

Your server is going to send all the visual data that the client needs to render the screen. You can not obscure this data away. No matter what you try a silled hacker can take your code and slow it down in the debugger editing it as he goes along until all he's left with is a primitive wrapper around your websocket. He let's you run the entire authentication but there is nothing you can do to stop him from stripping out any JavaScript you write from stopping him doing that. All you can achieve with that is limit the amount of hackers skilled enough of accessing your websocket.

So the hacker now has your websocket in a chrome sandbox. He sees the input. Of course your race course is dynamically and uniquely generated. If you had a set amount of them then the hacker could pre engineer the optimum race route. The data you send to visualise this map can be rendered faster then human interaction with your game and the optimum moves to win your racing game can be calculated and send to your server.

If you were to try and ban players who reacted too fast to your map data and call them bots then the hacker adjusts this and adds a delay. If you try and ban players who play too perfectly then the hacker adjusts this and plays less then perfect using random numbers. If you place traps in your map that only algorithmic bots fall into then they can be avoided by learning about them, through trial and error or a machine learning algorithm. There is nothing you can do to be absolutely secure.

You have only ONE option to absolutely avoid hackers. That is to build your own browser which cannot be hacked. Build the security mechanisms into the browser. Do not allow users to edit javascript at runtime in realtime.

Reside

Thanks for your input! Some of the points are pretty normal ways of cheat prevention, I would appreciate tips on some more advanced methods too. But all input is good. :) – Illuminism

@Illuminism advanced method are game domain specific ;) I can go into details of how to detect FPS cheaters but that doesn't help with detection the platformer cheaters which make almost impossibly perfect jumps. – Reside

@Illuminism Write a secure browser with a javascript game engine build into it ;) Whilst your at make it have p2p sockets and make the browser run as a node.js server. – Reside

Answer 2

At the server-side, there are 2 options:

1) Full server-side game

Each client sends their "actions" to the server. The server executes them and sends relevant data back. e.g. a ship wants to move north, the server calculates its new position and sends it back. The server also sends a list of visible ships (solving maphacks), etcetera.

2) Full client-side game

Each client still sends their actions to the server. But to reduce workload on the server, the server doesn't execute the actions but forwards them to all other clients. The clients then resolve all actions simultaneously. As a result, each client should end up with an identical game. Periodically, each client sends their absolute data (ship positions, etc.) to the server and the server checks if all client data is identical. Otherwise, the games are out of sync and someone must be hacking.

Disadvantage of the second method is that some hacks remain undetected: A maphack for example. A cheater could inject code so he sees everything, but still only sends the data he should normally be able to see to the server.

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At the client-side, there is 1 option: A javascript component that scans the game code to see if anything has been modified (e.g. code modified to render objects that aren't visible but send different validation data to the server).

Obviously, a hacker could easily disable this component. To fix that, you could force the client to periodically reload the component from the server (The server can check if the script file was requested by the user periodically). This introduces a new problem: the hacker simply periodically requests the component via AJAX but prevents it from running. To avoid that: have the component redownload itself, but a slightly modified version of itself.

For example: have the component be located at yoursite/cheatdetect.js?control=5. The server will generate a slightly modified cheatdetect.js so that in the next iteration, cheatdetect.js?control=22 (for example) must be downloaded. If the control mechanism is sufficiently complicated, the hacker won't be able to predict which control number to request next, and cheatdetect.js must be executed in order to continue the game.

Mythology

Answer 3

There's nothing you can really do to prevent anyone from modifying your JS or writing a GreaseMonkey script. However you can make it hard for them by minifying your script as well as making your code as cryptic as possible. Maybe even throwing in some fake methods or variables that do nothing but are used to throw an attacker off. But given enough time, none of these methods are completely foolproof, as once your code goes to the client, it is no longer yours.

Corespondent

Yes, I was actually looking for ways to prevent cheating despite code modification. Because that is of course what a cheater would do. :) I added a third paragraph with another assumption. – Illuminism

These methods of "delaying" attackers are just a nuisance for modders / users and maintainers of code base. – Reside

Security through obfuscation is never a good thing and advicing it in any place is downright stupid. Even you know it, so your answer should not have been written. :) – Beckett

Answer 4

They don't have to touch your client-side code -- they could just sniff and implement your Websocket protocol and write a tiny agent that pretends to be a human player.

Update: The problem has a few parts, and I don't have answers off the top of my head, but the various options could be evaluated with these questions in mind:

1. How far are you willing to go to prevent cheating? If you only care about casual cheating, how many barriers are enough to discourage the casual cheater? The intermediate Javascript programmer? A serious expert? Weighing this against the benefits of cheating, is there anything of real value at stake, like cash and prizes, or just reputation?
2. How do you get a high confidence that a human is providing inputs to your game? For example, with a good enough computer vision library I could model your game on a separate machine feed inputs to the computer pretending to be the mouse, but this has a high relative cost (not worth my time).
3. How can you create a chain of trust in your protocol such that knowledge of (2) can be passed to the server, and that your server is relatively confident your client code is sending the messages?

Sure many of the roadblocks you throw up can be side-stepped, but what is the cost to the player and you? See "Attrition warfare".

Signature

Yup, that is the exact assumption I'm looking for a solution to. I've read some papers that talk about this and come up with reasonable ways of mitigating it, but I'm looking for more input. – Illuminism

Answer 5

The only way I can even think of implementing this is by modifying your Javascript to function as a client and then designing a central server mechanism to validate data sent from that client. This is probably a big change to implement and will most likely make your project more complex. However, as was said earlier, if the application runs entirely on the client, the client can pretty much do whatever they want with your script. The only way to secure it to use a trusted machine to handle validation.

Shannon

This is the only real solution. Put any logic you can on the server. The server consumes events from each client and responds with updated state. If the client is simply rendering the current state and sending events for each user interaction the you have nothing to worry about. – Decoupage

@BobBaller as long as a javascript game is single player there is no point in placing validation into it. If a user wants to cheat then he ruins his own game experience. For single player javascript games that do not effect anything outside a single instance of that game you should allow the user to do whatever he wants. – Reside

@Reside that is true, but the original question specifically addresses multiplayer javascript games where validation is required to maintain fairness amongst all parties involved. – Shannon

@BobBailer if the game is already multiplayer then the logic should already be running on a central server. A p2p multiplayer game on the client would be a nightmare to control. – Reside

@Reside I just reread the question and I think it has been edited since my first post. I don't remember any initial mention of a server so I assumed it was p2p. – Shannon

@BobBailer how would p2p work in a client without 3rd party applications anyway? – Reside

Answer 6

Some other methods that can be implemented:

• Make the target elements difficult for a script to distinguish from other elements. Avoid divs with predictable class and id names if possible. Inject styling using JavaScript instead of using classes. Think like a hacker and make it hard on yourself.
• Use decoys that a script will fire on. For instance, if the threat vector is a screen scraping algorithm using pixel colors, throw some common pixel colors in non-target elements. Hits on these non-targets could seem inconsequential to the cheater, but would be detectable. You don't want the cheater to know why you know.
• Limit the minimum time between actions to slightly below the best human levels. The best players will hit that plateau, and it won't matter as much who's cheating, and immediately be able to detect anyone scripting faster than that by side-calling method calls.
• Random number generators are typically uniform. Human nature is not. Likely a random number generator will have values within a set limit and even distribution. Natural distribution is a Gaussian curve. If you sampled the distribution and it looks like a square wave in the x and y axis, 100% it's a cheater. This will be fairly difficult for the cheater to detect the threshold for the algorithm because it's a derivative of the random, and not the random distribution itself. You're also using aggregate data and not individual plays to detect it, so reverse engineering the algorithm would be extremely difficult without knowing your detection algorithm.
• Utilize entropy whenever possible. Avoid predictable game plays. Imagine a racing game on a set collection of race tracks. Each game play could have slightly differing levels of traction, horsepower, and momentum. The script would have to be extremely good to beat it. In a scrolling game, you can alter factors that are instinctual to humans, but difficult for computers, such as wind force, changes in gravity, etc. It would also make it more fun as a side benefit.
• Server generated tokens can be used to validate UI elements were used and not calls to the code itself. Validation can be handled in one call at the end of the game comparing events to hashed codes of UI elements. The token should be a hash with a server private key and some value of the UI element.
• Decoy the cheater with data they think you're using to detect cheats. Such as calls to a DetectCheat method with dummy calls to a fake backend. It's the old magician's trick. Wave your hand over here, while you slip a card into the deck with the other hand. Let them waste days on end in a maze that has no exit, with lot's of hair pulling.

Ineffective

Answer 7

I'd use a combination of minification and AJAX. If all of the functions and data aren't loaded into the page, it'd be more difficult to cheat.

On the other hand, modding turned out to be a very profitable tool for companies like Id Software. Perhaps allowing the system to be modded might make the game that much more enjoyable to the community at large.

Kandicekandinsky

Yeah the idea is that people should be allowed to see and modify the source code but still be prevented from cheating. There will of course always be benefits to the user modifying the code, but I'm looking for ways to decrease them without impacting gameplay. – Illuminism

Answer 8

Obfuscate your client exposed code as much as possible. Additionally, use some magic.

Fluoridation

This more look like joke than like real answer to question. – Estop

Answer 9

You can edit the javascript on the browser and make it work. Some people suggest that make a call to check with the server. So after making a call to the server, it will be validated in the server. Once validated, it will come to client side and do actions. But I think even this is not foolproof.

For eg.,. for a Basic login action : in angular while making a call to server, the backend validates username & pwd and if validated, it will come back to the client and let the user login using angular.

When I say login using angular, it is going to store things in cookies, like user objects and other things. But still the user can remove the JS code which is making the call to backend, and return TRUE(wherever needed) and insert user object(dummy) to cookies and other objects(whatever needed) and login. It is a very difficult thing to do, but it is doable. In many scenarios, this is not desirable even if it takes hours to edit/hack the code.

This is possible in single page applications, where JS files dont get reloaded for each page. To mitigate the possibility of getting hacked we can use minified codes. And I guess if actions like this is done in backend(like login in Django) it is much safer.

Please correct me if I am wrong.

Eirena