Are open source multiplayer games less secure? What have I not considered?

The problem of open source is, that cheaters are able to see and use all the source, even the server source code if available. They can easily try to use the source code to recompile a modified client version (enemy radar, see through walls, aiming bots etc) while faking necessary checksums, which is really hard to do with closed-source games. Further on, as side effect, open source is often low on budget and therefor they avoid commercial anti-cheat tool (eg punkbuster?).

The chances of cheats depends on the popularity and how easy it is to cheat. Therefor unpopular, closed source games might be most secure, while really popular games, regardless of being open or close source, are prone to cheating. Thought, open source games, which gains in popularity, will much likely be targeted by cheaters really quickly.

It is absolutely true that open source lowers the bar for certain kinds of cheats!

Open source also lowers the bar for security minded developers to help prevent cheating. Your job is to get as many secure-minded contributors as you get cheating players!

Separately, even if a game is not open source, a determined cheater will be able to cheat. De-compilation and assembly isn't THAT hard to work with.for a skilled programmer.

The draw-back is that it's much harder for a security-minded developer to help prevent cheating in the closed-source world. The relative incentives are less in favor of the developer.

The choice is not at all cut-and-dried. There's pluses and minuses on both sides. And cheating is only one small part of the giant challenge that is developing an online game -- if open source, or closed source, gives you a significant leg up in some other department (schedule, tools, robustness, cost, etc) then that's quite likely to be the deciding factor, rather than the different profiles of cheating/cheaters/mitigation/help.

Its true, but if closed-source software implements no other measures then the kind of "securtiy" it has is derisively called "security through obscurity" in secutrity/hacker circles -- that is to say, it has no security other than being obscured from plain view. It doesn't provide any protection from would-be cheaters as such, it just relies on hope that people will be detered by it not being readily accessible. History shows, however, that even moderately-popular software, closed-source or not, becomes a worthwhile target for crackers and cheaters eventually.

Its a bit like burying your money in the back-yard; instead of the real security of a bank-vault, you rely instead on the obscurity of its location in your yard. But if its known that the money is indeed in your yard, and its known that the score will be worthwhile, then you'll soon start to attract nefarious types.

Unfortunately, most open-source games implement little in the way of *actual* security either -- so in effect, many of these games are hiring sky-writers to publish the exact location and dollar amount of their hidden treasures for all to see and in the common tongue.

Its certainly easy to "crack" an open-source game or server, but its really a bit of a misnomer that that closed games are more secure simply because they are closed.

Open source makes finding exploits a lot easier. If you play it fast and loose, it'll be a lot quicker for hackers to find exploits within the code or even your design assumptions. If your game isn't particularly secure but the code cannot be reversed engineered easily, it can take more time. Ultimately, if your game is popular, you will be found out either way.

Why do you believe that all closed source games are built to make money, and all open source games are built as someone's hobby?
While I'm sure the correlation coefficient is greater than zero, I'm also sure that it's not one.

Regarding taking open source without giving back, I also know of companies that do that, but most of them are not as bad as Goldman Sachs, which reportedly rips out all copyright/license from libraries they take, and change it to Goldman Sachs proprietary, and then throw programmers in prison if they actually abide by the original license.

Anyway, yes, you find varying degrees of maturity in all walks of life, and there exists large game studios that are very mature, and others that aren't. You also find hobbyists that are very mature, and others that aren't. In the end, maturity is much more important to the quality of code produced than most other axes. And when it comes to security, code quality is a lot more important than most other factors. You can make a fun, commercially successful game on top of terribly misguided rendering code. You can't make a secure system on top of misguided security code.