I bet this is related to lulzsec's recent dump of 62,000 passwords. Password reuse anyone?
What is this?
Lulzsec is a group that have been hacking quite a number of well know networks and systems in the last week or so. They've exposed many security flaws, and gottn hold of many many username password combinations.
A lot of fairly new forum users have supposedly had their MT.Gox account hacked, and had their bitcoin taken, or if USD it's exchanged for bitcoin and taken.
There's nothing wrong with MT.Gox's security, only that a great number of users have been using the same username:password combination as another website that's been hacked.
If you look at the stickied thread in Newbies you can see that most people don't seem to reuse both their username and password on Mt. Gox.
EDIT
Another possibility is that the user with the hacked system had a password stealing trojan on their system.
I know that at least for me that is not the case.
The only options for what is happening are:
1)MTGox are themselves stealing users money
2)Users are reusing password/username combinations from other sites that have been hacked
3)Users have a compromised system that has resulted in their username/password being lifted.
4)MTGox has some major security holes
1 is not likely as MTGox make enough money as it is, also why then wouldn't they steal everyonese instead of just a few accounts worth?
I don't think Mt. Gox stole it themselves. Besides them indeed getting more gain from running a business, there are a lot more "invisible" ways to make money disappear from accounts if you have access to the system. So that's extremely unlikely.
4 is more likely but still not probable. MTGox have a simple but robust system that has been strengthened through attacks almost since it's inception.
They use username:password authentication over https, so that's not leaked.
They are vulnerable to a CSS history sniffing attack because they use GET requests for their forms, to just name a vulnerability I found (which can be thwarted by having a long non-dictionary password, by the way). So no, it's not as robust as you seem to imply.
Again because it's over https there is little to no chance of having your session hijacked.
They limit the number of password attempts so accounts cannot be brute forced.
I believe that that only works per IP, and that you have a practically infinite amount of attempts per account if you do distributed bruteforce (aka, let every bot in your botnet do 5 tries).
The system itself isn't likely easily hacked, otherwise everyones bitcoin in MTGox would be gone.
It would be a much better to stay relatively low-profile, and not give the impression that Mt. Gox were compromised, if it's indeed unsafe. That way you can slowly keep stealing more and more funds, while other people just attribute it to user error.
Options 2 & 3 are the most likely and most common in these situations.
I know that at least for me both 2 and 3 are not applicable. I don't reuse passwords, and I've turned my entire system pretty much upside down to see if there was anything suspicious - which there wasn't.