Occam's razor suggests that one mt. gox account being compromised indicates a client computer getting hacked or an account password getting phished and not some compromise of the exchange's server.

Yeah, as the others said, it's probably your computer, not MtGox being hacked. Try running a couple of different antivirus products, I'd recommend Avira, Comodo and possibly Malware Bytes.

You're going to have to do better than running off the shelf AV software.  If this is the action of some malware, it has to be captured before a sample can even be submitted AV software company.  A previous poster named some of the less effective AV programs according to this report: http://www.av-test.org/certifications

You need to have someone who knows what they are doing look at your system.

Don't count on AV software to save you from any bitcoin related payloads at this point.  Bitcoin related actions won't be part of any heuristic scanning at this point.

Yes, Linux should be more secure and it mines faster. There's no reason not to invest <$100 for a new hard drive to install some Linux distro onto if you're gonna invest heavily into Bitcoin.

+1

This exact same thing happened to me earlier.
I think my password was brute-forced.

Lesson learned is, use complex alphanumeric+symbols passwords, and change them frequently.

Mt. Gox also really needs to add some sort of secondary verification.

Are there truly no vulnerabilities in Linux?

There must certainly are some yet to be found and fixed, but Linux is not as big of target as Windows. Though it's true that Linux is highly used in corporate infrastructure, those systems are usually well-secured with other layers. Windows, by contrast, attracts criminals because it's the operating system of the computer incompetent. Windows boxes are easier "marks."

there are of course. over the years linux was basically swiss cheese. all nix in general.

local roots are quite common with a true remote root/or user access being quite uncommon now days.
one has to have some type of port listening. but even a kernel root is no longer unheard of. a proof
of concept was released a couple of years ago. if you hang around with the right circle of people
you come to the realization that a few folks have the knack for auditing code for vunls. For some
it is their job. Others a hobby.

the thing that has changed now days is that attackers no longer audit sendmail or ftp daemons for
issues... they are reviewing web browsers, plug ins, mail clients, flash, pdf, etc...
so it may not be linux at fault but other software ran on it.

why does an attacker need root? i bet permissions as your username would be just fine to do any
dirty deed. If not.. wait until spender releases another local root ;-)

None of this negates the possibility of a trojan already sitting on the networked machine intercepting everything.

Running from a USB stick is no guarantee either, an attacker who got access to your networked machine has various non-volatile attack avenues: such as BIOS rootkits: http://www.infoworld.com/d/security-central/hackers-find-new-place-hide-rootkits-252 - there is also working code for inserting modules into network card BIOS code (e.g. for disk password entry if your BIOS does not support it), this could easily be coopted as well.

Not many people are aware that BIOSes have a privileged execution mode (SMI/SMM) for running code in the background, used for low level hardware stuff (e.g. hw watchdogs), which is basiclly undetectable from within an OS running on top of it. It doesn't matter whether you use Windows or Linux (although for obvious reasons Windows is more likely to get compromised).

It's very likely Bitcoin specific malware is already out in the wild, perhaps as a module to an existing crimeware kit, perhaps something targeted. Virus scanners are useless in general since they work on the basis of guarding against infections that usually become known after the fact, and target masses, not spearhead phishing. And most of them have a somewhat poor detection rate even for generic attacks.