There is a Trojan virus that can hack your WOW account even if its protected by an Authenticator.
Okay don’t start to panic just yet and ditching your security as having an Authenticator and a decent anti-virus program still protects you 99% of the time.
The type of attack used is known as ‘Man in the Middle‘.
When you enter your Authenticator code the virus intercepts it and sends it to the evil bad people’s (trying not to swear here) servers and either crashes your UI or gives an incorrect login message.
What To Do?
Check your computer for a file called ‘emcor.dll’ and delete it, apparently it likes to reside in “C:\Users\(Your user name)\AppData\Temp” however do a full check just in case.
Below is the post from Blizz, click it to go to the full forum.
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Nice site. Thank you.,
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Lol already got hacked 3 times now
… Do I have to kill my authenticator? 
This helps to explain how my account was hacked after months of not playing it (and account was “inactive”). One day at work, I logged in to my Battle.net account to change my last name and update my e-mail address…”just in case” I ever played again. Within a few days, a guildie e-mailed me to tell me that my toon was logging on and not responding to whispers. I did the account recovery to get everything changed, but I have been completely baffled as to how my account was hacked since I never logged in to the game. Since that hack, I put my authenticator back on the account and my work computer was upgraded to Windows 7 (which required a hard drive wipe). However, I am now not entirely secure feeling about my company’s ability to keep this kind of crap off my work computer!