ArenaNet president Mike O'Brien has revealed that 1.5 per cent of Guild Wars 2 accounts were hacked prior to the introduction of password blacklisting, and asked all existing players to "please change [their] password".

O'Brien detailed the new security measure on the official ]Guild Wars 2 site, which has designed to prevent new Guild Wars 2 players from becoming victims of account hacks.

"Since we've been observing hackers constantly scanning accounts that don't even exist yet, waiting for someone to create those accounts, we obviously want to make sure that if those new customers do join the game, they don't use the password that the hackers are waiting for," stated O'Brien.

"Thus we're building a blacklist of all the passwords that hackers are scanning for - it's already at 20 million passwords and growing - and we're preventing new customers from choosing any of those passwords. (The blacklist contains passwords only, not account names.)

"This system has substantially eliminated hackers' ability to steal new accounts, as all new accounts now cannot possibly match what the hackers have been scanning for."

Since the added security has been in place, the amount of accounts being hacked has reduced to a far more respectable 0.1 per cent.

Helping out new Guild Wars 2 players is great, of course, but O'Brien knows that existing customers need protection too, which means changing passwords.

"It's difficult to distinguish between a login attempt by the real customer and a login attempt by a hacker. So we'll take the safe approach and ask all existing customers to change their passwords, and blacklist everyone's old password in the process.

"This all leads to the following request. All existing customers, please change your password. When you change it, the system won't allow you to pick your previous password, or any password that we've seen tested against any existing or non-existent account. Thus, after changing your password, you'll be confident that your new password is unique within Guild Wars 2."

If you're struggling to think of a new, secure password, O'Brien has a few words of advice.

"... We advise you to build a password out of four random words... Use a password like 'correct horse battery staple'... even if everyone selects their words from the same 2,000 most common words, that's still 16 trillion possible passwords. We'll soon introduce a random password generator to suggest passwords like that."

Guild Wars 2 has sold over 2 million units worldwide.

Security issues have plagued the game since its release last month.