Apparently quite a few people were hit by this:
Yoshi-P's statement on the matter:
Here is a message from Producer/Director Yoshida regarding the current security issue and action that will be taken from this point on for Market.
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This is Producer/Director Yoshida.
This is a thread, but I would like to mention the current situation and actions that we are going to take from this point on.
First of all, a few hours ago, we have received several reports about fraud in certain features of the Market operated by an outside source.
There are not many reports of casualties due to this, but to prevent any further cases, we performed an emergency maintenance.
We apologize for the inconvenience and that this has affected many of our players.
We are verifying the data and promise that everything will be kept safe.
This issue can only be seen in certain Worlds, Zones and circumstances, so it will not affect players that are not logged in.
Once we resolve the issue and assure the safety of the situation, alongside the recovery of the game, we will take action towards the outside fraud source and take security countermeasures.
Again, we apologize for the inconvenience that this may have caused.
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We are not planning to do any sizable data roll backs at this time for this case.
We are basically planning to correspond to the players whose data was affected.
We have obtained information of the route and IP address of the source and we are planning to take legal actions of this fraud.
Might be a good idea to check and make sure that all your gil is available at your next login, and that no fraudulent purchases were made at the market board by your character(s).