The hacking occured on the 16th May, in the night. Yahoo have also stated that as soon as they detected the problem, they cut all network access in order to carry out a internal investigation. During the down time, the traffic between yahoo and their internal systems would suggest that in the region of 22 million identities were stolen during the attack.
Yahoo have said that they dont know for certain is a file had been stolen, but advised the AFP that they certainly couldnt deny the possibility. The file in question is said to contain no passwords or other information necesary to reestablish a connection with yahoo, or indeed anything necesary to confirm identity.
Yahoo have been in contact with every person affected by the incident, urging them to change passwords. The company, Yahoo Japan is jointly owned by Yahoo and Softbank, and they have said that security measures have been upgraded in the days after the attack, and also that a full investigation will be launched in order to put in place more measures to ensure that this incident is not repeated.