Yahoo's data processing operations did not meet EU law at the time of a major breach.
A report into a hack that affected 500 million users in 2014 has been completed by Ireland's Data Protection Commission (DPC) today.
It noted the breach was one of the largest ever to impact EU citizens, affecting approximately 39 million European users.
Yahoo’s European headquarters are in Dublin’s Point Village in the docklands.
It is the largest breach which has ever been notified to and investigated by the DPC.
The breach was reported in 2016 and involved the unauthorised copying and theft of Yahoo users account data in 2014.
Yahoo is now required to take a number of steps in order to be compliant with EU law.
The DPC will monitor the web service provider.