Same way they have allowed OPM to get hacked and have over 21 million government workers security clearance information exposed to a foreign unfriendly government. In short life stories, spouses, children and friends names addresses phone numbers places of past and present employment social security numbers date of birth mothers maiden name everything needed to ruin someones life. The government fix was to "give you" 18 months of credit monitoring. 

Under the Obama administration cyber security has been terrible. The government spying program set up with Yahoo is probably the vehicle used by the hackers to gain access and hack Yahoo users information.