Yahoo agreed to a US government request to scan every incoming email to search for specific information of interest to the NSA or FBI. The snooping code was implemented in such a way that it could have allowed a hacker to basically read everyones Yahoo mail Ex-Yahoo security chief Alex Stamos is said to have taken particular issue with how poorly the scanning tool was installed. Yahoo CEO Marissa Mayer green lit the surveillance without consulting her security team, and there was an attempt to keep security experts out of the loop.”]
Source: https://grahamcluley.com/spy-program-given-hacker-access-yahoo-email-claims-employee/