The EU data protection authorities also wrote to Yahoo over a massive data breach that exposed the email credentials of 500 million users,
as well as its scanning of customers' incoming emails for specific information provided by U.S. intelligence officials.
The watchdogs asked Yahoo to communicate all aspects of the data breach to the EU authorities, to notify the affected users of the "adverse effects" and to cooperate with all "upcoming national data protection authorities' enquiries and/or investigations.
"The reports (about email scanning) are concerning to WP29 and it will be important to understand the legal basis and justification for any such surveillance activity, including an explanation of how this is compatible with EU law and protection for EU citizens," the watchdogs said in their letter to Yahoo.
The regulators will discuss the Yahoo and WhatsApp cases in November.