Malicious quiz apps were used to harvest thousands of users’ profile data, according to Facebook.
The firm says anyone who wanted to take the quizzes was asked to install browser extensions, which then lifted data ranging from names and profile pictures to private lists of friends.
These were installed about 63,000 times between 2016 and October 2018, it says.
Facebook is suing Andrey Gorbachov and Gleb Sluchevsky, of Ukraine, who worked for a company called Web Sun Group.
The quizzes, with titles such as “What does your eye colour say about you?” and “Do people love you for your intelligence or your beauty?”, gained access to this information via the Facebook Login system – which enables connections between third party apps and Facebook profiles.
While the system is intended to verify that such connections are secure, in this case, Facebook says, users were falsely told the app would retrieve only a limited amount of public data from their profiles.
“In total, defendants compromised approximately 63,000 browsers used by Facebook users and caused over $75,000 [£58,000] in damages to Facebook,” the company said in court documents first published by online news site The Daily Beast.
The BBC has contacted Web Sun Group for comment.
Andrew Dwyer, a cyber-security expert at the University of Oxford, said the court document suggested users who installed the browser extensions had “effectively opened up entry into their Facebook accounts”.
Facebook’s existing verification procedures would have struggled to recognise this kind of malicious activity before allowing the apps access to users’ profiles, he said.
“Fundamentally, this shows the failures of the app ecosystem – where there was little verification of what apps were doing,” he told BBC News.
“As the [alleged] malicious activity was outside of the app, the typical review process of verifying the app may not have caught this activity.”