Websites and apps featuring pirated movies and TV shows make about $1.3 billion from advertising each year, including from major companies like Amazon.com Inc., according to a study.

The piracy operations are also a key source of malware, and some ads placed on the sites contain links that hackers use to steal personal information or conduct ransomware attacks, according to the online safety nonprofit Digital Citizens Alliance and the anti-piracy firm White Bullet Solutions Ltd. While law enforcement officials have sought to stop some of the online criminality, the groups identified at least 84,000 illicit entertainment sites.

The study underscores just how tough a problem piracy is for both Hollywood studios and companies that distribute digital ads. The situation has been compounded by the COVID-19 pandemic, which has left more people watching films and television shows over the web, where criminals have a greater chance of successfully targeting victims.