Avast said it discovered the extensions last month and found evidence that some had been active since at least December 2018, when some users first started reporting issues with being redirected to other sites. Jan Rubín, Malware Researcher at Avast, said they couldn’t identify if the extensions had been created with malicious code from the beginning or if the code was added via an update when each extension passed a level of popularity. And many extensions did become very popular, with tens of thousands of installs. Most did so by posing as add-ons meant to help users download multimedia content from various social networks, such as Facebook, Instagram, Vimeo, or Spotify.
Avast said it reported its findings to both Google and Microsoft and that both companies are still investigating the extensions.
ZDNet’s article includes Avast’s lists of the 28 extensions which they’re recommending be uninstalled by users.
ZDNet also notes that “A day after Avast published its findings, only three of the 15 Chrome extensions were removed, while all the Edge add-ons were still available for download. A source familiar with the investigation told ZDNet that Microsoft has not been able to confirm the Avast report.”