According to The Wall Street Journal, this partnership was a direct result of the companies' regular meetings with European Union officials. European authorities have been putting the pressure on internet companies to do something to curb the spread of terrorist propaganda, since extremists have been using their platforms to spread their message. Twitter, for one, had to shut down hundreds of thousands of accounts associated with ISIS and other extremist groups this past year.

The other companies are pretty experienced in fighting off terror content, as well. Microsoft started auto-purging them from Xbox Live, Outlook Docs and its other services in May this year. In June, Reuters reported that YouTube and Facebook, among other internet companies, were using automated systems to find and remove terror-related images and videos. They reportedly uploaded their finds to a shared database, as well, though it's unclear whether that earlier experimental effort is associated with this collaboration. WSJ says the four companies will begin sharing their hashes next year and are open to welcoming new additions to the group in the future.