  We uncover a previously unknown, ongoing astroturfing attack on the
popularity mechanisms of social media platforms: ephemeral astroturfing
attacks. In this attack, a chosen keyword or topic is artificially promoted by
coordinated and inauthentic activity to appear popular, and, crucially, this
activity is removed as part of the attack. We observe such attacks on Twitter
trends and find that these attacks are not only successful but also pervasive.
We detected over 19,000 unique fake trends promoted by over 108,000 accounts,
including not only fake but also compromised accounts, many of which remained
active and continued participating in the attacks. Trends astroturfed by these
attacks account for at least 20% of the top 10 global trends. Ephemeral
astroturfing threatens the integrity of popularity mechanisms on social media
platforms and by extension the integrity of the platforms.
