Twitter historically identified authentic accounts by attaching a blue checkmark to the account name – a signifier that Twitter Blue recently allowed users to acquire via an $8 subscription. Following this, a number of fake accounts were created and used these blue checkmarks to appear authentic. As an example, an account claiming to be pharmaceutical firm Eli Lilly and Co. tweeted that insulin was free; which resulted in a loss of over $15bn in market cap and a drop of over 5% in stock price.

Source: one