First American Financial Corp. tumbled the most in nearly eight years amid concerns that a security flaw at the title insurer may have allowed unauthorized access to more than 885 million records related to mortgage deals going back to 2003.

Earlier, the company said that “at this time there is no indication that any large-scale unauthorized access to sensitive customer information occurred.” First American has curbed some access and hired an outside forensic firm to assess the damage.

Compass Point slashed its price target on the stock to a Street-low of $55.50 from $62, as First American likely faces increasing...