While most voters will cast their ballots at polling stations in November, online voting has been quietly and rapidly expanding in the United States over the last decade. Over 30 states and territories allow some form of Internet voting (such as by email or through a direct portal) for some classes of voters, including members of the military or absentees. Utah just passed a law allowing disabled voters to vote online; and Alaska allows anyone to cast their ballots online. And there were recent news reports that Democratic and Republican national committees are contemplating holding primaries and caucuses online. We estimate that over three million voters now are eligible to vote online in the U.S. But online voting is fraught with danger. Hackers could manipulate enough votes to change the results of local and national elections. And a skilled hacker can do so without leaving any evidence.