The major concern surrounding voting machine or ballot tampering is usually at early voting locations, since there it is typical to leave the ballot boxes and/or voting machines unattended overnight. The state I'm a poll worker in does have reasonable additional precautionary measures, such as printing and retaining daily open and close reports from the digital tabulators to guard against overnight ballot stuffing, but this does present an opportunity to do something like tampering with the machine so that it incorrectly tabulates future ballots.

Not really an easy attack but still a possible one, and these machines have not established a strong reputation for protection against something like a firmware implant

This state uses and retains paper ballots, so it would be yet more difficult to design an attack that would withstand an audit of the paper ballots. But some states don't...