Which of the following are ways a vote thief could cast multiple votes in an online election? (check all that apply)
a. Using a DoS attack to make a legitimate vote server unavailable and direct voters to fake vote servers
b. Purchasing pass codes from people willing to sell their right to vote
c. Setting up fake vote servers to collect pass codes of voters
d. Using social engineering techniques to get pass codes from targeted voters

All of the given options apply and are correct.          

Explanation:

a) The first option is correct because the intruder can use denial-of-service attack and by flooding the site with traffic to get the voter server unavailable. Then he can direct the voters to a fraud vote server.

b) The second options correctly applies too because the thief also buys the pass codes from the people who want to sell their right to vote and this way the thief can purchase multiple pass code and cast multiple votes.

c) The hackers can set up fake vote servers and then direct the voters to these servers or websites in order to collect pass codes of voters. These pass codes can then be misused by the thief to cast multiple votes in an online election.

d) Social engineering techniques can also be used to get pass codes from certain voters. By manipulating the targeted voters psychologically they can get their pass codes to cast multiple votes illegally.