Which of the following is the most common method for an attacker to spoof email?

A.
Back door

B.
Replay attack

C.
Man-in-the-middle attack

D.
Open relay

Explanation:
An open relay is the most common method for an attacker to spoof email.
An open relay is an SMTP mail server configured in such a way that it allows anyone on the Internet
to send e-mail through it. By processing mail that is neither for nor from a local user, an open relay
makes it possible for a spammer to route large volumes of spam.

Answer C is incorrect. Man-in-the-middle attacks occur when an attacker successfully inserts an
intermediary software or program between two communicating hosts. The intermediary software or
program allows attackers to listen to and modify the communication packets passing between the
two hosts. The software intercepts the communication packets and then sends the information to
the receiving host. The receiving host responds to the software, presuming it to be the legitimate
client.

Answer A is incorrect. A backdoor is a program or account that allows access to a system by skipping
the security checks. Many vendors and developers implement backdoors to save time and effort by
skipping the security checks while troubleshooting. A backdoor is considered to be a security threat
and should be kept with the highest security. If a backdoor becomes known to attackers and
malicious users, they can use it to exploit the system.

Answer B is incorrect. A replay attack is a type of attack in which attackers capture packets
containing passwords or digital signatures whenever packets pass between two hosts on a network.
In an attempt to obtain an authenticated connection, the attackers then resend the captured packet
to the system. In this type of attack, the attacker does not know the actual password, but can simply
replay the captured packet.