In computing, a backdoor is a secret point of entry to a computer program or computer. Legitimate backdoors may be created by a system developer, but more commonly they are used by hackers for penetrating a computer over a network. The most popular method of establishing a backdoor to a computer is through the 'Backdoor Mail Spam' in which an unsolicited email is sent to a victim with a
backdoor program attached to it. This backdoor program will be disguised (a Trojan) pretending to be perhaps a computer game or a screensaver. Further, the email may pretend to originate from a trusted source, such as the mail program of the network or a well-known computer company. When the recipient executes the attachment, the backdoor program installs itself secretly onto the computer and monitors a predefined port on the network waiting to allow a hacker in 'through the back door'.