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Image spam is unwanted advertising e-mail that contains images instead of text. Spam is a slang term for unsolicited commercial e-mail messages. Image spam is used as a means of circumventing filters used by anti-spam programs. These programs that protect against unwanted text e-mails have caused increasing use of image spam instead of text spam e-mails. Methods of combating image spam include sender verification and image permissions. On Internet message boards, image spam denotes when a message board poster posts a series of off-topic, repeated or unusually large images that disrupt the function of the message board or derail discussion in the message board thread.
Usually, image spam is a means of getting a spam message to a user whose e-mail program or host filters text-based spam. Because spam filter programs usually filter out unwanted e-mail advertising by identifying predictable words and word patterns used in spam e-mails, spamming advertisers often send image e-mails that circumvent text-only spam filters. Advanced spam filter programs are designed to detect graphics-based spam in addition to filtering text advertising e-mails
This type of spam is often successful in reaching the e-mail user because users often access an offline e-mail program that downloads e-mail messages automatically. When a user's e-mail program automatically downloads the message, it is displayed to the user. Some e-mail hosts or software programs automatically block images embedded in e-mail pending the user's approval of images sent by each sender. This way, users can avoid image spam, which can sometimes contain undesirable images or text.
One means of fighting spam involves attempting to verify that a human is sending a message. One example of this type of verification is CAPTCHA, which stands for Completely Automated Public Turing Test To Tell Computers and Humans Apart. To verify that a sender is human, CAPTCHA shows the sender an image made up of obscured letters and numbers and asks the user to respond by inputting the letters and numbers shown in the image. While some advanced spam-generating programs can circumvent basic CAPTCHA verification, advanced CAPTCHAs use animation and other technology to confuse computer programs designed to get past the verification.
Though image spam is not by nature a dangerous type of spam, it can be a nuisance. Other types of spam that can be dangerous include blank spam and backscatter spam. Blank spam is a seemingly blank email that can contain code that executes malicious software programs. Backscatter spam infects a user's e-mail box and sends itself to contacts listed in the user's address list. Usually, when one person gets hooked by a backscatter e-mail spam virus, it becomes an epidemic problem in that person's social group, with one user after the other unwittingly distributing spam emails as the problem spreads.