One of the Internet rites of passage is to telnet to port 25 and send fake mail to a friend. The SMTP daemon actually marks the forged mail with the IP number of the machine that connected to port 25, which makes it very easy to discern which machine sent the mail. 3 Spammers often forge mail headers so that the true source of a spam message is hard to discern. It is usually easy to detect forged mail by looking carefully at the header. RO This is forged mail.
Applets provide an interesting new twist on the standard approach to mail forging.
Because applets load across the network and run on a Web surfer's machine, a mail forging applet can cause the standard SMTP daemon monitoring port 25 to report mail coming from the Web surfer's machine-not the machine serving the applet. This can be leveraged to doubly forge mail. Imagine that Alice hits a Web page and an applet is consequently run on her machine. With many standard server configurations, this forging attack is possible. Figure 4.4 illustrates the difference between the standard port 25 attack and the revised applet attack.