ILOVEYOU was a computer worm that spread across millions of computers around the globe on and after May 5th, 2000. The worm was delivered via email messages with the subject line of “ILOVEYOU” and an attachment titled “LOVE-LETTER-FOR-YOU.TXT.vbs.” When the recipient of the email opened the attachment hoping to find a passionate love letter, the worm would execute its Visual Basic script to overwrite files within the computer’s directory and send copies of its malicious code to each of the contacts found in the infected computer’s email system.

ILOVEYOU was incredibly effective for two reasons. First, the worm exploited hidden file extensions in Windows operating systems. At the time, .vbs file extensions were often hidden by default on Windows computers. A recipient of the worm would therefore see .txt as the file extension for the email attachment they received, rather than .txt.vbs. Second, the worm hijacked legitimate email accounts to send itself to other email addresses. Recipients of the worm would see the email as coming from a trusted source with which they had emailed previously. Before account takeovers, spoofing, and social engineering became more prominent, end users were more likely to trust emails and their content, making ILOVEYOU one of the first widespread worms to cause a global infection of computer systems.