Drive-by spamming is a variation of drive-by hacking in which the perpetrators gain access to a vulnerable wireless local area network (WLAN) and use that access to send huge volumes of spam. Using the drive-by method allows spammers to save themselves the considerable bandwidth costs required to send that many messages legitimately, and makes it very difficult for anyone to trace the spam back to its source.
A drive-by spamming incident starts with war driving: driving around seeking insecure networks, using a computer equipped with a wireless Ethernet card and some kind of an antenna. A wireless LAN's range often extends beyond the building housing it, and the network may broadcast identifying information that makes access simple. Once the attacker finds an unprotected e-mail (SMTP) port, the attacker can send e-mail as easily as someone inside the building. To the mail server, the messages appear to have come from an authorized network user.
According to a report in Geek News, 60-80% of wireless LANS are vulnerable to a drive-by attack, mostly because administrators fail to change the default settings for network access points (devices that serve as base stations in a wireless network) when configuring the network.