A significant rise in the global volume of spam in the past two months has security analysts worried that bot nets are increasingly being used by spammers to stymie network defenses erected to curtail bulk email.
Estimates of the magnitude of the increase in junk email vary, but experts agree that an uncommon surge in spam is occurring. On the low side, Symantec, the owner of SecurityFocus, has found that average spam volume has increased almost 30 percent for its 35,000 clients in the last two months. Others have seen much more significant jumps: Spam black list maintainer Total Quality Management Cubed has seen a 450 percent increase in spam in two months, and the amount of spam filtered out every week by security software maker Sunbelt Software has more than tripled compared to six months ago.
While bulk emailers have, in the past, sent unwanted messages from a single server, increasingly the spam emanates from networks of compromised PCs, known as bot nets. The level of junk email has increased almost in lock step with the number of compromised systems used for spam, said David Hart, the administrator for Total Quality Management.
"What is most alarming is that new clients - internet addresses that we have never seen before and which could be new infections - have tripled since June," said Hart, who posted a chart tracking the growth on his Web site this week.
Bots and bot nets have rapidly emerged as one of the major threats on the Internet. Tens of thousands of compromised PCs are frequently counted among a single bot net's unwilling members, with some bot nets boasting as many as a million systems. Traditionally, the networks have been used to install adware on victims' machines or level denial-of-service attacks at online companies as part of an extortion scheme.
Now, spammers are frequently counted among the operators or the clients of bot nets. Last May, a spammer only identified as "PharmaMaster" used a bot net to target anti-spam provider Blue Security and its Internet service providers with a massive denial-of-service attack that blocked access to the companies for hours and, in the case of Blue Security, days. Because of the attack, the company exited the anti-spam business.
Many bot herders - as the criminals that infect computers with bot software are named - sell or rent bot nets to others to use, and spammers increasingly seem to be among their customers.