If you noticed a decrease in spam recently, there could be a good reason. This month, Microsoft took down the Rustok botnet.
Microsoft's Digital Crime Unit reported that its "research shows there may be close to one million computers infected with Rustock malware, all under the control of the person or people operating the network like a remote army, usually without the computer's owner even aware that his computer has been hijacked. Bot-herders infect computers with malware in a number of ways, such as when a computer owner visits a Web site booby-trapped with malware and clicks on a malicious advertisement or opens an infected e-mail attachment. Bot-herders do this so discretely that owners often never suspect their PC is living a double life."
These botnets aren't just the toy of young hackers who like causing mischief. They aren't trying to crash or disable the computer; in fact it's just the opposite. That stealth aspect to the bot infection is key to its success. The user has no reason to think they need to get their PC fixed, because a good botnet infection doesn't raise suspicion. That is the key to the botnet's survival.
A botnet is a huge money-making tool for its creators. When bot-herders take over a PC, they have many ways to turn a profit. One way is to grab information they find on the PC, or can extract by monitoring the user's keystrokes. This can give them access to bank accounts, credit cards, and login information to sites such as eBay or PayPal. Before the user can do anything to stop it, the botnet operator can transfer the PayPal money to another account. Or they can purchase expensive items with the user's eBay account and get the seller to send it to an address where the botnet operator can pick it up.
Perhaps the most valuable thing a botnet provides its handler is a large pool of "innocent-looking" IP addresses. In the case of the Rustok botnet, that's one million IPs. If the bot-controlled PC appears to visit a Web site, click on a Google Adwords ad, or send a few dozen emails, it's not possible to block that action based merely on the IP address. So Rustok's botnet could send 10 million spam messages by having each PC send just 10 emails, and nothing looks suspicious.
Click fraud is another endless source of money for botnet operators. By setting up some shallow content sites with Google Adwords or other ad networks, the bot-herder can have the bots visit those sites and click on the ads to generate revenue. The bot-herder can also use click fraud to attack competitors, clicking on their ads in order to drain their ad budgets. This type of fraud can be extremely difficult for the ad networks to spot if the botnet operator keeps the fraud at a low level and doesn't get too greedy.
When botnets started to emerge a decade ago, the creators of the botnets often used them directly and managed all the money-making schemes themselves. Now, many bot-herders rent out their botnet to other groups that have specific goals in mind, such as spam, click fraud, or targeted attacks. Underground message boards let bot-herders communicate with their customers to "sell time" on the botnet.
Botnets are a threat not only to businesses and consumers, but to governments as well. A botnet can be used as a huge army in cyberwarfare, effectively disabling communication channels by clogging critical Internet paths or Web sites. Unlike many weapons programs, a botnet can be self-funding and doesn't require technology that's embargoed by major nations like the United States. The commercial crime not only brings in money, but provides a "cover story" for why the botnet was created in the first place. At any point, however, the botnet can become a weapon of war if it is controlled by a country.
Microsoft has its own take on how to combat botnets: "It's like a gang setting up a drug den in someone's home while they're on vacation and coming back to do so every time the owner leaves the house, without the owner ever knowing anything is happening. Homeowners can better protect themselves with good locks on their doors and security systems for their homes. Similarly, computer owners can be better protected from malware if they run up-to-date software -- including up-to-date antivirus and anti-malware software -- on their computers.
Although anti-malware software can help, its effectiveness is far from perfect. The botnet creators are constantly working on ways to mask their infection vectors, and are often successful. Combine that with the gullibility of many users and some simple social engineering techniques ("free porn, don't worry about the antivirus warning, it's a known bug") and many PCs that are technically protected still become infected. Once it's established on the PC, the botnet software often disables any antivirus software, and may even turn off Windows Updates to prevent programs like the Microsoft Malicious Software Removal Tool from running.
Large enterprises can be a prime source of raw PC material for botnets, but they also have tools that consumers don't have for detecting and fixing botnet infections. The most important of these are network monitoring. Botnets have to communicate with a "controller" on the Internet in order to receive their marching orders. By analyzing the Internet traffic traveling through the corporate firewall, the network admins may be able to find suspicious patterns.
Botnet operators are often opportunistic in their attacks. If they happen to find that they have taken over a PC in an enterprise, they may sell the control of that PC to someone who would like to make a targeted attack on that company. At that point it's no longer just a case of your company's PCs being used for bad things. Your company's PCs have become a vector being used to attack the company itself. The potential for losses of both money and information are almost unlimited. That risk alone is the best justification for your company to actively monitor and combat its PCs being turned into botnet fodder.