Botnets - they're dangerous, deceptive, and very difficult to detect and deal with. What's more, according to recent surveys, the botnet threat is growing...rapidly.
Experts say it's imperative that businesses and end users become aware of the acute and growing dangers posed by botnets, and take decisive and effective steps to counter them before it's too late.
But that's easier said than done as botnets are insidious, and use stealth as a key weapon.
Short for robot, a bot is a captured and compromised computer; and of course botnets are networks of such computers. After being commandeered, these machines may be used for a range of nefarious purposes, including scanning networks for other vulnerable systems, launching denial of service (DoS) attacks against a specified target, sending spam emails, and keystroke logging as a prelude to ID or password theft.
Botnets are generally created through spam emails or adware that leaves behind a software agent, also sometimes called a 'bot'. Captured machines can be controlled remotely by the malware creator, referred to as the bot master or bot herder.
If additional software has to be downloaded to complete the capture process, the bot would first do that. "It may use any mechanism - FTP, PFTP, HTTP - to install the software," explains Jim Lippard, director of information security operations at network services provider Global Crossing, whose customers include more than 35 percent of the Fortune 500, as well as 700 carriers, mobile operators and ISPs.
The next thing the bot does is call home. It would "usually do a domain name server (DNS) lookup on a particular name used by the miscreant for that botnet. Then it will find the host for that name, and connect to it using standard Internet Relay Chat (IRC) protocol," Lippard says.
The larger a botnet, the more formidable the attack it can launch. For instance, when a botnet containing tens of thousands of captured machines is used to launch a denial of service attack, the consequences can be serious and irreparable.
There's the well publicised case of the botnet created by Christopher Maxwell that installed adware on vulnerable machines. It was estimated his botnet attacked more than 400,000 computers in a two-week period. Maxwell's attack, it was reported, crippled the network at Seattle's Northwest Hospital in January 2005, shutting down an intensive care unit and disabling doctors' pagers. The botnet also shut down computers at the US Department of Justice, which suffered damage to hundreds of computers worldwide in 2004 and 2005.
Maxwell pleaded guilty and was sentenced to three years in jail, three years of probation and a fine of $250,000.
The motivation of most bot herders is usually financial, say experts who follow this phenomenon closely. Botnets are sometimes rented out to spammers, scam artists or other criminal elements.
Lippard dubs bot software "the Swiss army knife of crime on the internet". There are multiple functional roles in the botnet economy, he says. For instance, there's the bot herder - the person who controls the bot. Lippard talks about two common ways bot herders make money. The first is by installing adware or clickware on to the systems they control.
"They then use [those programs] to show ads to the owners of the systems, or to click through advertisements for which they get credit under some sort of affiliate program."
He said a second way bot herders profit is by renting out use of the bots to spammers. "They aren't advertised as bots, but as proxies available for spammers to use. And the bot herder will charge a fee based on the number of proxies available in a rotating list on a daily, or even an hourly, basis."
Lippard said he's heard of individual bot herders making "as much as $10,000 a month between adware and clickware fraud and the selling of proxies".
Using botnets to launch denial of service attacks is also on the rise, Lippard says. The initial targets for these attacks used to be offshore gambling websites, and the credit card processors for those sites. "But it's been tried to a lesser extent against other businesses that depend on their websites for their operations to run."
Another common use of botnets is to scan captured machines for user names and passwords, software licence information, and contents of vital documents. Often keystroke loggers are loaded on these machines to retrieve user names and passwords, ID information, and anything that can potentially be used to make a profit.
Some variations of a botnet, known as Rinbot, may be used to steal registration keys for video games.
While Christopher Maxwell's case was widely publicised, experts estimate there are scores of botnets that go undetected. As stealth is their stock in trade, it's difficult to get precise statistics on the growth of this menace.
Some rather alarming numbers have been provided by Symantec's semi-annual Internet Threat Report, published in March 2007. According to that report, botnet activity is up around 10 per cent over the previous period, with the US hosting about 40 per cent of the command and control nodes.
As the motivation of most bot herders is financial, they are keen that compromised systems stay infected and are not detected or repaired by the owners. As many experts point out, botnets take their time spreading in order to remain undetected, as their creators - unlike some other malicious coders - aren't necessarily trying to take over thousands of computers in the shortest time possible.
The only way they could be detected relatively quickly is if the bot herder went overboard and installed so much malware that the system became extremely slow - to the point of being nearly unusable.
Lippard's colleague Bob Hagen notes in his blog that methods and mechanisms used today to detect and eradicate botnet controllers are being rendered obsolete.
"Historically, most botnets utilised Internet Relay Chat (IRC) as the communications mechanism between bots and their controllers," noted Hagen, who is Director of Security Development at Global Crossing. "The bots would stay persistently connected to an IRC server and listen on a designated channel for commands." He said intrusion-detection technologies used to be able to detect this communication channel quite easily.
"Unfortunately," says Hagen, "the era of simplistic botnets may be nearing an end". Realising the ease by which bot communications can be discovered, he said bot herders are starting to use encrypted IRC communications, HTTP tunnelling, and peer-to-peer networking. He noted, however, that most enterprises don't view IRC as a critical business application and configure their firewalls to block it.
"HTTP, on the other hand, is rarely blocked. Therefore, HTTP tunnelling has grown in popularity as a method for botnet communications. Even scarier still, the centralised botnet controller is giving way to the decentralised peer-to-peer model used by file transfer networks such as Gnutella and BitTorrent."
Protecting against these strategies requires a change of tactics on the part of network creators and operators, he suggested.
"Carrier networks must be built with intelligent, flow-based sensors within [their] core that will detect distributed DoS attacks in progress and automatically implement measures to reduce their effects."
He said corrective measures could include applying rate limiting access controls lists on area routers or even null-routing the traffic via border gateway protocol (BGP) route advertisements.
"More importantly, the flow data collected by these sensors can be aggregated to provide a comprehensive view of the distributed DoS attack, enabling the attacking bot to be discovered and reported to their respective internet providers."
Finally, Hagen says sensors should also be tightly integrated with network management systems so operations teams can be alerted as soon as a DDoS attack is detected.
He recognises that such an approach to network security and countering botnets may be criticised as being too defensive.
"While this defensive posture may seem like a set-back, it will force the security community to develop new offensive tactics by which the number of bots and botnets may be reduced," he said.
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