The practice has been a topic of conversation among spam fighters on internet discussion groups since late June, said Joe Stewart, senior intrusion analyst with LURHQ, a Chicago-based managed security services company.
Experts observed that one spammer who was sending out spam e-mail pointing to spoofed PayPal websites and Russian pornography sites appeared to be able to change the addresses of his websites every few minutes, according to Richard Smith, an Internet security and privacy consultant.
Smith stumbled upon the problem in early July while investigating e-mail messages pointing to a phony PayPal site being used to harvest personal financial information from customers of the online payment service.
After reporting the address of the site that he believed was the source of the phony website to the Internet service provider responsible for that address, Smith was surprised to see the same web domain associated with a different Internet address belonging to a different ISP a few minutes later, and still another address a few minutes after that.
After writing a program to monitor the websites associated with the pornography and bogus PayPal domains, Smith collected the IP addresses of hundreds of computers being used as hosts for the illicit content, each for only a few minutes at a time.
The trick lies in a sophisticated trojan program placed on the remote systems and used by the spammer, according to Stewart, who obtained a copy of the program from an infected system belonging to an employee of one of LURHQ's enterprise customers.
The program, which Stewart has called "migmaf", acts as both a proxy server for spam and a reverse proxy server for a master web server serving the spoofed and pornographic content.
Domain names and e-mail addresses for the pornography sites point to Russia as the source, Smith said.
In its capacity as a proxy server, the trojan forwards outgoing spam from its source to the intended recipient, replacing the source address with its own IP address and covering the spammer's tracks.
As a reverse proxy server, the trojan receives requests from spam recipients who, for example, click on a link to a pornographic website, and passes that along to the master web server. That server responds with the requested web page and sends that content along to the compromised computer, which then serves it to the requesting machine.
Users never know where the content they are receiving is really coming from, and the website's owners are shielded from pressure by their ISP to shut down the site, according to Smith and Stewart.
Because such behind-the-scenes activity might eventually arouse the suspicions of victims, each compromised user machine acts as a DNS (Domain Name Service) host for the illicit web domains for only 10 minutes, before being replaced by another compromised system known to the spammer.
To move web properties around continually, the spammer installs DNS software on the compromised machines, turning them into their own DNS servers. Then, using features of DNS, the spammer sets a short expiration, or "time to live" setting on what is referred to as the DNS "host name mappings," which specify a relationship between a domain name, such as www.ebay.com, and a numeric internet address, Smith said.
Using online domain registration services such as Network Solutions and automated scripts, the spammer updates the host mapping information at regular intervals, replacing the DNS address for one compromised machine with that of another, Smith said.
Such techniques are attractive to spammers who are looking to bypass IP address blacklists, which are the most widely used antispam technology, according to Linus Upson, a spam expert for US antispam company Qurb.
"As a spammer, you care about getting spam into people's mailboxes. A solution like this nullifies the most widely used antispam technology," Upson said.
And, for spammers involved in fraudulent activity, hiding the source of the spam is a way to avoid getting caught, he said.
Neither Stewart nor Smith knew how the trojan came to be installed on the affected systems.
A virus such as W32.Sobig could have dropped it on systems that it infected, or a malicious Active X control on a website could have planted it on vulnerable machines, Smith said. Alternatively, the program could have been distributed through the IRC (Internet Relay Chat) network or peer-to-peer (P-to-P) networks like Kazaa.
While the new trojan cannot spread itself like a virus, migmaf has a number of features that report the statistics of systems it compromises back to the master web server, according to an analysis written by Stewart and posted on the LURHQ website. The trojan can report statistics and information about its state back to the master server and monitor the available bandwidth on the infected system, he said.
By dissecting a copy of the trojan, Stewart was able to trace the location of the master web server back to a machine owned by Houston web-hosting company Everyones Internet.
Everyones Internet did not respond to a request for comment, but Stewart said that the master web server has been deactivated and Smith said that his monitoring shows that the PayPal and pornography websites are down. Nevertheless, the new distribution system will make it extremely difficult to track down the source of future illicit content and spam.
"It took Joe Stewart seven days to locate that server. It usually takes a couple minutes," Smith said.
Like Kazaa and other P-to-P networks, the spam network is distributed and lacks a single point of failure, which will make it difficult to dismantle.
The sample trojan program has been passed along to major antivirus companies, which are developing signatures to detect the stealth program, Stewart said. However, multiple versions of the migmaf trojan probably exist, many of which will not have antivirus signatures developed for them, he said.
Smith and Stewart advised usersto install personal firewalls on any unprotected home computers, especially those with "always on" broadband internet connections.
Even if it does not prevent users from having the new trojan installed, firewall software will prevent the spammers' master server from communicating with an infected host and becoming a distribution point for spam or pornography, they said.
Paul Roberts writes for IDG News Service