Four-fifths of spam now emanates from computers contaminated with Trojan horse infections, according to a study by network management firm Sandvine out this week. Trojans and worms with backdoor components such as Migmaf and SoBig have turned infected Windows PCs into drones in vast networks of compromised zombie PCs.
Sandvine reckons junk mails created and routed by "spam Trojans" are clogging ISP mail servers, forcing unplanned network upgrades and stoking antagonism between large and small ISPs.
Using its own technology, Sandvine was able to identify subscribers bypassing their home mail servers and contacting many mail servers within a short period of time - a sure sign of spam Trojan activity - over sustained periods. It also looked at SMTP error messages returned, which helps to clarify the total volume of spam within the service provider network. "After comparing those data points with the total volume of legitimate messages passing through the service provider's mail system, we are able to arrive at our percentage of 80 per cent," explained Sandvine spokesman Mark De Wolf.
Sandvine's analysis, cross referenced with data from SORBS, to determine what IP space is assigned to residential subscriber pools of global service providers, shows most spam now originating from residential broadband networks.
Instead of using open mail relays or unscrupulous hosts (so-called 'bullet-proof' hosting - in reality, ISPs in developing countries who pull the plug on spammers when enough complaints are received by their upstream provider), spammers are using compromised machines to get their junk mail out. Many security firms reckons many of the most well-publicized worm attacks in recent months (such as MyDoom and Bagle) were launched expressly to install spam Trojans on unsuspecting end users' machines - waiting to be utilized later as a spam delivery relay. This expanding network of infected, zombie PCs can also be used as platforms for DDoS attacks, such as those that many online bookies have suffered in recent months.
The behaviour of spam Trojans on the network taxes ISP infrastructure and, in the case of smaller ISPs, creates the perception that some networks are generating more than their fair share of spam and other types of malicious traffic. The mounting scope of the problem means ISP need to begin filtering traffic - rather than leaving the problem up to end users - if spam is to be contained, Sandvine argues.
"While spam filters can provide an effective treatment, the scale & scope of the spam problem means additional remedies are needed", said Marc Morin, co-founder and chief technology officer of Sandvine. "As a complement to existing mail server and client based tools, service providers need to arm themselves with network-based anti-spam defences and combat this growing form of malicious traffic." ®
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Phatbot arrest throws open trade in zombie PCs
The illicit trade in compromised PCs
Trojans as spam robots: the evidence
SoBig-F blamed for massive increase in spam
Sandvine's trend analysis The Effects of Spam Trojans on Service Provider Networks (PDF)