Spammers are combining their circumvention techniques into a "triple threat" to increase the efficacy of their attacks, according to Symantec.
Paul Wood, MessageLabs intelligence senior analyst, Symantec, said in a statement on Tuesday that spammers are combining three more established spam techniques — Captcha-breaking, social-networking spam and using web mail — in an attempt to fool spam filters.
Spammers have for the past few years used sophisticated character-recognition software to break the Captcha system. Captcha is a verification system commonly found on email account sign-up pages, which displays a set of graphically-distorted numbers and alphabets, meant to be readable to a human but harder for automatic software to recognise and register for an account.
The use of web mail also shows that cybercriminals are using more reputable websites which are less likely to get blocked, debunking a "common misconception that [spammers] are more likely to use less-reputable websites... to hide malware," noted MessageLabs.
The anti-spam company said the number of new sites hosting malware was slashed from 3,561 in April to 1,149 in May, showing that more established-domains were being used to host malicious content. Furthermore, a huge 84.6 percent of domains blocked for hosting malware had been established for over a year.
Wood explained that newer domains tend to get flagged as suspicious because they are typically temporary sites set up just to host spam and malware, and also because they are shut down faster. Trustworthy older domains can be compromised through SQL injection attacks, he said.
The trend of abusing trust also extends to the proliferation of social-networking spam, said Wood. Social-networking sites pose a number of varying threats to users, from the availability of personal data on these sites increasing the likelihood of phishing, to the higher chances of users clicking on spam links because they trust friends who 'send' it to them.
According to MessageLabs, May saw the global ratio of spam rise by 5.1 percent over April to hit 90.4 percent. Hong Kong was the most-spammed economy at 92.3 percent.