Spammers have started setting up bogus URL shortening services to act as relay points for traffic generated by their emails, Symantec's MessageLabs division has reported in its May 2011 Intelligence Report.
Criminals have for some time embedded URLs created by legitimate URL shortening services as a way of attempting to fool spam filters, which see a real URL rather than a suspect one. Following the link leads to a spam website.
Because this trick is now known to security companies and short URL companies, Symantec has noticed the rise of a new evasion technique that works by embedding a legitimate short URL, which in turn points to a short URL system set up by the spammers. This then hands traffic to the final site, introducing an extra layer to beat filters.
To make sure, spammers have even started setting up chains of these sites to make the links harder and harder to analyse and even registering the domains months in advance of their use to get round domain analysis (bogus domains are often very recent, legitimate ones older). Currently these have mostly been using .ru domains.
According to MessageLabs' figures, the trick of using short URLs in different guises appeared in 2009 before spiking in May 2010 when as much as 20 percent of spam used it. Its use had been less popular in recent months.
"With legitimate URL-shortening services attempting to tackle abuse more seriously, spammers seem to be experimenting with ways to establish their own services to better avoid disruption," said Paul Wood, MessageLabs Intelligence senior analyst. "However, as long as new URL-shortening services are being created, we expect spammers to continue abusing them."
"What is unique about the new URL-shortening sites is that the spammers are treating them as 'stepping stones' - a link between public URL-shortening services and the spammers' own sites," he said.
A interesting feature of the bogus short URL services is probably their relative invisibility. They don't appear in search engine results and can't be used visitors. They exist solely to forward spam, the company said.