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Spam levels – hit by recent botnet takedown efforts – have begun to return to their previous noxiously high levels.

The average global spam rate was 89.1 per cent, an increase of 1.4 percentage points on 2009, according to a study by Symantec. Global spam rates peaked at 92.2 per cent in August, largely powered by an aggressive campaign designed to distribute the Rustock botnet, one of the main global conduits of junk mail. Botnets in general accounted for 88.2 per cent of all spam for 2010 as a whole.

Botnet takedown and the closure of spam affiliate, Spamit, in early October 2010, meant that the proportion of spam that could be traced back to botnets slid by 11 percentage points to 77 per cent. However, by the end of 2010, the total number of active bots had returned to roughly the same numbers as at the start of the year, as hackers re-established junk mail communication channels and compromised proxies (zombies). The total number of botnets worldwide is between 3.5 and 5.4 million, Symantec Hosted Services estimates.

The top three botnets for spam distribution have not changed much despite the upheavals in the underground economy (botnet takedowns, Zeus suspect arrests etcetera) during the second half of 2010. Rustock remains the Big Daddy of botnets, with a spam output that has doubled over the course of the last 12 months to reach 44 billion spam emails per day. The botnet is reckoned to have compromised at least a million (zombie) hosts. Grum and Cutwail are the second and third largest botnets, respectively. Each is also associated with the distribution of malware by spam.

Cybercrooks who control botnets have experimented with different command and control structures, moving from traditional IRC controls (which can be easily blocked at firewalls) to web-based controls. Some have experimented with using social networks such as Twitter as a command channel this year.

MessageLabs recorded an average rate for malware in email traffic of one in 284.2 emails (0.352 per cent) during 2010, virtually unchanged from the proportion of malware in email it recorded last year (0.349 per cent). However, the number of different strains of malware in these blocked email grew by a factor of 100 over the last 12 months to reach 339.673 for 2010. The change reflects the increased industrialisation of malware production, according to the security firm.

Symantec reckons even more sophisticated approaches are in the pipeline with controls hidden in plain view, using steganography, likely to emerge – perhaps within images or music files distributed through file sharing or social networking websites. The tactic will allow botherders to "surreptitiously issue instructions to their botnets without relying on an ISP to host their infrastructure" and thus limit the chances that they will be discovered. ®

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