Global spam volumes plummeted by as much as one-third during March when the Rustock botnet was taken down, but other spammers have been quick to take up the slack.

That's according to Symantec's March 2011 MessageLabs Intelligence Report, which reveals that, prior to its takedown, Rustock was sending up to 13,82-billion spam e-mails daily, accounting for an average of 28,5% of global spam sent from all botnets in March.
When the botnet ceased sending spam, global spam volumes fell by one-third. However, since then, other botnets have stepped up their activities to take advantage of the gap in the market, with Bagle now the most active spam-sending botnet in 2011.
MessageLabs Intelligence identified that global spam volumes fell by 33,6% between March 15 and 17 following legal action against command and control hosts used by the Rustock botnet. In the days following the takedown of Rustock, spam accounted for approximately 33 billion emails per day, compared with an average of 52-billion per day in the previous week.
“It remains to be seen whether the criminals behind Rustock will be able to recover from this co-ordinated effort against what has become one of the most technically sophisticated botnets in recent years,” says MessageLabs Intelligence senior analyst:, Paul Wood. “Rustock has been a significant part of the botnet and malware landscape since January 2006, much longer than many of its contemporaries.”
Notably, Bagle did not appear in the top 10 spam-sending botnets at the end of 2010 as reported in the MessageLabs Intelligence 2010 Annual Security Report. By the end of 2010, Rustock had been responsible for as much as 47,5% of all spam, sending approximately 44,1-billion e-mails per day.
Also in March, MessageLabs Intelligence analysed the spam traffic from the top ten major spam sending botnets. Since the end of 2010, the more-active Bagle botnet has sent approximately 8,31-billion spam e-mails each day, the majority linking back to pharmaceutical products. Bagle does not have as many bots under its control, or spikes of traffic as large and dominating as Rustock, but its output has been more consistent.
In March, 83,1% of global spam was sent from botnets, an increase of 6.,1 percentage points compared with the 77% at the end of 2010. During 2010, botnets sent an average of 88,2% of global spam.
 “Botnets have been and remain a destructive resource  for cyber criminals and through the years have become the spammers’ air-supply, without which it would be very difficult for them to operate. Botnets are also used for other purposes such as launching distributed denial of service attacks, hosting illegal web site content on infected computers (known as bots), harvesting personal data from them and installing spyware to track the activities of their users,” Wood says.
Other report highlights include:
* Spam: In March 2011, the global ratio of spam in email traffic from new and previously unknown bad sources decreased by 2% (one in 1,26 emails).
* Viruses: The global ratio of e-mail-borne viruses in email traffic from new and previously unknown bad sources was one in 208,9 e-mails (0,479%) in March, an increase of .134 percentage points since February. In March, 63,4% of email-borne malware contained links to malicious websites, a decrease of .1 percentage points since February.
* Endpoint Threats: The endpoint is often the last line of defense and analysis. The threats found here can shed light on the wider nature of threats confronting businesses, especially from blended attacks. Attacks reaching the endpoint are likely to have already circumvented other layers of protection that may already be deployed, such as gateway filtering. Threats against endpoint devices such as laptops, PCs and servers may penetrate an organisation in a number of ways, including drive-by attacks from compromised websites, Trojan horses and worms that spread by copying themselves to removable drives. Analysis of the most frequently blocked malware for the last month revealed that the Sality.AE virus was once again the most prevalent. Sality.AE spreads by infecting executable files and attempts to download potentially malicious files from the Internet. MessageLabs deployed techniques such as heuristic analysis and generic detection, to correctly identify and block several variants of the same malware families, as well as identify new forms of malicious code that seek to exploit certain vulnerabilities that can be identified generically. Approximately 15.7 percent of the most frequently blocked malware last month was identified and blocked in this way, using endpoint security protection.
* Phishing: In March, phishing activity was one in 252,5 e-mails (0,396%), a decrease of 0.065 percentage points since February.
* Web security: Analysis of web security activity shows that an average of 2 973 web sites each day were harbouring malware and other potentially unwanted programs including spyware and adware, a decrease of 27,5% since February. 37%of malicious domains blocked were new in March, a decrease of 1,9 percentage points since February. Additionally, 24,5% of all web-based malware blocked was new in March, a decrease of 4,2 percentage points since last month.
In South Africa, spam accounted for 79,5% of e-mail traffic.