One of the world's most active spam botnets -- Grum -- was crippled after two of its command and control (CnC) servers hosted in the Netherlands were taken down, according to researchers from security firm FireEye.
"These two CnC servers were responsible for pumping spam instructions to their zombies," said FireEye senior staff scientist Atif Mushtaq in a blog post on Tuesday. "With these two servers offline, the spam template inside Grum's memory will soon time out and the zombies will try to fetch new instructions but will not able to find them."
If Grum stops sending spam, it will have a significant impact on the global spam volume, Mushtaq said. However, this might be just a temporary victory, because the botnet's creators still control two CnC servers hosted in Russia and Panama.
Grum relies on two types of control servers for its operation. One type is used to push configuration updates to the infected computers and the other is used to tell the botnet what spam emails to send. It's only the second type of servers that were shut down.
This means that Grum's operators can still theoretically use the two remaining configuration servers to update the botnet and direct it to new spam template servers. However, that hasn't happened so far.
"No action has been taken by the bot herders so far," Mushtaq said. "There is complete silence from their side."
FireEye is trying to get the remaining servers shut down. However, the ISPs in Russia and Panama that host the abusive servers have not responded to the company's notifications.
"I don't know if the security community will eventually be able to take down the rest of the Grum botnet, but we are trying and trying very hard," Mushtaq said. "We did not give up after the first failed attempt and will continue to contact the Russian and Panamanian authorities through different channels."
The first versions of the Grum malware appeared in early 2008, which makes Grum one of the oldest botnets still active.
Over the years, the botnet has managed to fill the void left in the spam distribution market by the successful takedowns of other botnets like Srizbi, Rustock or Mega-D/Ozdock.
According to spam statistics published by security firm Trustwave, since the beginning of the year, Grum, Cutwail and Lethic have been the most active spam sending botnets in the world. Grum alone was responsible for nearly 35 percent of the world's spam traffic observed last week.
The takedown of the two Grum spam template servers came after FireEye published a blog post on July 9 in which Mushtaq revealed technical details about the botnet and the infrastructure supporting it.
"In my opinion, taking down the top three spam botnets -- Lethic, Cutwail, and Grum -- is enough for a rapid and permanent decline in worldwide spam level," Mushtaq said at the time. "We still have to deal with small players, but I am sure that, after seeing the big players being knocked down, they will retreat as well."