After being stranded for weeks, a monster botnet responsible for an estimated 40 percent of the world's spam was able to briefly reconnect to its mothership in a tense international duel playing out online that could have a dramatic effect on the amount of junkmail flowing into inboxes everywhere.

The rogue network dubbed Srizbi was able to establish ties to a new master control channel using an emergency mechanism built into the 500,000 or so machines infected by the bot. Botherders designed the pseudo random domain name generator in the event their network got disconnected from the previous channel. That's precisely what happened earlier this month, when a network provider known as McColo was yanked offline.

At time of writing, most of Srizbi's connection to the outside world had once again been severed, thanks to decisive actions taken to shut down servers located in Estonia. A single server located in Germany continued to host some nodes of the network, as researchers scrambled to get it shut down as well.

"An onslaught of spam was certainly averted," said Alex Lanstein, a researcher at intrusion detection system prover FireEye, who has spent the past four weeks closely monitoring Srizbi. "Estonia stepped in in record time and kicked these guys off line."

According to Lanstein and other researchers at FireEye, computers infected by Srizbi are automatically armed with a mathematical algorithm that generates random-looking domain names that are invoked in the event their main channel is knocked out of commission. The bots then endlessly cycle through a list of domains to contact until a connection is finally made. (FireEye has a wealth of technical details about the domain name algorithm here.)

For weeks, the researchers were able to thwart the emergency backup measure by generating the domain names such as qpqduqud.com themselves and then snapping up the addresses ahead of the bad guys. The cat-and-mouse standoff ended this week after FireEye researchers decided they could no longer afford to spend the money buying the domains.

Most of Srizbi's new command and control servers were located in Estonia and all of its domains were registered in Russia. For about 13 hours, some 100,000 or so infected machines had the ability to connect to those servers, though it's not clear exactly how many of them did so, since many of them were likely not powered on, Lanstein said.

The Estonian servers were quickly disconnected in a take-down action Lanstein said he was not at liberty to discuss in detail.

At time of writing, a single server located in Frankfurt continued to act as a command and control channel for the rogue network.

Since the shutdown of McColo, junk-mail levels have plummeted, according to anti-spam companies and anecdotal evidence. Levels have gradually crept upward since then after a separate spam botnet dubbed Rustock, also orphaned by the McColo shutdown, was able to reboot itself last week.

Srizbi's resurrection had the potential to see spam levels spike to previous levels. After successfully reconnecting to the new servers, infected machines immediately got back to the business of sending spam. While the Estonian servers were active, botmasters updated templates for sending spam to Russian-speaking recipients, most likely as a way to test if the bots were still working. The botmasters were stopped before they could do much else.

"If Estonia hadn't taken it down I'm sure they would have pushed out an English spam template and these things would have fired up again, but they didn't have time to," Lanstein told The Register. ®