Spam. (credit: Sean MacEntee)

A botnet that enslaved about 4,000 Linux computers and caused them to blast the Internet with spam for more than a year has finally been shut down.

Known as Mumblehard, the botnet was the product of highly skilled developers. It used a custom “packer” to conceal the Perl-based source code that made it run, a backdoor that gave attackers persistent access, and a mail daemon that was able to send large volumes of spam. Command servers that coordinated the compromised machines’ operations could also send messages to Spamhaus requesting the delisting of any Mumblehard-based IP addresses that sneaked into the real-time composite blocking list, or CBL, maintained by the anti-spam service.

“There was a script automatically monitoring the CBL for the IP addresses of all the spam-bots,” researchers from security firm Eset wrote in a blog post published Thursday. “If one was found to be blacklisted, this script requested the delisting of the IP address. Such requests are protected with a CAPTCHA to avoid automation, but OCR (or an external service if OCR didn’t work) was used to break the protection.”