The researchers reported on Wednesday that thousands of computers running the Linux and FreeBSD operating systems have been infected over the past seven months with a sophisticated malware. The malware make part of a perfidious network blasting the internet with spam. The malware likely infected many more machines during the five years it’s known to have existed.
The malware known as Mumblehard has been designed by experienced and highly skilled programmers. It includes a backdoor and a spam daemon, which is a behind-the-scenes process that sends large batches of junk mail. These two main components are written in Perl and they are obscured inside a custom “packer” which is written in assembly, a low-level programming language that closely corresponds to the native machine code of the computer hardware it runs on.
The researchers have revealed evidence that Mumblehard may have links to Yellsoft, a company that sells DirecMailer, which is Perl-based software for sending bulk e-mail. The block of IP addresses for both Yellsoft and some of the Mumblehard C&C servers share the same range. The pirated copies of DirecMailer silently install the Mumblehard backdoor. The pirated copies are also obscure by the same packer used by Mumblehard’s malicious components.
The Eset researchers suspect that the malware may take hold by exploiting vulnerabilities in the Joomla and WordPress content management systems. They also think that the infections are the result of installing pirated versions of the DirecMailer program.
Eset observed around 9,000 IP addresses which cannot be directly correlated to the number of machines that were infected by Mumblehard, since in some cases more than one server may share an address and in other cases a single server may give up an old address and take up a new one.