Researchers at ESET have identified a family of malware targeting web servers running the Linux and FreeBSD operating systems.
The malware has been dubbed ‘Mumblehard’, and gives attackers a backdoor that allows them to control the systems they compromise. According to ESET, the malware goes back to at least 2009, and has a module for sending spam messages.
“Mumblehard components are mainly Perl scripts encrypted and packed inside ELF binaries,” the firm noted in a whitepaper. “In some cases, the Perl script contains another ELF executable with the same packer in the fashion of a Russian nesting doll.”
ESET researchers sinkholed the malware’s backdoor module and collected statistics on the infected servers. During a seven-month period, they observed Mumblehard queries from a total of 8,867 unique IP addresses. The highest number of unique IP addresses seen in a single day was 3,292.
“There are two components in the Mumblehard malware family: a backdoor and a spamming daemon,” blogged ESET’s Marc-Etienne M.Leveille. “They are both written in Perl and feature the same custom packer written in assembly language. The use of assembly language to produce ELF binaries so as to obfuscate the Perl source code shows a level of sophistication higher than average.”
“Monitoring of the botnet suggests that the main purpose of Mumblehard seems to be to send spam messages by sheltering behind the reputation of the legitimate IP addresses of the infected machines,” he explained.
The researchers discovered what they believe is a connection the spam campaign and a company called Yellsoft, which sells software known as DirectMailer for sending bulk e-mail messages. According to ESET, command and control (C&C) servers hardcoded in the Mumblehard samples are all located in the IP range 126.96.36.199 to 188.8.131.52.
“If you check out the two next IP addresses, 184.108.40.206 and 220.127.116.11, you will find that both are name servers for yellsoft.net,” according to ESET’s whitepaper. “Also, the yellsoft.net web server is hosted at 18.104.22.168. If you dig further, the five IP addresses, from 162 to 166, will answer the same NS and SOA record for this IP range, despite the fact that in reality, this range is served by rx-name.com. This strongly suggests that the five IP addresses are hosted on the same server.”
In addition, pirated copies of DirectMailer install the Mumblehard backdoor, according to ESET. Another possible infection vector noted by the firm is the use of Joomla and WordPress exploits.
Victims should look for unsolicited cronjob entries for all the users on their servers, the researcher noted in his blog.
“This is the mechanism used by the Mumblehard backdoor to activate the backdoor every 15 minutes,” he wrote. “The backdoor is usually installed in /tmp or /var/tmp. Mounting the tmp directory with the noexec option prevents the backdoor from starting in the first place.”
The whitepaper on the malware can be read here.