'Bashlite' Malware Leverages ShellShock In BusyBox Attack

Researchers at Trend Micro are warning that a new version of the Bashlite malware is using the ShellShock vulnerability (CVE-2014-6271) to gain control of devices using BusyBox.

BusyBox is built on top of the Linux kernel and is used by devices such as routers. According to Trend Micro, recent samples of Bashlite (ELF_BASHLITE.SMB) scan networks for devices and machines running on BusyBox, logs in using a set of usernames and passwords, and then runs a command to download and run bin.sh and bin2.sh scripts to gain control over the BusyBox system.

"Remote attackers can possibly maximize their control on affected devices by deploying other components or malicious software into the system depending on their motive," blogs Rhena Inocencio, threat response engineer at Trend Micro.

Since news about ShellShock first broke, attackers have used it in attempts to compromise systems. Just recently, the cross-browser testing service BrowserStack said an attacker leveraged ShellShock to compromise an unpatched server that was not in active use.

Last month, security researchers at Trend Micro warned about ShellShock exploits targeting SMTP servers, while Akamai Technologies said attackers were building botnets out of vulnerable systems. In addition, researchers with Solutionary's Security Engineering Research Team (SERT) reported that 67% of traffic captured with ShellShock signatures was related to previously known bad sources within 24 hours of their release -- meaning that attackers had quickly altered their tactics to include ShellShock exploits.

"When news of the ShellShock vulnerability broke out at the end of September, we spotted several attacks that leveraged the said vulnerability, thus manifesting the prevalence or even evolution on how attackers used the exploit," Inocencio blogs. "For instance, attackers used ShellShock to target SMTP servers, launch botnet attacks, and even to download KAITEN source code among others."

The first variant of Bashlite (ELF_BASHLITE.A) was capable of launching distributed denial-of-service attacks, as well as brute-forcing logins to get a hold of user credentials. The latest version (ELF_BASHLITE.SMB) arrives on a system as a file dropped by other malware or as a file downloaded unknowingly by users when visiting malicious sites. It executes commands from a remote malicious user, effectively compromising the affected system, according to Trend Micro.

The previous Bashlite sample (detected as ELF_BASHLITE.A) used BusyBox just to echo the string "gayfgt" if the remote malicious user invokes the command Scanner On, Inocencio blogs. This is done to check if the device runs BusyBox.

If the command Scanner On is received, the telnet forked process will attempt to connect to port 23 of a randomly generated IP address to check it can log in remotely using telnet. Once the connection is successful, the malware tries to log in using a default set of usernames and passwords, including "root," "admin," and "support." From there, the newest version of the malware sends commands to download and execute a file in the remote computer.

"We strongly advised users to change the default usernames and passwords and disable remote shell if possible to these devices," Inocencio blogs.