Criminal hackers are actively exploiting the critical shellshock vulnerability to install a self-replicating backdoor on a popular line of storage systems, researchers have warned.

The malicious worm targets network-attached storage systems made by Taiwan-based QNAP, according to a blog post published Sunday by the Sans Institute. The underlying shellshock attack code exploits a bug in GNU Bash that gives attackers the ability to run commands and code of their choice on vulnerable systems. QNAP engineers released an update in October that patches systems against the vulnerability, but the discovery of the worm in the wild suggests a statistically significant portion of users have yet to apply it.

"The attack targets a QNAP CGI script, /cgi-bin/authLogin.cgi , a well known vector for Shellshock on QNAP devices," Johannes B. Ullrich, dean of research at Sans, wrote. "This script is called during login, and reachable without authentication. The exploit is then used to launch a simple shell script that will download and execute a number of additional pieces of malware."

Infected systems are equipped with a secure shell (SSH) server and a new administrative user, giving the attackers a persistent backdoor to sneak back into the device at any time in the future, Ullrich said. The worm also patches the Shellshock vulnerability, presumably to prevent other attackers from taking over the compromised device. Infected devices also execute a script that makes them carry out a click-fraud scam against an advertising network. A variety of other scripts are also installed.

"Infected devices have been observed scanning for other vulnerable devices," Ullrich said.

He was unable to locate the specific script responsible for the scanning, but it is presumed it automatically infects any unpatched systems it finds. As indicated in one of the links above, researchers from FireEye warned in early October that Shellshock exploits were likely to persist for a long time, particularly on network-attached devices from QNAP. Admins who have not already done so should take inventory of all devices that run any type of embedded Linux to make sure the Bash vulnerability has been patched.