Two different malware families are regularly downloading new variants of each other to re-infect the compromised machines and make cleanup much harder, Microsoft said.
The first threat comes from a family of worms called Vobfus, and it spreads via removable drives and network shares, Hyun Choi, a malware researcher with Microsoft, wrote on the Malware Protection Center blog. Once Vobfus has infected the computer, it then downloads Beebone, a downloader Trojan capable of downloading other types of malware, including Vobfus, Zbot, Sirefef, Fareit, Nedsym, and Cutwail. Beebone also communicates with a remote command-and-control server to join the computer to a botnet.
Beebone downloads additional Vobfus variants, usually brand-new ones that security tools don’t recognize yet, creating a “cyclical relationship,” Choi said. Even if Vobfus is detected and removed, it would have already downloaded Beebone by then. And Beebon can continue to download Vobfus variants, making it difficult to eradicate Vobfus entirely.
This is why Vobfus “may seem so resilient to antivirus products,” Choi said.
Most malware with self-updating mechanisms can be eradicated because once removed from the system, they cannot download newer versions to re-infect the machine. With Vobfus, so long as it keeps downloading versions of Beebone that antivirus and other security tools don’t recognize, the cycle of infection will continue.
Because Beebone downloads multiple malware families, the “cumulative side-effects” are present in the compromised machine, making the cleanup process even more challenging.
In a network environment with lots of mapped network usage or data -sharing via removable drives, Vobfus spreads by copying itself and an autorun.inf onto new locations, Choi said. It also creates a runkey in the system registry to ensure it runs every time Windows starts.
“In the wild, we have observed that Vobfus maintains a very successful removable-drive infection rate, thus supporting its spreading,” Choi said.
With Beebone on the computer, thieves gain remote control of the machine to send spam or launch other attacks against other machines. The malware also has data-collection capabilities and may look for specific pieces of information that are transferred to remote servers and sold on the underground market, Choi said.
Administrators should have the operating system and all installed software up-to-date. They should also make sure the “autorun” feature is disabled on Windows machines so that Vobfus can’t use that method when an infected USB drive is attached to the computer. Since Vobfus sometimes use drive-by-download links as an infection vector, users need to be cautious about what links they are clicking on, Choi said. It’s also important to make sure the antivirus tools are regularly updated to detect the latest malware versions right away.