Spammers are using cloud-based storage services to store malware, allowing them to circumvent e-mail spam filters, according to security experts at Kaspersky Lab and MX Lab.
Kaspersky Lab detected the click-fraud Trojan, a variant of the Trojan-Dropper.Wind32.Drooptroop family, which has been in circulation since the beginning of December, said Vicente Diaz, a Kaspersky Lab expert. There are over 7,000 variants of this particular family, according to Kaspersky. As with other types of malware that took advantage of the holiday season, the executable file for this Trojan was named gift.exe, Diaz said.
The security firm detected more than 1,000 infections using this technique to distribute this variant, according to Diaz.
The Trojan is stored on Rapidshare, a cloud-based file-sharing and storage service. The spam messages that users receive in their in-box have no text, just a single link pointing to a valid Rapidshare URL. These messages get past spam filters because there are no malicious files attached, the domain name is not considered a "bad" one, and executables hosted on Rapidshare aren't automatically classified as a threat, said Diaz.
There was also a recent fake antivirus spam campaign that included a Rapidshare link pointing to surprise.exe, according to security firm MX Lab. The executable file downloads and installs the fake AV Security Shield on the user's computer, which runs after the computer is rebooted.
Once downloaded, there's no guarantee that authentic antivirus products will detect these Trojans. According to MX Lab, only 16 of the 43 major antivirus products detected surprise.exe as a Trojan or as fake AV.
Malware is getting harder to detect, as authors "run through 40 or more antivirus software [programs] to make sure the viruses they are developing don't get detected before releasing it," Neil Daswani, CTO of Dasient, told eWEEK.
The Drooptroop Trojan hooks into the spoolsv.exe process and intercepts network traffic to and from the browser, redirects user requests, and allows a remote user to gain access to the system, said Diaz. The Trojan also has a fake AV component, as it displays a fake AV scan inside a browser window to scare the user into buying the counterfeit security product.
Scammers seem particularly attracted to Rapidshare for hosting malware. Sophos warned about spam messages with Rapidshare links pointing to a fake AV back in 2009. Jerome Segura of Pareto Logic also reported several pieces of malware being stored on the site. Those files were downloaded by other Trojans, not by a user, Segura said.
"This is a major risk and flaw in file hosting services" and "rather convenient" for the bad guys, Segura said.
Because of the holiday, Rapidshare was not available for comment.
Enterprises work with cloud storage providers to reduce IT operating costs and simplify maintenance. Spammers are apparently thinking along similar lines, such as not having to deal with bandwidth costs, said Segura. Using a free service makes the fake AV scam even more profitable, said Diaz.
Malware has taken advantage of other online cloud services in the past, said Diaz. Examples include using Twitter as a communication channel for botnets, Amazon EC2 to host remote control servers and advertising networks to distribute malicious ads.