The world's largest spam botnet has recently been found sending bogus email with links to the Stels Android Trojan, an indication that the malware business on mobile devices is leaving startup mode.
For years, cybercriminals have distributed Android malware through forums or rogue app stores, where the malicious code is hidden in a free game. Last year, security researchers at Microsoft reported seeing a new wave of pharmacy, penny stock and e-card spam emails being sent by an Android botnet.
In the latest development, the Stels creators are paying for use of Cutwail, which is used primarily to distribute spam and malware to Windows PCs, experts say. This is a significant step in the maturing of the Android malware business, because it shows criminals are developing services that others can rent.
"They've now found a good distributor and they've outsourced that part as a service," said Sean Sullivan, security researcher for F-Secure. "It's an evolution of Android into more like Windows [malware] distribution methodologies."
Cybercriminals that want to target Windows PCs have a broad selection of development tools, exploit kits, botnets, even pre-built malware to choose from. In time, many of the same services will be available for Android, as cybercriminals follow the shift in the consumer computer market from notebooks and desktops to smartphones and tablets.
"The cybercriminals are taking note of the increase in mobile devices and they're taking advantage of that," said Brett Stone-Gross, a senior security researcher for Dell SecureWorks' Counter Threat Unit. "The guys that run these botnets, they go where the money is at."
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Stels is a multi-purpose Android Trojan capable of harvesting contact lists, sending and intercepting text messages, making phone calls to premium numbers and installing additional malware, said Dell Secureworks. The creators are sending fake emails from the Internal Revenue Service, using the upcoming April 15 individual tax return filing deadline as a lure.
The link in the emails takes the victim to a compromised website, which displays a fake Adobe Flash Player update page. The page says the update is needed to view the content of the page. Clicking on the update gets a prompt asking victims if they want to install this application. Clicking install loads the malware, which can operate on nearly all versions of Android.
When targeting Windows PCs, the Cutwail operators have shown lots of versatility. The botnet has been used to distribute the Gameover ZeuS banking Trojan through bogus email. ZeuS is used in stealing banking credentials and credit-card numbers.
In addition, Cutwail has been used in phishing operations that target Facebook, AT&T, Verizon and UPS users. Recently, the botnet was used to target customers of security firm Trustwave with a fake PCI DSS compliance scan.