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Trusteer Uncovers New Banking Malware Using Advanced Evasion Techniques

Tilon Malware

Security researchers at Trusteer have uncovered another a new variant of new financial malware with advanced evasive capabilities.

A stealth variant of the Silon financial malware, the new variant is named “Tilon” and injects itself into the browser to launch "man in the browser" attacks, according to Amit Klein, CTO of Trusteer. It also installs as a service on targeted Windows machines with a genuine-looking name and a random executable name so that the process wouldn't be easily be flagged as suspicious, Klein said. Once executed, it injects malicious code into various Windows processes, and then terminates itself so that there is no trace of the malicious process in memory.

Tilon also starts a "watchdog thread" inside one of the Windows native processes to monitor the entries for the installed "genuine-looking" service in the registry and the malicious executable files stored on the disk. If any of these are tampered with, Tilon restores them within three seconds, making it hard for security products to remove the infection, Klein noted in a blog post.

"The net result is very low AV detection of the Tilon dropper," Klein wrote.

Tilon also has a "peculiar" way of hooking browser functions to grab form data and inject code into Web pages, Klein said. It uses the privileged system instruction Clear Interrupt Flags to throw an exception and then hook into the browser processes.

"This unorthodox hooking technique is likely used to evade security products that look for 'traditional' hooking techniques on browser functions," Klein explained.

Tilon will also not install on a virtual machine. While there are other malware types that refuse to install on virtual machines, Tilon goes one step further and installs a "fake system tool" scareware. Instead of terminating the installation or running in the background, Tilon pretends to be scamware so no one would suspect what it is actually capable of, and discover its true nature. A look at VirusTotal shows many of the antivirus tools detected the dropper as a fake system tool instead of financial malware, according to Klein.

Tilon's financial fraud capabilities are on par with other MitB malware. It is capable of fully controlling the traffic to and from the browser to the Web browser. It can also take Web pages as they are served up by the Web browser and use a "search and replace" mechanism to replace certain URLs and sections of the page with its own text. It also collects login credentials and information about various transactions by capturing form submissions, logging the data, and transmitting the information to a remote command-and-control server.

"Tilon merely provides more or less what Silon did back in 2009, and what Zeus, SpyEye, Shylock and others are capable of today," Klein wrote.

Silon, discovered in 2009, was compromising customers who had two-factor authentication enabled on their online bank accounts. The malware had been quiet for the past year, leading Tristeer researchers to wonder if Silon’s perpetrators were "taking a long vacation in prison."

Trusteer first detected Tilon last month.

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Fahmida Y. Rashid is a Senior Contributing Writer for SecurityWeek. She has experience writing and reviewing security, core Internet infrastructure, open source, networking, and storage. Before setting out her journalism shingle, she spent nine years as a help-desk technician, software and Web application developer, network administrator, and technology consultant.