Hackers have been caught spreading the Winnti malware to hijack control of web users' systems using a new backdoor contained in the legitimate Aheadlib analysis tool.
Security firm Trend Micro reported discovering the backdoor while examining reports of a fresh Winnti outbreak on Thursday.
"We found this particular threat via feedback provided by the Smart Protection Network; we detect it as BKDR_TENGO.A. It passes itself off as a legitimate system DLL file, winmm.dll, like most of the Winnti samples," wrote Trend Micro's Eduardo Altares. "We believe that this was done using a legitimate tool called Aheadlib, which is a legitimate analysis tool."
The news is troubling as Aheadlib is a valid tool used by several businesses to construct C code from DLL files. The criminals reportedly used the tool, which is connected to various parts of the network it is analysing, to create a backdoor they can use to bypass the system's security protocols.
"Aheadlib accepts any DLL file and is able to construct C code to hook all the functions provided by the original library. This is very useful in analysing malware, but can also be abused to help create files that pass themselves off as legitimate system libraries," explained Altares.
Winnti is an espionage-focused malware commonly used by hackers believed to stem from China. Altares said that there is evidence the hackers have already used the backdoor to successfully to steal data from a number of targets.
"We suspect that this was used in a targeted attack. Despite this, however, the file is not encrypted and neither was it particularly hard to analyse. Its main behavior is to steal Microsoft Office, .PDF, and .TIFF files from USB drives inserted into the system," he noted.
"These stolen files are stored in the $NtUninstallKB080515$ under the Windows folder. It also creates a log file named Usblog_DXM.log. The files can be retrieved by the attacker at a later time. Aside from retrieving files, it has several backdoor commands which allow the attacker to take control of the system."
Altares said that there is no way to know if the latest Winnti attack stems from China, as the IP addresses linked to it provide conflicting information about its origin.
"Two of these IP addresses proved to be of particular interest, namely 18.104.22.168 and 22.214.171.124. They are located in the United States, but multiple Chinese-language domains point to them. All of these have been blocked as command-and-control servers," he said. "This attack highlights how information theft can be performed even with malware that is not particularly advanced or sophisticated. It also shows some of the challenges in attributing attacks of this nature."
The attack discovery follows a fresh wave of allegations against China. Most recently the US Department of Defense accused the country of mounting several sophisticated attacks on it networks, in its Military and Security Developments Involving the People's Republic of China 2013 report to Congress on Monday.