A recently observed variant of the Asruex backdoor acts as an infector by targeting old vulnerabilities in Microsoft Office and Adobe Reader and Acrobat 9.x, Trend Micro reports.
Asruex was initially discovered in 2015 and was previously associated with the spyware DarkHotel. In addition to backdoor capabilities, the malware also appears to be able to inject code into Word and PDF files by targeting two old vulnerabilities tracked as CVE-2012-0158 and CVE-2010-2883.
These unique infection capabilities potentially make attacks more difficult to detect, given that security researchers might not consider checking files for an Asruex infection, but only keep an eye for its backdoor abilities.
“The use of old, patched vulnerabilities could hint that the variant was devised knowing that it can affect targets who have been using older versions of Adobe Reader (versions 9.x up to before 9.4) and Acrobat (versions 8.x up to before 8.2.5) on Windows and Mac OS X,” Trend Micro points out.
To infect machines, Asruex uses a shortcut file with a PowerShell download script. To spread, the malware uses removable drives and network drives.
The newly observed variant was initially discovered in the form of a PDF file, which was not created by the actors behind the threat, but was infected by the Asruex variant.
The file would drop and execute the infector in the background if opened using older versions of Adobe Reader and Adobe Acrobat. The contents of the original PDF host file are still displayed, which means that the user is unlikely to observe anything suspicious.
For this behavior, a specially crafted template that takes advantage of the CVE-2010-2883 vulnerability while appending the host file is leveraged.
The dropped executable contains several anti-debugging and anti-emulation functions. It detects if avast! SandboxWINDOWSsystem32kernel32.dll exists and also checks whether it runs in a sandbox environment by verifying computer names, user name, exported functions by loaded modules, file names, running processes, module version of running process, and certain strings in disk names.
The file also injects into a legitimate Windows process memory a DLL responsible for the malware’s infection and backdoor capabilities.
“It infects files with file sizes between 42,224 bytes and 20,971,520 bytes, possibly as a parameter to narrow down host files into which their malware code could fit,” Trend Micro notes.
The CVE-2012-0158 vulnerability, on the other hand, provides attackers with the ability to execute arbitrary code remotely, via a Word document or web site. The infection process is similar to the one leveraging PDFs.
The malware was also found to infect executable files, in addition to Word documents and PDF files. The malware compresses and encrypts the original executable or host file and appends it as its .EBSS section. Thus, it can drop the infector and also execute the host file like normal.