If your printers start printing garbage characters until they run out of paper, it’s a sure sign your network has been hit by the Milicenso Trojan. Help Net Security reports that Symantec (SYMC) researchers have found that the garbled printouts are just a side effect of the infection and not its goal. The malware’s last variants have an extremely low detection rate – only 4 of the 42 solutions used by Virus Total detect them at the moment.
The article says the Milicenso Trojan is actually a backdoor used to deliver other malware on the affected machines. The infection vectors are links and malicious attachments in unsolicited emails, as well as websites hosting malicious scripts that trigger the download of the Trojan. “The Trojan creates and executes a dropper executable, which in turn creates a DLL file in the %System% folder”, shared the Symantec researchers.
The heavily encrypted DLL file creates a number of EXE and DLL files, and uses a number of routines to discover whether the execution environment is a virtual machine, public malware sandbox or a black-boxing site. The Trojan also drops a piece of adware, whose aim is to serve as a decoy for AV solutions present on the machines. The blog says the Adware.Eorezo has only one goal: to point Internet Explorer to an ad-relater URL.
Help Net Security explains the trojan triggers the massive printing by exploiting the Windows default print spooler directory. “During the infection phase, a .spl file is created in [DRIVE_LETTER]\system32\Spool\ \[RANDOM].spl. Note the Windows’ default print spooler directory is %System%\spool\printers.”
The researchers explained “The .spl file, while appearing to be a common printer spool file, is actually an executable file and is detected as Adware.Eorezo. Depending on the configuration, any files, including binary files, created in that folder will trigger print jobs.”