It is getting harder for cybercriminals to deliver malware via email, so attack methods have had to become more sophisticated; the latest attempt uses a malicious PNG file to deliver a banking Trojan.

Simply sending malware as an attachment in a spam email is certain to result in some unsuspecting users’ computers being infected, but cybercriminals are now having to use more advanced techniques to evade detection and get past spam filters and antivirus software. The latest attack method is an example of how attackers are using much more sophisticated methods to evading detection.

Malicious PNG File Used to Infect Windows, OS X, and Linux Machines

A new campaign has been discovered by SecureList which is being used, at present, to attack computers in Brazil. However, while the majority of victims are located in Brazil, the malware is also being used to attack users in Spain, Portugal, the United States and beyond.

To evade detection, the attackers have encrypted a malicious payload in a malicious PNG file – a common image format many people do not usually associate with malware.

The image file is not attached to an email and sent in a spam message, instead the initial attack takes place using a PDF file containing a malicious link. The PDF file is sent out in spam emails which use social engineering techniques to fool users into opening the attachment. The PDF file does not contain any malicious code, instead it uses a link to infect users. Clicking the link in the PDF file initiates the infection process.

The link is used to get users to download a malicious Java JAR file, which in turn downloads an infected ZIP file. The zip file contains a number of other files, including a malicious PNG file, or file with a PNG header. Researchers analyzed the binary file and determined that the PNG file size was much larger than it should be for the size of the image.

Further analysis showed how the malicious PNG file was loaded to the memory – using a technique called RunPE which is used by hackers to hide malicious code behind a legitimate process. In this case that process is iexplore.exe.

The malicious PNG file cannot infect a user on its own, as a launcher is required to decrypt the contents of the file. The attackers send the PDF file to start the infection process. Since the zip file contains the PDF extension, users downloading the file are likely to double click to open, thus infecting their systems. Since the malicious code in the PDF file is encrypted, it is not picked up by antivirus software. However, SecureList points out that the malicious files used in this attack are picked up by Kaspersky Lab products.