Malware authors like to play hide-and-seek. Hiding executable files inside PDFs and Microsoft Office documents then emailing them as attachments are nothing new, but sometimes one layer isn’t enough. This Avast Virus Lab analysis peels back the layers of a new threat.
Malware authors continually surprise us with their creativity. In an effort to trick banking customers into revealing the login credentials for their online account, cycbercrooks are using the trust people have in Microsoft Office to make them execute banking malware on their own computers. Here’s how it works:
Typically, spam emails contain executable files that can harm a victim’s computer and steal private information. In the layered version, they have PDFs or Microsoft Office documents attached that contain a malicious executable file. We recently found an email that had an added layer and decided to analyze the email.
The email, disguised as a financially-related message from a legitimate company, informed the recipient that an invoice was due and had a PDF file attached. Embedded inside the malicious PDF was a Microsoft Office document and simple java script that dropped and executed the DOC file.
Inside the DOC file we found malicious macro code, which users must activate, as the code is disabled by Microsoft Office by default. The code obfuscates DOC files by creating new documents with unique methods names, variable names, and URLs, making it difficult to detect the malicious files.
When we analyzed the malicious macro code, we found some hints that helped us with our analysis. In this sample it was a function called MICHEL.
We already knew this function would open the URL with the malicious file, and when we found this function in one of the modules, we were able to find the download path.
The address is stored as a GUADALUPE variable. The URL is unique for each sample and leads to the download of a malicious PE file.
The PE file would act as an information stealer, stealing login credentials from banking sites like
- Santander, whose principal market is in the Northeastern United States
- Ulster bank, based in Ireland
- From Google accounts
How to protect yourself from banking malware
Our number 1 recommendation is keep your security software updated. Avast streams hundreds of updates every day to your devices, so you will stay protected. For example, the executable file downloaded by the malicious Microsoft Office document belongs to a banker family evolved from infamous Zeus. This variant is also known as a Dridex Botnet. At the time of writing this post, the botnet is still active, but the malware itself is inactive. Avast detects it as Win32: Pierre-A.
Clever cybercrooks use social engineering to manipulate their victims. Use extreme caution when opening emails related to your finances until you can verify the legitimacy.
Samples related to this analysis:
Half of all avast! users are running an older versions of Adobe Reader on their computers that are vulnerable to a variety of malware attacks.
The avast! Virus Lab found that 49.41% of avast users were using the older Adobe Reader versions as of end-April. The number was also surprisingly stable, dropping by around five percentage points from the early March level of 55.71%.
“The numbers were a surprise to us,” said Jiri Sejtko, head virus analyst. Read more…
A new method of producing malicious PDF files has been discovered by the avast! Virus Lab team. The new method is more than a specific, patchable vulnerability; it is a trick that enables the makers of malicious PDF files to slide them past almost all AV scanners.
Overall, PDF specifications allow many different filters (such as ASCII85Decode, RunLengthDecode, ASCIIHexDecode, FlateDecode, …) to be used on raw data. In addition, there is no limit on the number of the filters used for a single data entry. Anyone can create valid PDF files where the data uses, for example, five different filters or five layers of the same filter. All of these features are based on extremely liberal specifications, a fact which allows bad guys to utilize malicious files in a way that does not allow antivirus scanners access to the real payload.
The new trick is based just on one filter, so it doesn’t sound exciting, does it? So what’s the reason for posting this blog post?
The filter used to encrypt text data is meant to be used only for black and white images. And apart from avast!, probably no other AV scanner is currently able to decode the payload because no other AV can detect those PDF files.