Recently, Alliacom cybersecurity team has received a Java malware sample with the name of “Scan_20170620_094711_pdf.jar”. The malware is a JAR file that transferred as an attached file in e-mail spam campaigns.
Through the analysis of the malware, it reveals that it is a RAT (Remote Access Trojan) called “JRAT” which is a commercial RAT tool with malicious functions. The fact that the malware is written in Java gives it the ability to run on multiple platforms having JVM (Java Virtual Machine) environment installed.
The malware is composed of two main parts: droppers and the JRAT tool. The droppers are used to decrypt and install the JRAT tool on the target machine.
In the following sections, we focus on how the droppers inside the malware install the RAT tool on the victim machine and our analysis on the RAT tool ending with the conclusion.
To evade detections of anti-spam or anti-virus measures, the malware hides its encrypted payloads in files with .tiff extension which is normally used to store graphics images. When the user double-clicks the jar file and in case the JVM environment is installed on the user’s machine, the “main” method in “com.total.Main” class will be executed. The class and resources files in the “Scan_20170620_094711_pdf.jar” are shown in Figure 1.
Actually, the “com.total.Main” class is the first dropper used to decrypt and load other classes from the encrypted .tiff file. This dropper calls later three other droppers and finally install its payload (the real JRAT jar file) on the victim machine as shown in Figure 2.
After decompiling the java package “com.total”, we find that the main class is obfuscated and implements 71 subclasses within it. To protect itself from being analyzed, the main class uses an object array to hold all data generated in runtime and a long string composing with digital numbers used as indexing in the previous array as shown in Figure 3.
To get a more readable code, we replace the old index values in all occurrences of the object array “N1427_.N1426_[ ]” by a new calculated index values. We find that the “N2327_” class is used to decrypt a .tiff file and to get another class named “qeaqtor.Loader”. The decryption of the class bytecode is simply a XOR operation on each byte of .tiff file (see Figure 4 below).
The decrypted “qeaqtor.Loader” class bytecode is then loaded as a java class by calling the “defineClass” method in “java.lang.ClassLoader”. Figure 5 shows that the “go” method in the “qeaqtor.Loader” class is finally invoked.
In the “qeaqtor.Loader” class, three methods named “resource”, “criminal”, “bytes” are related to the decryption of .tiff files.
The decryption process is similar to the one used in stage 1, except for the XOR buffer, which is generated from the combination of class name hash, class name length and a hard-coding seed. After the XOR operation, the buffer is then decompressed as .zip file to get class bytecodes. The main decryption mechanism is implemented in “bytes” method as shown in Figure 6. In total, this dropper loads four new classes and calls the “go” method in the “qeaqtor.Header” class.
As shown in Figure 7, the third dropper reuses the decryption method implemented in “qeaqtor.Loader” to load the “operational.Jrat” class from .tiff files, then invokes the “main” method in it.
The final class named “operational.iiiiiiiiii” is decrypted (using the same decryption method as the one in stage 3) in this dropper. Although it is decrypted as a java class file, it is actually a jar file which contains the JRAT tool.
The dropped jar file is identified as a commercial RAT tool named “JRAT”. It is composed of a number of files and directories as shown in Figure 9. Most of these files are encrypted. As the JRAT jar has already been analyzed by many other security analysts , we know that the RAT jar has also a dropper to drop the real payload. Using the decryption tool available on the github , it is possible to manually obtain the dropped payload which consists of files shown in Figure 10.
As shown in our analysis, the malware is probably developed based on the commercial RAT tool called JRAT written in Java. The Cybercriminals added multiple protection layers to avoid detection by anti-spam products. However, the dropped JRAT is easily detected by anti-virus products based on its signature since the malware writes them directly on the disk.
Rédigé par : Xu Jianqiao