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Introduction
Having meticulously dissected the intricate delivery methods employed by BumbleBee in our previous blog post , we embarked on a journey through the multifaceted and complex delivery chains that enable its stealthy penetration. From the covert utilization of seemingly innocuous files to ingenious tactics that evade detection, we navigated through the diverse arsenal BumbleBee wields to gain access.
Now, our exploration of this malicious loader takes us deeper into its heart — its behavior within the targeted environment. As we journey through its maneuvers, we seek to unravel the layers of complexity that lie beneath its surface. With a focus on unveiling the true essence of BumbleBee’s actions post-infiltration, we uncover the evolution and adaptation that contribute to its resilience.
BumbleBee Blog Series – 2
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Unpacking Executables
Malicious applications need to stay undetected as long as possible so victims can be infected without being caught by malware researchers, antivirus scanners or dynamic behavior-based analysis engines. To protect against the first two, malware is often packed, encrypted or obfuscated. For one, signature-based antivirus engines might fail to trigger even on known malware families when the sample is manipulated to a certain degree. Obfuscating the sample also increases the workload of threat researchers analyzing the malware manually, thus this either slows them down or potentially prevents researchers from dissecting the malware altogether.
This is why one major advantage of dynamic behavior-based analysis is that the malware is executed in a simulated environment which allows us to dynamically observe it’s behavior at runtime. As packed and encrypted samples will at some point execute the original, unprotected executable, our system is able to automatically save a memory dump of the unpacked binary. This is such a powerful feature that threat researchers rarely have to manually unpack a malware to analyze it’s behavior. Instead, they can let VMRay Platform perform the unpacking.
To demonstrate this feature, we let VMRay Platform analyze a BumbleBee sample which is often protected by such measures. Once the dynamic analysis is done, we try to identify the memory dump containing the unpacked binary.
As we support multiple trigger reasons to dump a memory region (for example, when the memory content changes, when the first network connection is established, when the application is terminated, etc.) there could be multiple memory dumps to select from, many of which are not fully unpacked yet. The easiest method to find a good memory dump is to select one with a YARA match, which implies that there must be enough strings and code snippets to classify it as belonging to a certain malware family (see Figure 1 and Figure 2)
An even better approach is to find the memory dump for which the config extractor was triggered (see Figure 3). A successful config extraction is a stronger indicator for an unpacked binary.
This memory dump can now be analyzed by popular tools such as IDA Pro. It might be necessary to run it through pe_unmapper first. To benefit from the information extracted during the analysis, we also have an IDA plugin that enriches the disassembly and decompilation with runtime information, such as function parameters observed during execution (see here for more information ).
While this approach of unpacking malware has some limitations, e.g., packers employing virtual machine-based protection such as VMProtect may never “unpack” the original executable as some of the code was replaced by virtual operations by the protector, and thus this will prevent a memory dump containing the original instructions, VMRay Platform will still be able to dynamically observe the malware unhindered, as the original behavior will remain the same.
Malicious behavior
The main goal of a malware loader is to infect the system with additional malware, but most loaders also try to ensure that the malicious executables are not downloaded or executed while being observed either by threat researchers during manual analysis or by a behavior-based dynamic analysis system (see Figure 4).
For this purpose, BumbleBee includes more than 50 evasion techniques, ranging from tracking mouse movements to checking for artifacts of common virtualization tools such as VirtualBox. Our analysis shows that these techniques were mostly copied from Al-Khaser, an open source collection of known evasion techniques.
We provide a list of all functions imported from Al-Khaser we have identified so far in the Appendix.
Network
To download additional malware, BumbleBee makes contact with a remote C2 server. The oldest BumbleBee sample we could find uses HTTP as the network protocol while more recent versions have adopted WebSockets. Once the connection is established, the infected system waits for one of seven possible commands.
Command name: shiFunction: Shellcode injection
Command name: dijFunction: DLL injection
Command name: dexFunction: Download & Execute
Command name: sdlFunction: Uninstall
Command name: insFunction: Persistence
Command name: gdtFunction: Execute shell commands
Command name: plgFunction: Load plugins
We have been able to observe that the last two commands were added somewhere around April and August 2022. Interestingly, the “plg” instruction is just a reference to the “dij” command, a hint that this functionality might be updated in the future.
Evasion techniques
In it’s most recent version, BumbleBee employs a total of over 50 evasion techniques, a majority of which are imported from the Al-Khaser open-source project.
They can be summarized in the following categories:
Detecting virtualization software and related tools
Most evasion techniques we have discovered are related to virtualization tools often used either during manual analysis by threat researchers (VirtualBox, VMWare, Xen) or potentially used during dynamic, behavior-based analysis, such as QEMU and KVM. One of the techniques, for example, checks if certain files such as “System32\drivers\VBoxMouse.sys” exist on the system to detect the presence of VirtualBox based on its drivers (see Figure 13).
There are also functions related to detecting Wine, a compatibility layer on Linux to execute Windows applications.
Detecting reverse engineering tools
Particularly in older BumbleBee versions, we found checks for OllyDbg, a popular tool used by threat researchers to analyze malware and other Windows executables.
This check was removed in more recent versions, but newer samples still check for Process Explorer, another useful utility sometimes used by threat researchers to explore processes running on the system.
Detecting sandboxing artifacts
To detect some sandboxing solutions, BumbleBee checks if the currently logged-in username or it’s own filename is a name primarily used by sandbox technologies, for example “sample.exe” (see Figure 6).
This is one of the reasons VMRay Platform can randomize the username, the hostname and the filename (see Figure7). Additionally, one common evasion technique is checking if the filename is the hash of the file itself, which is often the case in dynamic analysis engines. This can also be prevented by randomizing the filename.
Detection via hardware configuration
In addition to searching for evidence that the sample is running inside of a virtualized or simulated environment, BumbleBee also checks the hardware of the system for specific configuration artifacts mostly associated with sandboxing.
One method checks if the RAM and the disk size are above a certain threshold, another checks if information about the CPU fan can be retrieved.
Detection via user behavior
Not all evasion techniques rely on detecting the environment itself. Some smart techniques revolve around detecting the presence of a user, which is usually not the case for automated sandbox solutions. In particular, BumbleBee checks if the mouse pointer moves (see Figure 8).
VMRay employs a variety of techniques to simulate user behavior to avoid these evasion techniques from detecting the monitored environment. For example, the mouse is moved regularly and we even simulate interactions with the user interface. Additionally, our customers can also manually interact with the virtual environment during the analysis as if they were sitting in front of it – right from the web browser.
Custom evasion techniques
We have discovered one evasion technique seemingly not covered by Al-Khaser: BumbleBee checks if it was started from the desktop (see Figure 9).
The reasoning here is that BumbleBee’s delivery chain usually results in the sample being located in, for example, the temp directory or on a CD-ROM drive mounted from an ISO file, and not the desktop.
VMRay Platform allows customers to set the sample directory while uploading submissions (see Figure 10).
Note that having multiple evasion techniques can actually backfire for the malware, as performing the evasion techniques themselves can be detected as malicious behavior. For example, our VMRay Platform is able to observe the checks for virtualized environments, which contributes to the final malicious verdict.
The evolution of BumbleBee
We have tracked BumbleBee since it first appeared in the wild and were able to create a rough timeline of when particular important changes were introduced.
In summary, the network communication was changed from HTTP to WebSockets, and two new C2 operations were introduced (see Figure 11) alongside nearly 20 additional evasion techniques. Notably, we have identified samples where all evasion techniques were removed for a period of time around November 2022, but they were reintroduced later on (see Figure 12).
Conclusion
In the world of cybersecurity, staying informed is our strongest shield. Our exploration of BumbleBee’s delivery methods and malicious behavior has unveiled a landscape of complexity, where seemingly innocuous file formats are manipulated for malicious intent. With over 50 evasion techniques in its arsenal, BumbleBee showcases the evolving art of deception employed by threat actors.
In our upcoming article, we’ll delve deeper into BumbleBee’s behavior, exploring its configurations and the clusters emerging from our analysis. Stay tuned for the insights that fortify our understanding of this intricate menace.
Appendix
Evasion Techniques imported from Al-Khaser
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