How To Build An Effective Malware Analysis Sandbox
Automated malware analysis systems (or sandboxes) are one of the latest weapons in the arsenal of security vendors. Such systems execute an unknown malware program in an instrumented environment and monitor their execution. While malware analysis sandbox systems have been used as part of the manual analysis process for a while, they are increasingly used as the core of automated detection processes. The advantage of the approach is clear: It is possible to identify previously unseen (zero day) malware, as the observed activity in the sandbox is used as the basis for detection.
For a high level overview of this topic, please read Next-Generation Sandbox Offers Comprehensive Detection of Advanced Malware.
Goals of a dynamic analysis system (sandbox)
A good malware analysis sandbox has to achieve three goals: Visibility, resistance to detection, and scalability.
First, a sandbox has to see as much as possible of the execution of a program. Otherwise, it might miss relevant activity and cannot make solid deductions about the presence or absence of malicious behaviors. Second, a sandbox has to perform monitoring in a fashion that makes it difficult to detect. Otherwise, it is easy for malware to identify the presence of the sandbox and, in response, alter its behavior to evade detection. The third goal captures the desire to run many samples through a sandbox, in a way that the execution of one sample does not interfere with the execution of subsequent malware programs. Also, scalability means that it must be possible to analyze many samples in an automated fashion.
What information should a sandbox collect?
In this post, we discuss different ways in which a sandbox can monitor the execution of malware that runs in user mode (either as a regular user or administrator). This leaves out malicious code that tampers with the kernel, such as rootkits. We leave those for a future post. Also, the vast majority of malware runs as regular user mode processes, and even rootkits typically leverage user mode components to install kernel drivers or to modify the operating system code.
When monitoring the behavior of a user mode process, almost all malware analysis sandboxes look at the system call interface or the Windows API. System calls are functions that the operating system exposes to user mode processes so that they can interact with their environment and get stuff done, such as reading from files, sending packets over the network, and reading a registry entry on Windows. Monitoring system calls (and Windows API function calls) makes sense, but it is only one piece of the puzzle. The problem is that a sandbox that monitors only such invocations is blind to everything that happens in between these calls. That is, a sandbox might see that a malware program reads from a file, but it cannot determine how the malware actually processes the data that it has just read. A lot of interesting information can be gathered from looking deeper into the execution of a program. Thus, some sandboxes go one step further than just hooking function calls (such as system calls or Windows API functions), and also monitor the instructions that a program executes between these invocations.
Emulation versus virtualization
Now that we know what information we want to collect, the next question is how we can build a sandbox that can collect this data in a way that makes it difficult for malware to detect. The two main options are virtualization and emulation.
An emulator is a software program that simulates the functionality of another program or a piece of hardware. Since an emulator implements functionality in software, it provides great flexibility. For example, consider an emulator that simulates the system hardware (such as the CPU and physical memory). When you run a guest program P on top of this emulated hardware, the system can collect very detailed information about the execution of P. The guest program might even be written for a different CPU architecture than the actual CPU that the emulator runs on. This mechanism allows, for example, to run an Android program, written for ARM, on top of an emulator that runs on an x86 host. The drawback of emulation is that the software layer incurs a performance penalty. The potential performance impact has to be carefully addressed to make the analysis system scalable.
With virtualization, the guest program P actually runs on the underlying hardware. The virtualization software (the hypervisor) only controls and mediates the accesses of different programs (or different virtual machines) to the underlying hardware. In this fashion, the different virtual machines are independent and isolated from each other. However, when a program in a virtual machine is executing, it is occupying the actual physical resources, and as a result, the hypervisor (and the malware analysis system) cannot run simultaneously. This makes detailed data collection challenging. Moreover, it is hard to entirely hide the hypervisor from the prying eyes of malware programs. The advantage is that programs in virtual machines can run at essentially native speed.
Leveraging emulation and virtualization for malware analysis
As mentioned previously, the task of an emulator is to provide a simulated (runtime) environment in which a malware program can execute. There are two main options for this environment. First, one can emulate the operating system (this is called OS emulation). Intuitively, this makes sense. A program runs in user mode and needs to make system calls to interact with its environment. So, why not simply emulate these systems calls? While the malware is running, one can get a close look at its activity (one can see every instruction). When the malware tries to make a system call, this information can be easily recorded. At this point, the emulator simply pretends that the system call was successfully executed and returns the proper result to the program.
This sounds simple enough in theory, but it is not quite as easy in practice. One problem is that the (native) system call interface in Windows is not documented, and Microsoft reserves the right to change it at will. Thus, an emulator would typically target the Windows API, a higher-level set of library functions on top of the native system calls. Unfortunately, there are tens of thousands of these Windows API functions. Moreover, the Windows OS is a huge piece of software, and emulating it faithfully requires an emulator that has a comparable complexity of Windows itself! Since faithful emulation is not practical, emulators typically focus on a popular subset of functionality that works “reasonably well” for most programs. Of course, malware authors know about this. They can simply invoke less frequently used functions and check whether the system behaves as expected (that is, like a real Windows OS). OS emulators invariably fail to behave as expected, and such sandboxes are quite easy for malware to detect and evade. Security vendors that leverage OS emulation are actually well aware of this limitations. They typically include OS emulation only as one part of their solution, complemented by other detection techniques.
As the second option for an emulator, one can simulate the hardware (in particular, CPU and physical memory). This is called (whole) system emulation. System emulation has several advantages. First, one can install and run an actual operating system on top of the emulator. Thus, the malware is executed inside a real OS, making the analysis environment much more difficult to detect for malware. The second advantage is that the interface offered by a processor is (much) simpler than the interface provided by Windows. Yes, there are hundreds of instructions, but they are very well documented, and they essentially never change. After all, Intel, AMD and ARM want an operating system (or application) developer to know exactly what to expect when she targets their platform. Finally, and most importantly, a system emulator has great visibility. A sandbox based on system emulation sees every instruction that a malware program executes on top of the emulated processor, and it can monitor every single access to emulated memory.
Virtualization platforms provides significantly fewer options for collecting detailed information. The easiest way is to record the system calls that programs perform. This can be done in two different ways. First, one could instrument the guest operating system. This has the obvious drawback that a malware program might be able to detect the modified OS environment. Alternatively, one can perform system call monitoring in the hypervisor. System calls are privileged operations. Thus, when a program in a guest VM performs such an operation, the hypervisor is notified. At this point, control passes back to the sandbox, which can then gather the desired data. The big challenge is that it is very hard to efficiently record the individual instructions that a guest process executes without being detected. After all, the sandbox relinquishes control to this process between the system calls. This is a fundamental limitation for any sandbox that uses virtualization technology.
How is our sandbox built?
We think that more visibility is better, especially facing malware that becomes increasingly aware of virtual machines and sandbox analysis. We have seen malware that tries to detect the presence of VMware for many years. Even if one builds a custom sandbox based on virtualization technology, the fundamental visibility limitations remain. Of course, when a malware program checks for specific files or processes that a well-known hypervisor like VMware introduces, these checks will fail, and the custom sandbox will be successful in seeing malicious activity. However, virtualization, by definition, means that malicious code is run directly on the underlying hardware. And while the malicious code is running, the sandbox is paused. It is only woken up at specific points, such as system calls. This is a problem, and a major reason why we decided to implement our sandbox as a system emulator.
Why does not everybody use system emulation, since it seems such a great idea? The reason is that one needs to overcome two technical challenges to make a system emulator work in practice. One challenge is called the semantic gap, the other one is performance. The semantic gap is related to the problem that a system emulator sees instructions executed on the CPU, as well as the physical memory that the guest OS uses. However, it is not immediately clear how to connect CPU instructions and bytes in memory to objects that make sense in the context of the guest OS. After all, we want to know about the files that a process creates, or the Windows registry entries that it reads. To bridge the semantic gap, one needs to gain a deep understanding into the inner workings of the guest operating system. By doing this, we can then map the detailed, low level view of our system to high level information about files, processes and network traffic that are shown in our report.
The second question is about performance. Isn’t emulation terribly slow? The answer is yes, if implemented in a naive way. If we emulated every instruction in software, the system would indeed not scale very well. However, we have done many clever things to speed up emulation, to a level where it is (almost) as fast as native execution. For example, one does not need to emulate all code. A lot of code can be trusted, such as Windows itself. Well, we can trust the kernel most of the time – of course, it can be compromised by rootkits. Only the malicious program (and code that this program interacts with) needs to be analyzed in detail. Also, one can perform dynamic translation. With dynamic translation, every instruction is examined in software once, and then translated into a much more efficient form that can be run directly.
A sandbox offers the promise of zero day detection capabilities. As a result, most security vendors offer some kind of sandbox as part of their solutions. However, not all sandboxes are alike, and the challenge is not to build a sandbox, but rather to build a good one. Most sandboxes leverage virtualization and rely on system calls for their detection. This is not enough, since these tools fundamentally miss a significant amount of potentially relevant behaviors. Instead, we believe that a sandbox must be an analysis platform that sees all instructions that a malware program executes, thus being able to see and react to attempts by malware authors to fingerprint and detect the runtime environment. As far as we know, Lastline is the only vendor that uses a sandbox based on system emulation, combining the visibility of an emulator with the resistance to detection (and evasion) that one gets from running the malware inside the real operating system.