Last year, we published a paper titled Hot Knives Through Butter, Evading File-Based Sandboxes. In this paper, we explained many sandbox evasion methods-and today’s blog post adds to our growing catalog.

In the past, for example, we detailed the inner workings of a Trojan we dubbed UpClicker. The malware was notable for a then-novel technique to evade automated dynamic analysis systems (better known as sandboxes). UpClicker activates only when the left mouse button is clicked and released — a sign that it is running on a real, live, human-controlled PC rather than within an automated sandbox.

If the malware determines it is running in a sandbox, it lies dormant so that the sandbox doesn’t observe any suspicious behavior. Once the sandbox incorrectly clears it as a benign file, UpClicker goes on to a real computer to do its dirty work.

Last year, our colleague Rong Hwa shared the technical details of another sandbox-detecting Trojan called BaneChant. Like UpClicker, BaneChant uses human interaction to ascertain whether it is running in a virtual-machine environment, activating only after it detects more than three left clicks.

Since then, we’ve seen a spate of new malware that pushes the concept further. The newest sandbox-evading malware counters recent efforts to mimic human behavior in sandbox environments.

This blog post describes three tactics that FireEye has discovered in recent attacks.

Sandbox evasion: a primer

Most sandbox-evasion techniques fall into one of three categories:

Human interaction. This category includes malware that requires certain actions on the user’s part (a sign that the malicious code is running on a real-live PC rather than within an automated sandbox).

Configuration specific. Malware in this category takes advantage of the inherent constraints of file-based sandboxes. For example, knowing that a sandbox can spend, say, five minutes running suspicious code for analysis, malware creators can create code that automatically lies dormant for a longer period. If the code is still running after that, it’s probably not in a sandbox.

Environment specific. In this category, malware checks for telltale signs that its code is running in widely used VM environments. It checks for obscure files unique to VMware, for instance, or VM-specific hardware drivers.

The first category of sandbox evasion is one of the trickiest to counter. To fool sandbox-detecting malware, some vendors now simulate mouse movement and clicks in their virtual-machine environments to mimic human activity. But malware authors are upping the ante with even more sophisticated sandbox detection and evasion techniques.

To scroll is human

One malware we discovered lies dormant until the user scrolls to the second page of a Rich Text Format (RTF) document. So simulating human interaction with random or preprogrammed mouse movements isn’t enough to activate its malicious behavior.

Here’s how it works:

RTF documents consist of normal text, control words, and groups. Microsoft’s RTF specification includes a shape-drawing function, which includes a series of properties using the following syntax:

{\sp {\sn propertyName } {\sv propertyValueInformation}}

In this code, \sp is the control word for the drawing property, \sn is the property name, and \sv contains information about the property value. The code snippet in Figure 1 exploits a vulnerability that occurs when using an invalid \sv value for the pFragments shape property.

Figure 1: Code exploiting vulnerability in the RTF pFragments property

A closer look at the exploit code reveals a series of paragraph marks (./par) that appears before the exploit code.

Figure 2: A series of \.par (paragraph marks) that appears before the exploit code

The repeated paragraph marks push the exploit code to the second page of the RTF document. So the malicious code does not execute unless the document scrolls down to bring the exploit code up into the active window—more likely a deliberate act by a human user than simulated movement in a virtual machine.

When the RTF is scrolled down to the second page, then only the exploit code triggers, and as shown in Figure 3.0, it makes a call to URLDownloadToFileA function from the shell code to download an executable file.

Figure 3: Exploit code

In a typical file-based sandbox, where any mouse activity is random or preprogrammed, the RTF document’s second page will never appear. So the malicious code never executes, and nothing seems amiss in the sandbox analysis.

Two clicks and you’re out

Another sandbox-evading attack we spotted in recent attacks waits for more than two mouse clicks before executing. The two-click condition aims to more accurately determine whether an actual person is operating the targeted machine—most people click mouse buttons many times throughout the day—or a one-time programmed click designed to counter evasion techniques.

Here’s how this technique works:

The malware invokes the function Get AsyncKeyState in a loop. The function checks whether any mouse buttons are clicked by looking for the parameter 0×01, 0×02, or 0×04. (The parameter 0×01 is the virtual key code for the mouse’s left button, 0×02 is the code for the right button, and 0×04 is the code of the middle button.)

The instruction “xor edi edi” sets the edi to 0. If any of the buttons is pressed, the code invokes the instruction “inc edi,” as shown in Figure 4. After that, the instruction “cmp edi,2” checks whether the left, right or middle mouse buttons have been clicked more than two times. If so, code exits from the loop and gets to its real work. Otherwise, it stays under the radar, continuously checking for more mouse clicks.

Figure 4: Assembly code for evasion employing left, middle or right mouse clicks

Slow mouse, fast sandbox

Another recently discovered evasion technique involves checking for suspiciously fast mouse movement. To make sure an actual person is controlling the mouse or trackpad, malware code checks how quickly the cursor is moving. Superhuman speed is a telltale sign that the code is running in a sandbox.

This technique also makes use of the Windows function GetCursorPos, which retrieves the system’s cursor position. In the example malware code shown in Figure 5, GetCursorPos returns 614 for the x-axis value and 185 for the y-axis value.

Figure 5: Malware making first call to API GetCursorPos

After few instructions, malicious code again calls GetCursorPos to check whether the cursor position has changed. This time the function returns x= 1019 and y = 259, as shown in Figure 6.

Figure 6: Malware making second call to API GetCursorPos

A few instructions after the second GetCursorPos call, the malware code invokes the instruction “SUB EDI, DWORD PTR DS:[410F15]”. As shown in the figure 5.0, the value in EDI is 0×103 (259 in decimal) and DS:[410F15] = 0xB9 (185 in decimal). The value 259 and 185 are the Y coordinates retrieved from the two GetCursorPos calls. If the difference between the two Y-coordinate measurements is not 0, then the malware terminates.

Figure 7: Subtracting the Y coordinates to detect whether the cursor is moving too quickly to be human-controlled

In other words, if the cursor has moved between the two GetCursorPos calls (which are only a few instructions apart), then the malware concludes that the mouse movement is simulated. That’s too fast to be a real-world mouse or track pad in normal use, so the code must be running in a sandbox.

A growing challenge

Cybersecurity is a constant arms race. Simulating mouse movement and clicks is not enough to fool the most advanced sandbox-evading malware. Now malware authors are incorporating real-world behaviors into their evasion strategies.

Simulating these behaviors—the way actual people scroll documents, click the mouse button, and move the cursor— is a huge challenge for cybersecurity. Anticipating future evasion techniques might be even tougher. Expect malware authors to employ more novel techniques that look for that human touch.

In the paper “Hot Knives Through Butter: Evading File Based Sandboxes,” we’ve outlined 15 prior evasion techniques that have been used by advanced malware in real attacks to bypass file based sandboxes. We plan to continue updating the evasion series as we come across new techniques used by the latest threats and advanced malwares to bypass file based sandboxes.