Code Transposition Anti-Disassembly

Code transposition is a technique used by malware authors to evade detection and analysis by rearranging the instructions of a piece of code without changing its behavior. This technique is used to make the code more difficult to read and understand for disassemblers and reverse engineers, as well as to hide the true intent of the code.

There are …

CPU Counting Sandbox Evasion

In many sandboxes, due to virtualization constraints, the number of CPUs may be limited to one. However, in the real world, all x86 CPUs have been sold with multiple cores or at least hyperthreading for over a decade.

As a result, it is sometimes possible to detect virtualization or sandboxing by examining the number of CPUs on the system. …

Call to Interrupt Procedure Anti-Debugging

This anti-debugging technique involves using the INT n instruction to generate a call to the interrupt or exception handler specified with the destination operand.

To implement this technique, the int 0x03 instruction is executed, followed by a ret (0xCD03, 0xC3) nested in a __try, __except block. If a debugger is present, the except block will not be executed, …

VPCEXT Sandbox Evasion

The VPCEXT instruction (visual property container extender) is another anti–virtual machine trick used by malware to detect virtual systems. This technique is not documented. If the execution of the instruction does not generate an exception (illegal instruction), then the program is running on a virtual machine.

RDTSC Anti-Debugging

The Read-Time-Stamp-Counter (RDTSC) instruction can be used by malware to determine how quicky the processor executes the program's instructions. It returns the count of the number of ticks since the last system reboot as a 64-bit value placed into EDX:EAX.

It will execute RDTSC twice and then calculate the difference between low order values and check it with …

GetTickCount Anti-Debugging

This is typical timing function which is used to measure time needed to execute some function/instruction set. If the difference is more than fixed threshold, the process exits.

GetTickCount reads from the KUSER_SHARED_DATA page. This page is mapped read-only into the user mode range of the virtual address and read-write in the kernel range. The system clock tick updates …

GetLocalTime, GetSystemTime, timeGetTime, NtQueryPerformanceCounter Sandbox Evasion Anti-Debugging

When a debugger is present, and used to single-step through the code, there is a significant delay between the executions of the individual instructions, when compared to native execution.

Interrupts Anti-Debugging

Adversaries may use exception-based anti-debugging techniques to detect whether their code is being executed in a debugger. These techniques rely on the fact that most debuggers will trap exceptions and not immediately pass them to the process being debugged for handling.

By triggering an exception and checking whether it is handled properly, the adversary's code can determine whether it …

Obscuring Control Flow Anti-Disassembly

Obscuring control flow is an anti-disassembling technique that involves using methods of flow control that are difficult or impossible for disassemblers and debuggers to follow. This can make it more difficult for analysts to understand the program's behavior and can also make it more difficult for other tools, such as debuggers, to accurately interpret the program.

One example of …

Impossible Disassembly Anti-Disassembly

Impossible disassembly is an anti-disassembling technique that involves inserting data bytes after a conditional jump instruction in order to prevent the real instruction that follows from being disassembled. This technique takes advantage of a basic assumption in disassembly, which states that one byte is only interpreted in the context of one instruction. By inserting a byte that is the opcode …

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