GREM Bypassing Packers and Obfuscation Questions and Answers

Question 1

An analyst is manually unpacking a malware sample and, after stepping through the packer's stub code, encounters a `JMP EAX` instruction.
The value in the EAX register points to a memory region that was recently allocated and written to by the stub.
What does this instruction most likely represent?

Accepted Answer

The transfer of execution to the Original Entry Point (OEP).

Answer

A call to an obfuscated API function.

Answer

An anti-debugging check to verify memory integrity.

Answer

An exception handling routine to mislead the analyst.

Question 2

A reverse engineer encounters a conditional branch in a program that always resolves to the same path at runtime, but is constructed in a way that is difficult for static analysis tools to determine. For example, a condition like `if ((x * x) + 1 <= 0)` is used to guard a malicious code block. What is this control flow obfuscation technique called?

Accepted Answer

Opaque predicate

Answer

Instruction substitution

Answer

Dead code insertion

Answer

API hammering

Question 3

While dynamically analyzing a packed executable, an analyst observes the program allocate a new memory section with `PAGE_EXECUTE_READWRITE` permissions, write data into it, and then prepare to execute it. To reliably identify the Original Entry Point (OEP) at the moment of transition, which of the following is the most effective action?

Accepted Answer

Setting a hardware breakpoint on execution for the newly allocated memory section.

Answer

Running a string utility on the memory dump of the new section.

Answer

Disassembling the initial entry point of the packed file.

Answer

Examining the PE header for an unusual number of sections.

Question 4

During static analysis, an analyst finds no meaningful ASCII strings. However, when debugging, they observe a small loop that runs immediately before a Windows API call. This loop uses a multi-byte XOR key to decode a block of data on the stack, which then resolves to a string like "kernel32.dll". This is a common implementation of what technique?

Accepted Answer

Stack string obfuscation

Answer

Process hollowing

Answer

Return-Oriented Programming (ROP)

Answer

API hashing

Question 5

Which of the following is a classic indicator of a PE file packed with a standard, non-modified version of UPX (Ultimate Packer for eXecutables)?

Accepted Answer

The PE header contains section names like `UPX0` and `UPX1`.

Answer

The file size is significantly larger than its unpacked version.

Answer

The Import Address Table (IAT) is stored in the `.rsrc` section.

Answer

The entry point code immediately calls `VirtualAlloc` and `VirtualProtect`.

Question 6

A malware author chooses to resolve Windows API function addresses at runtime by calculating hashes of function names and comparing them to pre-calculated values, rather than listing them in the Import Address Table (IAT). What is the primary anti-analysis benefit of this technique?

Accepted Answer

It prevents static analysis tools from quickly identifying malware capabilities by inspecting the IAT.

Answer

It allows the malware to execute significantly faster by caching function pointers.

Answer

It dramatically reduces the overall size of the malware executable on disk.

Answer

It enables the malware to run on non-Windows operating systems without modification.