Callfuscated

Callfuscated: Reverse Engineering a Stack-Based VM Protected by MBA, Opaque Predicates and Call-Based Jumps

A detailed technical write-up examines the HackTheBox challenge Callfuscated, which combines virtualization, mixed Boolean-arithmetic expressions, opaque predicates and junk instructions. The author first converts call-pop sequences into direct jumps by proving that register r8 is never read, then removes the resulting NOPs to obtain clean code. The resulting binary reveals a classic stack-based virtual machine whose handlers invoke heavily obfuscated functions. Triton is used to lift these functions into AST form, after which the CoBRA solver simplifies the MBA expressions into the original operations: addition, subtraction, multiplication, XOR, OR and AND. With semantics recovered, the author captures the bytecode at runtime, writes a C translator that emits readable operations, compiles the output and obtains a clean decompiled routine that checks the flag.

187,064 Instructions for One Flag: Reverse Engineering HTB Callfuscated Insane Challenge

A detailed technical breakdown of the HackTheBox Callfuscated reversing challenge reveals an extremely heavy obfuscation scheme built around a custom virtual machine, mixed Boolean-arithmetic transformations, opaque predicates, and call-based junk code. The binary implements a password checker that executes 187,064 instructions even on a short input because every real operation is wrapped inside thousands of call/pop gadgets. The author bypassed static analysis by building a ptrace-based tracer, dumping the 586-cell VM program array, and writing a Python emulator that faithfully replays the recorded execution trace. After fixing several edge cases involving rand() return addresses and operand-size detection, the emulator reproduced the exact register state of the original binary. Dynamic analysis exposed that the VM performs simple big-endian word construction followed by XOR operations with eight constant pairs, allowing the flag to be recovered directly without further symbolic execution.