MC 2 (Dynamic Analysis)

Platform: CUSTOM | Category: Reversing | Type: Challenge | Difficulty: Easy | OS: N/A | **Author: XESXOR Techniques: random_seed_analysis, static_analysis

Summary

Walkthrough of the CUSTOM Reversing MC 2 (Dynamic Analysis). Language: C. Binary only. "Guess the random number, and app will give you the flag."

Description

Language: C. Binary only. "Guess the random number, and app will give you the flag."

ELF 64-bit, dynamically linked, not stripped.

Analysis

The program generates a "random" number using srand/rand with a predictable seed (e.g., fixed or based on time(NULL)). When the number is guessed correctly, the program outputs the flag.

Solution

Analysis of the disassembled code reveals the seed and generation logic. We calculate the number or patch the check. The flag is printed by the program upon correct input. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Key Takeaways / Lessons

• Mastering random_seed_analysis, static_analysis was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-03-09 • CUSTOM • MC 2 (Dynamic Analysis) • Difficulty: Easy