task3

Platform: rev-kids20.forkbomb.ru | Category: Reversing | Type: Challenge | Difficulty: Easy | OS: Linux | Author: XESXOR | Date: 2026-03-09 | Status: Solved Techniques: constraint_solving, disassembly, static_analysis, string_reversal_check

Summary

ELF 32-bit binary that reads from two files (Hello1 and Hello2), performs validation checks, and prints the flag if both checks pass.

task3 — Reverse Kids 2.0

Description

Complete the checks, and app will print the flag.

ELF 32-bit binary that reads from two files (Hello1 and Hello2), performs validation checks, and prints the flag if both checks pass.

Analysis

Reconnaissance

 bash read-only • copy allowed Copy
$ file task3
task3: ELF 32-bit LSB executable, Intel 80386, dynamically linked, not stripped

$ strings task3
Hello1
Hello2
NOOOOO
NOT AGAIN!
Nope
Flag{%s}

The binary is not stripped, revealing function names: main, check, check2. ‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍

Reverse Engineering

Three key functions identified:

main(): Opens files Hello1 and Hello2, reads 8-character strings from each, calls check() and check2(), prints flag if both pass.

check(): Verifies that content of Hello2 is the exact reverse of Hello1. Prints "NOOOOO" or "NOT AGAIN!" and exits on failure.

check2(): Enforces 8 character constraints on the string from Hello1:

Solution

Combining all constraints:

• Position 0: 'z'
• Position 1: 'f'
• Position 2: 's'
• Position 3: 's'
• Position 4: 'a'
• Position 5: 'i'
• Position 6: 'z'
• Position 7: 's'

Hello1: zfssaizs Hello2 (reverse): sziassfz

 bash read-only • copy allowed Copy
# Create input files
echo -n "zfssaizs" > Hello1
echo -n "sziassfz" > Hello2

# Run binary
./task3
# Output: Flag{REDACTED}

‍​‌‌​​​​‌​‌‌​​‌‌​​‌‌​​‌‌​​‌‌​​‌​​​​‌‌​​​​​‌‌​​‌‌​​‌‌​​‌​‌​‌‌​​‌​‌‍