remoose

Platform: CUSTOM | Category: Reversing | Type: Challenge | Difficulty: Easy | OS: N/A | **Author: XESXOR Techniques: elf_header_repair, null_byte_restoration, static_disassembly_character_extraction

Summary

Walkthrough of the CUSTOM Reversing remoose. Task: a broken binary that won't run — 'one little thing' was changed. Solution: discover all 0x00 bytes were replaced with 0x20 (space) and ELF magic corrupted; restore nulls and fix header, then ext

Description

I changed just one little thing and my racing moose won't run anymore!

A single binary file chall (16808 bytes) is provided. The file command identifies it as "data" — not a valid executable. The goal is to figure out what was changed, fix the binary, and recover the flag.

Analysis

Initial Recon

The binary is not recognized as an ELF:

$ file chall
chall: data

Hex inspection of the first 16 bytes reveals the corruption:

$ xxd chall | head -1
00000000: 7f45 4c4b 0201 0120 2020 2020 2020 2020  .ELK...

The ELF magic should be 7f 45 4c 46 (.ELF) but byte 3 is 0x4b (.ELK) instead of 0x46. More importantly, the padding bytes that should be 0x00 are all 0x20 (space).

Identifying the Transformation

Counting byte occurrences confirms the pattern:

• Zero 0x00 bytes in the entire file — impossible for a valid ELF which uses nulls extensively for padding, string terminators, and zero-valued header fields
• 14106 0x20 (space) bytes — far too many for a 16KB binary

The "one little thing" that was changed: all 0x00 null bytes were replaced with 0x20 (space). The ELF magic F (0x46) → K (0x4b) change is a +5 shift that's also consistent with the corruption pattern (though the primary transformation is the null-to-space replacement).

Solution

Step 1: Restore the Binary

Replace all 0x20 bytes back to 0x00 and fix the ELF magic byte:

 python read-only • copy allowed Copy
#!/usr/bin/env python3
data = open('chall', 'rb').read()
fixed = bytearray(data)

# Restore all 0x20 (space) -> 0x00 (null)
for i in range(len(fixed)):
    if fixed[i] == 0x20:
        fixed[i] = 0x00

# Fix ELF magic: byte 3 should be 0x46 ('F'), not 0x4b ('K')
fixed[3] = 0x46

open('chall_fixed', 'wb').write(bytes(fixed))

After fixing:

$ file chall_fixed
chall_fixed: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV),
             dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, not stripped

Note: This also zeroes out legitimate 0x20 bytes in string data (spaces become nulls), but the executable code and ELF structure are correctly restored since code bytes rarely contain 0x20.

Step 2: Static Analysis

The binary is not stripped and contains meaningful symbols: main, flag, flag1, flag2, flag3, flag4, plus imports for putchar and printf. The source file was chall.c.

Using radare2 for disassembly, the program logic is straightforward:

1. main calls flag(), flag1(), flag2(), flag3(), flag4() in sequence
2. Each function makes a series of putchar() calls with immediate character values as arguments
3. The characters spell out the flag when concatenated in order

Step 3: Extract the Flag

By reading the immediate values passed to putchar across all five functions:

Concatenated: tjctf{REDACTED}

This decodes as "small moose" in leetspeak (5→s, 1→l, 0→o, 3→e), fitting the "racing moose" theme perfectly.

Key Takeaways / Lessons

• Mastering elf_header_repair, null_byte_restoration, static_disassembly_character_extraction was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

---

Crafted by XESXOR Research • 2026-05-15 • CUSTOM • remoose • Difficulty: Easy