The Challenge

You’re given flag3000.zip. Inside is flag2999.zip. Inside that is flag2998.zip. This nesting continues all the way down to flag0.zip, which finally contains flag.txt with the flag. Manually unzipping 3000 times is not an option.

Approach

The pattern is entirely regular: flagN.zip contains flag(N-1).zip. Starting from 3000 and working down to 1, extract each archive to the same output directory. At each step, the next archive to process is flag{i-1}.zip in the output directory. After the innermost extraction, flag.txt contains the flag.

Python’s zipfile module handles this cleanly in a loop. No external tools needed, no error handling complexity — just iterate from 2999 down to 1 (since flag3000.zip is the starter and flag1.zip is the last one that contains flag0.zip or flag.txt), extract each zip to the working directory, and read the final file.

Solution

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import os
import zipfile

# Path to the directory containing the zip files
zip_folder = '/home/user/Downloads/flag3000.zip'

# Path to the destination folder where the extracted files will be saved
destination_folder = '/home/user/Downloads/zipdir/'

file_path = '/home/user/Downloads/flag3000.zip'
for i in range (2999, 0, -1):
    file_path = '/home/user/Downloads/zipdir/flag' + str(i) + '.zip'
    with zipfile.ZipFile(file_path, 'r') as zip_ref:
        zip_ref.extractall(destination_folder)
        
for i in range (0, 3000):
    os.remove("/home/user/Downloads/zipdir/flag" + str(i) + ".zip")

The script starts by pointing file_path at the outermost archive and then loops down from 2999 to 1, extracting each zip into destination_folder. At each step flag{i}.zip extracts flag{i-1}.zip into the same directory, so the path variable just counts down. The second loop removes leftover zips after extraction to keep the folder clean.

The paths are hardcoded to /home/user/Downloads/ — this was written quickly during the competition to just run once and get the flag out.

What I Learned

Nested archives are a standard CTF misc trope. The key is recognizing the pattern immediately and writing the loop before you even try to manually unzip anything. The zipfile module’s extractall to a single directory approach is cleaner than creating per-level subdirectories.