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extensions

  • Description: This is a really weird text file TXT? Can you find the flag?
  • Difficulty: Medium

🔎 Solution​

In this challenge, we are given a .txt file. At first glance, it appears to be a plain text file.

Opening it in a hex editor, however, immediately reveals something interesting: the file begins with the byte sequence 89 50 4E 47 0D 0A 1A 0A - which is the magic header of a PNG image file.

> xxd flag.txt      
00000000: 8950 4e47 0d0a 1a0a 0000 000d 4948 4452  .PNG........IHDR
00000010: 0000 06a1 0000 0260 0802 0000 0085 ad5e  .......`.......^
00000020: 9a00 0000 0173 5247 4200 aece 1ce9 0000  .....sRGB.......

With this in mind, the solution is straightforward. By simply changing the file extension from .txt to .png, we can open it as an image. Viewing the image reveals the hidden content, which contains the flag.

🚩Flag​

picoCTF{now_you_know_about_extensions}