TFC CTF 2023 - DOWN BAD

Description

• Category: Forensic

The flag is right there!

Download:

• down_bad.png (right-click and Save link as)

Resolution

When we open the image, there is an error.

Let’s find the error and repair it:

$ pngcheck down_bad.png
down_bad.png  CRC error in chunk IHDR (computed 1d9c52c0, expected a9d5455b)
ERROR: down_bad.png

We change the CRC using ImHex:

And we can now open it:

It seems that the challenge and the content of the picture don’t want us to look down, so let’s do that.

I’m using TweakPNG which is a great tool for manipulating PNGs easily (only available for Windows but you can run it with WINE):

Let’s increase the height to see what’s below:

We save our modification, and now we can see the flag: TFCCTF{28ae25c96850245ffdd70a880158f9f3}