Ph0wn 2018 - Track the hacker


Category: Misc

Challenge description

This challenge was the next step of Save the factory. We were told that the hacker found a way to store data, some kind of file, and we were asked to find it.

Challenge author: m0eukh

Challenge resolution

Like previously, we discovered that many nodes were standing out. They were named frg_<number>, like “fragment number…“.

Their values were hex encoded. We retrieved them and sorted the nodes based on the number in their name.

But a simple mistake cost us a lot of time: the leading zeros were missing from the output (i.e. a 2-bytes value 0x0123 would have been displayed 123 with the first 0 missing).

After adding the leading zeros, we concatenated all values (using the Hxd hexadecimal editor on Windows). The file command on the output file looked promising:

We expanded the file with gunzip and spotted the flag directly in the output:

The flag was mixed with garbage since it was actually contained in a Tar archive:

Author: cnotin Clément Notin | @cnotin

Post date: 2018-12-16