capture the flag, hacking, web application security

InfoSec Institute CTF #13

Another day, another challenge…

Today’s challenge is from the InfoSec Institute.

Going to the following link, we see the following:


Doing a right click, view page source we see the following:


So from the hint we’re looking for a back-up file.

Since this is on a Linux box, let’s see what what the naming conventions are for backup files.

Let’s see if there’s a backup folder.


That led us to a dead-end.

Let’s try adding .old at the end of the file.

Adding the “.old” at the end of the URL and pressing enter we get the following:


Hmm… this looks like another file. Let’s open it.

Opening the file in a text editor we get the following:


Looking at the file we see the the first paragraph, which matches our first screenshot.

Next we see commented out code, that is asking us to download a mysterious file, “iamadecoy”.

Let’s navigate to this file and see what we find.

After the file downloads, we try to open it.

Hmm… that’s weird when clicking on the file a prompt is shown asking what type of file this is.

Since we don’t know what type of file it is, let’s go to this site here, to find out.

After uploading our file we determine that is a pcap file.


We’re going to need Wireshark for this one…
Opening Wireshark, and opening our file we’re presented with the following:


The beginning of the file is DNS queries that are rejected we can ignore that.

Searching through the file we notice some HTTP requests that are getting files, in particular – HoneyPy.png

Going to packet 633


We can reconstruct this exchange.

Going to File –> Export Objects –> HTTP

We get the following:


Our file is highlighted in the above screenshot, so let’s click Save.

Opening the file we get our flag!


Lessons learned:

Use the hints that are provided! We knew that the file we were looking for was a backup.  After playing around with the filenames we discovered that the file we were looking for ended in an “.old”. Once we opened the file we noticed there was another file “imadecoy”. After downloading that file and trying to open it our operating system was confused on the file type. Uploading our file to the above link we determined that the file had a pcap (packet capture) extension, and we would need to use Wireshark.

Opening Wireshark, we determined that the file we needed was inside of an HTTP packet. Reconstructing the packet we were able to download the file we needed. After opening that file we received our flag. This challenge was a multi-step process. It’s very important to pay attention to detail.