DEV Community

DaNeil C
DaNeil C

Posted on • Updated on

0x00SEC CTF - Exercise #6

This weekend I took a dive into 0x00Sec's new bi-monthly CTF.

Note::: NO, I won't be posting the found flag{}, but I will be posting the methods I used.

  • CTF Name: Exercise #6
  • Resource: 0x00SEC
  • Difficulty: Easyish
  • Number of Flags: 1


  • Hint:
    1. Note in the Source Code of

<!-- TODO: -->
<!-- * Test IP whitelisting security -->

  • Acquired By:
    • Step 1: Is to see the hint in the code and look at the site. There really isn't much to this site.Alt Text
    • Step 2: So I need to figure out how to get on their whitelist. "IP whitelisting is a security feature often used for limiting and controlling access only to trusted users." (2) So, if we look at the response we can see that it is a "Server: nginx/1.17.5" and "X-Powered-By: PHP/7.3.14" in the header. php 7.3.14 has a whitelist but as I was looking at this and trying different URL parameters I found information about the X-Forwarded-For header and how it "is a de-facto standard header for identifying the originating IP address of a client connecting to a web server" (3) and if I make a request with this in the header then I can change the IP address that is displayed at the rejected one. How to exploit this though?
    • Step 3: I started sending different requests in the X-Forwarded-For header and a lot of the simple things like quotes and slashes did nothing. I did find that a payload of X-Forwarded-For: "XOR(if(now()=sysdate(),sleep(6),0))OR” produced an interesting output that was missing information. Alt Text
    • Step 4: Now that I have a point of change I can see what else I can do. I did notice that there was no sleeping so this isn't going to work, but what will happen if I change this to something else? It seems like maybe it wants a few inputs that maybe I can leverage. To do this I take a while and over think it.

Originally, I was assuming that this was something more complicated as I was not getting anything back. I thought that I was working with some blind injection but after 2 hours I realized I just needed to try a few local IP addresses. I did Fuzz a few of the local 192.x.x.x ip range but forgot about the 127.x.x.x range. Eh. Once I tried a "" I got access to the flag. Alt Text


This CTF taught me about a common security misconfiguration, header spoofing, and that there are a lot of other ways that this header misconfiguration can be used to exploit a system.
I find it interesting that this header could be used to exploit a database if the database is logging it. I did not find out if it can be used if the host is not doing anything with it but that will be an interesting new research point for me.

Happy Hacking


Please Note that I am still learning. If something that I have stated is incorrect please let me know. I would love to learn more about what I may not understand fully.

Top comments (0)