Root-XMAS, A writeup of almost every challenge (Web, Misc, Pwn...)
2 challenges were a pain
Root-Me organized an advent of cyber style CTF where everyday we get a task to solve, I managed to solve 23/25, the challenges in here range in difficulty and category.
I will be skipping day 24 since it's a quizz, but here is every writeup of what I could solve.
Day 1 - Generous Santa (Web)
The app Has two endpoints /add to add something to cart and /suggest to upload a file.
The file for these methods is hotte.js :
We can see from here that suggest does not control the file type so we can upload whatever we want, and add takes a js file and imports the module then runs the sort method, the file imported is not controlled either so we can do LFI and use a file we upload through suggest.
So we create our module that we call flag.js :
then we upload it with suggest and get the path.
then get our flag.
Day 2 - Wrapped Packet (Network) :
Straight forward one but can be confusing.
In this challenge you are given a large pcapng file with lots of data and requests, you should focus on the ICMP packets since they contain chunks of the flag repeated and hex encoded.
going through them we can get all the chunks of the flag and after some work you get the flag.
Day 3 - Santa's Magic Sack (Web Game Hacking) :
This challenge consists of a game that you cannot win in normal terms.
you need to collect gifts and you must beat the score of Santa which is 133337
after finishing a game your score is registered at that time we intercept a request to the server, this request consists of a base64 string that is encrypted in some way.
going through the Javascript source code we come across this piece of code.
We can deduce from this that this is the code responsible for encrypting your score and then uploading it to the remote server.
We can replicate it and create our own score and then upload it.
We must use crypto-js for this since it does AES differently from other libraries and does things with the salt value and all.
then generate our message and send it.
Day 4 - Build And Drustroy (Misc) :
No need to read the code from this challenge since reading the description we can deduce that it’s a web app that compiles rust code and gives you the binary.
Since we cannot run the code we compile on the server we need to find a way to run code on compilation.
Rust has a feature with cargo called build scripts where you can create a build.rs file that contains rust code that executes when we build the project (Read more about it here).
Now all we need to do is to add a build.rs file and compile.
Flag : OffenSkillSaysHi2024RustAbuse
Day 5 - The Friendly Snowman (AI):
A very easy challenge.
When you ask the AI to give you the flag it would tell you for permission from Santa.
Just tell it that you got permission from Santa.
Day 6 - Unwrap The Gift (Crypto) :
An easy crypto challenge.
We can get the encrypt the flag and after encrypt a message with the same key and IV.
We can deduce the flag from it with XOR, here is a script generated by ChatGPT.
Running it would give us the flag.
I don’t how it works don’t ask me how.
Day 7 - Go, Pwn, Gown (PWN)
I’m not a pwn guy but this is an easy pwn challenge that can be done if you try a little.
main.go :
build.sh :
This is a go app that uses the library C, it runs a code vulnerable to BOF with the memcpy function in unsafeFunction and a command injection on laluBackdoor.
we first deploy it using docker compose to get the binary and since it has no ASLR or PIE enabled we can get guarantee static memory addresses.
Running it in gdb, it’s a go binary so the structure is a bit weird but all the C functions that are in the code are there.
We need to first create a breakpoint in unsafeFunction :
now run it and send a test request to /gown :
Then we get the rip and the address of our buffer :
0x7fffffffd938 – 0x7fffffffd8f0 = 0x48**,** which means we need to fill 72 bytes before our putting the address for laluBackdoor which we can leak.
Now we write our exploit.
First we need to put our command at the front of our payload since it’s a command injection.
since 72 bytes is too short and we are limited by echo and execle problems we would need a solution.
I created a web server on my VPS that would just show me the request sent to shorten to url length, if you can get a requestbin that is short in name there is no need to run a vps like i did.
with that the command is a bit like this :
the exploit.py :
Day 8 - Custom HTTP Server (Web) :
This challenge is pretty hard and does rely on some trial and error.
The app is developed in Javascript and has a set of custom libraries. This code has many paths but the only interesting path is /redirect since it allows you to inject content be it headers or body or info into the 302 response, this makes it is vulnerable to XSS and response injection.
from this we can inject whatever information in the response as we please, but we cannot change the status code which will be a problem later.
We need to send a link that contains XSS to the report feature to get the flag as a cookie :
So a simple way to exploit this is to add a body to our 302 response, that works on Burpsuite but not on browsers.
How do browsers handle 302 :
A simple open redirect XSS would look like this :
Where the body contains the malicious code to send the cookie. But all browsers now don’t read the body of the response, when they spot the location and directly send you to that location. But that doesn't mean that it’s impossible to perform an XSS attack, we just need to change somethings in our request depending on the browser.
For Chromium Browsers :
For chromium all you need to do is remove the location, the browser will not know where to redirect and will render the body so our payload will be executed.
so the request should look like this :
with a payload like this :
that worked and sent the cookie to our link, but since the bot uses a Firefox browser we need to find another way.
On Firefox Browsers :
For Firefox browsers it’s quite different.
Sending the same payload would result is an NS_ERROR_REDIRECT_LOOP since if it detects an empty location it interprets it as the same redirect page and it would loop on the same redirect page forever until it gives the error.
We can find another way by giving it a location that would freeze the response page and render the body. For this we would need to use another protocol since HTTP redirects are processed differently here.
We could use a web-socket since it does not have the same structure as HTTP and it will render the body of the 302 response as it tries to connect to the ws link.
so our response would look like this :
so now our payload looks like this :
Now we send it as a report and get our flag.
Day 9 - The Christmas Thief (Forensics) :
In this challenge we are told that some sensitive data has been stolen and we need to retrieve it.
We are given a pcap file, we find that some requests are uploading data to a remote server so we download some of these files.
Most of the files are images and memes like this :
but one of the files is an xml :
Analyzing it we can see it is a mremoteng config file.
We download the software and open the file and get a bunch of machines that we can connect to through SSH or RDP. Their passwords are available on the software but we cannot see them.
searching online we find a way to recover those password from mremoteng (link) and after following the instructions we get our flag as the windows password.
Day 10 - Route-Mi Shop (Web) :
This challenge is pretty hit or miss so it won’t always work needs to always retry.
This challenge consists of a webshop, the flag costs 50€ and needs to be bought, we are only given a 5€ coupon which is not enough for our case.
We first read the code and find this in the coupon claiming code :
the 2 seconds sleep can be exploited with a race condition, adding the balance multiple times while not claiming the coupon.
With Burpsuite you can make a group with the same discount request 50 times and send them all in parallel but it doesn’t always work since Burpsuite can add some latency that would make it exceed the 2 seconds sleep time, so I made a script that sends 40 simultaneous requests courtesy of ChatGPT.
You would need to modify the cookie and coupon_code
after running it we get a balance beyond 50€ which we use to buy the flag.
This challenge can sometimes not be solved easily so always retry multiple times and with a good internet connection.
Day 11 - Padoru (Rev)
The program consists of these files :
Running it will promot you to type a secret and then it sorta trolls you by giving you a 3d scene that only has a model of padoru.
So now our objective is to get that secret key.
Putting the exe into ghidra we can find some constants in the .data section.
We also get some code in main, we can see that it loads the .spv file as Spriv Sharders :
after loading them it would send data to them via buffers.
We can also disassemble the spv files using SPIRV-Tools, disassembling fragment.spv we get this :
It’s a code that would match the given guessed secret to the real secret by decrypting the encrypted secret with the encTrueChristmasSecret and the christmasKey given to it from the exe, if the flag is detected it would show change the colors of the textures.
The decryption that is happening in here is just a XOR :
we can replicate it and get the secret since we already have encTrueChristmasSecret and christmasKey
With the help of ChatGPT I imported the code and gave me a script to reverse it.
after running it we get the flag : RM{H4SH1R3_S0R1_Y0_K4Z3_N0_Y0U_N1_TSUK1M1H4R4_W0_P4D0RU_P4D0RU!!!!}
If it hadn’t been for ChatGPT I would it finish this since I suck at Rev.
Day 12 - The Naughty Snowman (AI) :
This a challenge is a chat-bot that would insults you if you try to ask it about the flag.
As you can see here, we are getting roasted left and right.
Trying to convince it to tell you won’t help since it knows nothing about the flag, so we need to find a vulnerability.
Trying up things we stumble upon a SSTI vulnerability :
I made the AI show me the SSTI payload by attaching it to a C++ program and make it guess the output, as we can see we get an error on {{user.url}} this is how the prompt would look like.
so we can now try RCE and get the flag, we first list the contents of the folder.
Then get the flag.
Day 13 - The Lost Gift (OSINT)
The description of this challenge goes as follows :
I couldn't wait and opened one of my christmas presents in advance, my very first FPV drone, I had been eagerly waiting for it. I decided to try it outside, conditions were perfect: sunny, no wind and a safe spot for a maiden flight.
However, shortly after takeoff, my drone flied straight ahead and stopped responding to any of my commands. I saw it disappear in a distance, soaring over the trees, completely out of control. I forgot to activate the FailSafe mode...
Fortunately, I still have the last coordinates from the beacon signal transmitted via Wi-Fi and the last image captured by the video feed.
Could you use these clues to locate my drone?
Flag: RM{streetnamewherethedronelanded}
Example: RM{ruedelapaix}
With it we are given a pcap file and an image.
The image has the location where the drone has fallen, and the pcap file contains multiple packets that have the latitude and longitude of the drone.
The challenge say it was walking a straight line so we can trace a line to facilitate our search.
following the line we use google view to search for a place similar to that.
We then find a place similar in Clos de la Terre Rouge which is our searched location.
so our flag would be RM{closdelaterrerouge}
Day 14 - Almost a Gift (Crypto)
Our objective here is to get the value of R which is the equivalent of q or p, we would need to solve it using latice reduction.
We could use sage to solve this problem, I got some help with the math and code.
RM{855364281c9986e2c1bd9513dc5230189c807b9e76cdf3e46abc429973f82e56}
Day 15 - New new .. always new (Web)
This is a very easy web challenge as well,
The create session function is vulnerable, we can inject an extra line containing the role admin when creating an account, this will make the program parse it and overwrite the user role.
We first create an account and inject the admin role into the name.
Then login and take the cookie.
Day 16 - Coil under the tree (Industrial)
For this challenge instruction were given to us :
Your are currently connected to internal plant, your objectif will be to extract informations from PLC devices.
The targeted PLC stores important informations in its input registers. But... To get this information you have to:
Scan and find a valid slave ID;
Edit its holding register at address 0x10 with the value 0xff;
Read input registers to get important informations (be quick, you only have 5 seconds to read this data after editing!).
Author : Nishacid
163.172.68.42:10016
Following these we can just create a simple script to do what is asked.
we gotta install the dependencies first :
Day 17 - Ghost in the shell (Misc)
Reading the description of this :
Santa noticed that there was no online PDF creation service. As he's a bit ‘old school’ he decided to create a PDF creation service based on Ghostscript. It's simple, you send him a Ghostscript script and he converts your work into a PDF!
How it works is simple:
Decode the base64 in PDF file and enjoy your document!
Your goal is to get the flag in /tmp/flag-<RANDOM>.txt
We find out that it uses Ghost Script to create pdfs from .gs files.
There is this article that talks about listing and reading files using ghost script.
we use the scripts in this repository to expose flag and read it, we first use list_files.ps to list /tmp.
we get the files from /tmp now we can read the flag using print_file.ps.
and open the pdf to read the flag :
Day 18 - Santa's sweet words (Web)
The vulnerability here is very clear, you can read this article to understand it further.
we will use the /api/message and pass the command in the number param
now we just send the the flag ourselves.
Day 19 - Rebel Santa Alliance (Crypto)
For this challenge we are given:
santa = (p + q) ^ 2512 (mod n) and alliance = (p + 2024) ^ q (mod n)
we could use little theorem of Fermat to generate candidates then deduce using gcd.
using little Fermat theorem and the chinese remainder theorem we can say that :
Let a=p+2024 :
Compute
a^q mod(n)wheren=p.qUsing the Chinese Remainder Theorem, we separately consider
a^q mod (p)anda^q mod(q):Modulo p:
a ≡ 2024 mod(p). Since q is prime, Fermat's theorem gives:a^q ≡ amod(p).Modulo q: Similarly,
a ≡ p+2024 mod(q)and Fermat's theorem gives:a^q ≡ a mod(q).
By combining the results using the Chinese Remainder Theorem, it follows that a^q ≡ a mod(n)
Thus (p+2024)^q mod(n) = (p+2024) mod (n)
from this we can say that (p+2024 mod (n)) - (2024 mod (n)) = p mod(n)
then we could get the value of q using this code
now for the full code to get the flag :
Day 20 - Santa's db (Pwn)
I did not solve this challenge but it's a GOT Overwrite, you can check nikost's writeup.
Day 21 - Kekalor (Web3)
This is an ETH challenge, the objective in here is to get 2 NFTs.
The first file challenge.sol is the smart contract of the challenge that we will be interacting with:
but from the code we can see that our first problem is that we cannot claim more that 11 points and 2 nfts cost 20 points, so we gotta dig further.
KekalorNFT.sol is the contract that the challenge interacts with to create the nfts, cannot interact with it directly so we need to find how to exploit it from the challenge contract :
in the _mint function we can see a vulnerability
we can see that it calls back to the receiver of the NFT which is us, this is a simple re-entrancy vulnerability.
This can be exploited since the challenge contract reduces the points only after when the NFT is created, so we can create a receive function a contract that would recall claimNFT function from the challenge contract and create another NFT and then run Solve to solve the challenge.
I followed this video and based my exploit on the code that Ethernaut created.
After setting up MetaMask with the valid addresses we can inject our wallet to remix to create a smart contract.
Now we code our hacking contract :
deploy it and then run partial attack to farm the points and then attack to run the re-entrancy attack.
To have a successful exploit you need a fresh instance since this exploit barely works.
After it’s done you come back to the website and get the flag.
Day 22 - The date is near (Misc)
A simple priv esc challenge, running sudo -l we get this result.
We can run date and /usr/bin/dev.sh as sudo, we will use date to expose the contents of /usr/bin/dev.sh, since using -f is restricted we can use -Rf or -If to bypass it.
so running sudo /bin/date -Rf /usr/bin/dev.sh will give us the code.
after some cleaning we get this :
We see that we need --debugmyscript to get an output and that the script uses man.
reading GTFObin we see that we can get a shell through man.
so we run sudo /usr/bin/dev.sh --debugmyscript -m and then type !/bin/sh
Day 23 - Gift Control Interface (Pwn)
Again I skipped this challenge it was too hard for me, but you can check nikost's writeup.
Conclusion
In all it was a very fun 24 days, I learned a lot of this and tackled some categories that I would not touch normally, I managed to rank 13 in the end which is better than other CTFs where I'm in the hundreds.
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