bookmark_borderCC7F Round 1., during Hacktivity 2021

In a joint project we step to the next level with two rounds of CCTF. Harder challenges and highers prices wait for the players.

You can read more about us in the media:

https://finance.yahoo.com/news/cctf-organizers-awalcon-cryptall-partner-131600682.html

https://markets.businessinsider.com/news/stocks/cctf-organizers-awalcon-and-cryptall-partner-with-ecox-to-host-the-largest-blockchain-hacking-event-so-far-1030838702

bookmark_borderCCTF 5 Writeups – Part 1.

RTFM (50 points)

As is the case with all CCTF editions, there’s a quasi-challenge that points people in the direction of the manual, and which is supposed to give an initial feel for how the search for a so-called flag goes. The accepted solution was one of the example flags shown in the manual.

Author: SI

unk (80 points)

A real, entry-level challenge. A file (named “unk”) was presented. It appears to be a damaged Microsoft Word document. A Word document is actually a Zip archive containing things like XML and thumbnail image files. Different Zip implementations work differently on damaged archives; e.g. GNU Zip was able extract the damaged Zip archive to a sufficient extent. The flag was in the thumbnail image. — Sometimes thumbnails are not in sync with the working text of the document, and may contain sensitive data: snapshots of rendered text that was deleted.

Author: SI

CryptoFriend (80 points)

Another entry-level challenge. A file was presented under the name “a_friend.zip”. This was also an invalid Zip archive.

tl;dr: the file turned out to be a concatenation of a Zip archive and a PNG image, where the latter contained the flag. But on to the goose chase leading to this discovery:

E.g. FreeBSD’s Zip implementation extracted a single “a friend.docx” file. This file was a valid Word document which read “i hid my location, come help me”. OK, let’s do this.

No, the thumbnail didn’t contain a flag. According to metadata in the document, the document was prepared by Kevin Chung on 2017-09-11. Let’s find this guy. Searching the internet for his contact information, cryptocurrency addresses, etc, but most importantly, flags, it came to light that he’s behind CTFd, the web application powering the CCTF contest’s portal — the plot thickened. He also runs a blog. One of his blog posts, dated closest to 2017-09-11, really contains a string of the form “FLAG{…}”. Tough luck: wrong answer. In fact, Kevin Chung appeared the same way in the “unk” challenge. So this was a witch hunt.

One other thing was eye-poking: potentially subliminal data in the Word document’s XML attributes, e.g. “paraId="3B2827CE"” — is there real content embedded in these IDs, or is this totally random crap?

The “a_friend.zip” file was actually ~19MiB in size, while the extracted “a friend.docx” was only ~24KiB, so it makes more sense that the flag was to be hidden outside the latter. Inspecting the file (using tools like: strings, hd, etc), one could stumble upon the following:
• There was EXIF data containing latitude–longitude coordinates. However, those pointed to Andromeda, so this lead had to be scrapped as well.
• There was an EXIF user comment with a code of some sort: “FM0 FC000000000:zzzzzz1 f144 078043881a29e1e816c14c0 bac 87152…”. Well, whatever.

Upon further inspection, it was then apparent that the EXIF structure was actually part of a PNG image that appeared without compression in the “a_friend.zip” file.

Author: SI

Vault (100 points)

Given some smart contract — the source code and a deployed instance, displayed by the etherscan.io service —, the task was to determine the correct call to the contract that would exfiltrate the 0.1337 ETH (of the Ropsten test network kind) from the contract.

In the constructor, the `pin` variable was initialized with the value of `block.timestamp % 10000`, and then never changed. And there was the fund retrieval function taking a single number argument, which would compare the number against the `pin` variable. The latter just signifies that we were looking for the value of `block.timestamp` at the time of construction.

There were multiple techniques to tackle this problem:

• A hacky approach was to try to call the contract with every possible value from 0 to 9999, and see which worked — after all, this was on Ropsten, a gas giant.
• There are many tools to interact with deployed contracts, including to read values of global variables. The `pin` variable was private, so if a particular tool didn’t permit reading the value directly, it was conceivable to trick the tool into thinking that the contract’s source code / ABI was slightly different: where the `pin` variable wasn’t private.
• The smart solution was to note the date and time when the contract was created (corresponding to when the block, that included the creating transaction, was mined), and convert it to seconds-since-the-Epoch.

Author: SI

bookmark_borderEvents in May 2021: Blockchain Budapest 4.0 and CCTF

BDAY is organized again in 2021 with great talks, presentations and the CryptoCurrency (is) The Flag hacker game.

This is another chance to meet the best in the largest cryptocurrency and blockchain gathering of Central Europe. Topics as follow:

  • NFTs and their real values
  • DeFi and its future
  • Largest Hungarian Blockchain projects such as CCTF or ILGON
  • …and the first Satoshi Statue 🙂
Website: https://blockchainbudapest.hu/

The presenters and those who own a VIP ticket will be able to take part in the networking. We hope to see you there!

bookmark_borderCCTF4 Hacktivity Writeups 2.

Don’t be eval

The task was to somehow break a specified website. The HTML markup of the website contained the text “Figwheel”. A quick web search will reveal that Figwheel is a software package for developing websites — live¹ — in the Clojure programming language.

On the website, the only item of interest was the link anchored to the text “do you even REPL, bro?”: the URL contained an argument of “(cons 1 2)”, which looks like Clojure code (a lot like Lisp). Along with the challenge’s name of “Don’t be eval”, these all gave the suggestion that the web request’s single parameter was taken as Clojure code to be evaluated, and indeed it was.

Clojure has access to the full Java ecosystem, including IO functions. By sending in appropriate code snippets (in the URL parameter), it was possible to list the contents of the current directory; it contained a file called “flag.txt”. Then that file could be printed, which contained the flag.

¹ to get a feel for what raz, the creator of this challenge, does for a living, see https://www.youtube.com/watch?v=XSIy8gmjmgY#t=1204s

Author: Mr. SI

Pwncoin challenge

For this challenge, a host was specified, and it was suggested that one ought to use Netcat. It was also blatantly stated that one should try overflowing the “meaning of life” (i.e. 42). The solution was to send an arbitrary string exactly of length 43 (not more, which might be weird, but is realistic), over a plain TCP connection; this revealed the flag.

On top of that — and this is something that even the creator of this challenge didn’t think about —, one could discover that the service served at most 1 client at a time, denying other connections while one is open. This permitted a shrewd contestant to prevent other contestants from even attempting to solve this challenge thereafter, by leaving a connection to the server hanging without submitting anything — it wasn’t me! :trollface.jpg:

Note from six for this solution: it was a wargame! 🙂

Author: SI

Thank you SI for submitting the writeups!

bookmark_borderCCTF 4 Hacktivity Writeups 1.

We have received many requests for the CCTF game writeups. Here is the first one, the challenge was called “BIPolognese”. Be careful, spoilers follow.

Challenge: BIPolognese

BIPolognese (100 points)
Crypto Wojack (beginner)

Crypto Wojack was considerate again and made a cold backup of his wallet seed so Bogdanoff can't hak it again.
Meanwhile, he was lost in eating ₿10.000 pizza.

Look at that picture! Can you get the account address?

The hints

  1. The challenge’s name itself suggests a BIP seed
  2. Cold backup
    • These are copied somewhere offline, but before it is shown on the screen
  3. Doing something during eating
    • Crypto Wojack is doing something with the BIP seed and a wallet
  4. Look at the picture
    • You will find the BIP seed on the right laptop’s screen

The solution

Note the BIP44 seed words from the screen, open a web browser, install MetaMask.

After you have installed it, this screen will appear:

Choose the “Import wallet” option and use the seed phrase from the picture.

You find that the wallet it empty, but the flag is the first address as mentioned in the Rules.

Writeup by: six

bookmark_borderGuide for CCTF4 Hacktivity

We have organized (Awalcon, H.A.C.K. and QAN) CCTF for the fourth time and decided to provide a beginner guide. This helps you to get started with hacking a Ethereum smart contract. While going through the guide you will understand more about the logic of cryptocurrencies, blockchain and smart contract. For the crypto only part, please refer to the links at the end of the post.

Guide intro

Cryptocurrencies such as Bitcoin and Ethereum allow you to handle digital assets on decentralized networks. In most cases the assests are simply coins, but here is where Ethereum can provide you more: smart contracts. Imagine coding a program that you broadcast once to a decentralized network, it gets stored on the blockchain and can be called anytime in the future. Or imagine a business that does not have physical contracts, just virtual ones. All these are secure as long as somebody successfully cheats the consensus or exploits vulnerabilities in contrats. It may sound complicated for the first, but it is not. Let’s see a practical example.

Creating a wallet

Most of the Ethereum hacks (breaking smart contracts, accessing accounts without or with weak authentication, phishing) do not require more than a web browser and a bit of coding skills. For the start, only a FireFox or a Chromium/Chrome web broswer will be enough to connect to the test network and to use an addon which allows interacting with (maybe) your first live smart contract.

Open your FireFox or Chromium browser and install the MetaMask extension: https://metamask.io/. Currenly, MetaMask is the most commonly used software by end users to interact with Ethereum systems. After getting the extension, it either automatically opens up or you can open it from the top right bar in your browser. The first step of using MetaMask is to generate a wallet. At this point you are asked to provide a password. Preferably, use a passphrase that is like a sentence, but does not include words from dictionaries (example: “HaxxA11co|ns”). Choose wisely. Move on and read the phishing warning carefully. Finally you need to make sure the secret backup phrase that allows restoring the wallet is secured somewhere, of course in a place only you can access and see.

All is set, now you have an Ethereum wallet, an account.

Changing networks and faucets

MetaMask allows you to change between Ethereum networks. Please change to Ropsten and note that the game will be played there too.

Faucets provide free Ethereum for you on the test networks. Now it is time to get some from: https://faucet.ropsten.be/ (if it does not work, you can find other Ropsten faucets).

Coding and compiling a smart contract

Let’s compile an example smart contract and interact with it. Open https://remix.ethereum.org/ where you get an example contract written in Solidity language. Remix website has an inbuilt compiler and if you click on “Start to compile” it will compile the code. Now you can swith to the “Run” tab. If you have MetaMask running, then you should see “Injected web3” in the environment.

MetaMask injects the so called web3.js into each website you visit, that way the website can communicate with MetaMask (also think about that: is it a good idea to inject to all sites?).

Deploying a smart contract

Make sure your MetaMask account is unlocked, switched to “Ropsten” and you got a coin from the faucet. Then click on “Deploy”.

MetaMask pops you up a transaction which is actually the deployment of the compiled smart contract to the Ropsten Ethereum test network. Now you may wonder what “gas fee” is? Gas limits the computational efforts of the smart contract, meaning you cannot deploy a computational heavy infinite loop for free. You can only use a smart contract if enough gas is provided. If you are ready, click on “Confirm”.

Wait until the transaction changes from “Pending” state to “Confirmed”. This is indicated in MetaMask. The network needs time to make sure your conract is broadcasted and mined successfully. If you click on the transaction, you have a button “View transaction on Etherscan”: click on it and have a look at what happened.

Interacting with the contract functions

By going back to https://remix.ethereum.org/ you can start playing with the “Deployed Contracts”, under the “Run” tab. You can call the deployed smart contract’s functions one by one. Each call you initiate takes a transaction. The executed code runs on all of the Ropsten Ethereum nodes.

Congratulations, you have compiled your first smart contract and interacted with it.

Congratulations

You have seen the very basics now and it is time to think about what can go wrong… Weak passwords, MetaMask seeds all over the place, programmers making mistakes in smart contracts that you can call, logic, broken crypto problems and so on.

References, to learn more

Bitcoin white paper

Cryptography tutorial

Ethereum white paper

Ethereum beige paper (a readable version of the yellow paper)

History of Ethereum Security Vulnerabilities, Hacks and Their Fixes (2017 Sept.)

bookmark_borderCCTF3 – The official CTF game of Bday 3.0

CCTF is organized the third time, now for Bday 3.0 (Blockchain Day) which is one of the largest cryptocurrency related event in Central-Europe.

CCTF is a “Capture The Flag” game where the participants need to hack realistic challenges related to cryptography and cryptocurrencies. The best ones will get rewarded by some presents and QARK tokens, offered by QAN.

If you would like to participate, you can do so by registering here: https://cryptoctf.org/

What about the previous CCTF events? An archive will be created including all the three of them after the last finishes. It will be available on the CCTF’s website.

Who creates these events? The founder of the CCTF project is six from Awalcon who initiated it in 2019 by calling fellow hackers into the project. It is a joint project where the creators include not just Awalcon, but also members from H.A.C.K. and this year Silur from QAN.

six speaking at BDAY3.0 conference