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J.A.C.

Why/What Blockchain Exploitation?

In this blog series we will analyze blockchain vulnerabilities and exploit them ourselves in various lab and development environments. If you would like to stay up to date on new posts follow and subscribe to the following:
Twitter: @ficti0n

Youtube: https://www.youtube.com/c/ConsoleCowboys
URL: http://cclabs.io
http://consolecowboys.com

As of late I have been un-naturally obsessed with blockchains and crypto currency. With that obsession comes the normal curiosity of "How do I hack this and steal all the monies?"

However, as usual I could not find any actual walk thorough or solid examples of actually exploiting real code live. Just theory and half way explained examples.

That question with labs is exactly what we are going to cover in this series, starting with the topic title above of Re-Entrancy attacks which allow an attacker to siphon out all of the money held within a smart contract, far beyond that of their own contribution to the contract.

This will be a lab based series and I will show you how to use demo the code within various test environments and local environments in order to perform and re-create each attacks for yourself.

Note: As usual this is live ongoing research and info will be released as it is coded and exploited.

If you are bored of reading already and just want to watch videos for this info or are only here for the demos and labs check out the first set of videos in the series at the link below and skip to the relevant parts for you, otherwise lets get into it:

Background Info:

This is a bit of a harder topic to write about considering most of my audience are hackers not Ethereum developers or blockchain architects. So you may not know what a smart contract is nor how it is situated within the blockchain development model. So I am going to cover a little bit of context to help with understanding. I will cover the bare minimum needed as an attacker.

A Standard Application Model:

In client server we generally have the following:
Front End - what the user sees (HTML Etc)
Server Side - code that handles business logic
Back End - Your database for example MySQL

A Decentralized Application Model:

Now with a Decentralized applications (DAPP) on the blockchain you have similar front end server side technology however

Smart contracts are your access into the blockchain.
Your smart contract is kind of like an API
Essentially DAPPs are Ethereum enabled applications using smart contracts as an API to the blockchain data ledger
DAPPs can be banking applications, wallets, video games etc.

A blockchain is a trust-less peer to peer decentralized database or ledger

The back-end is distributed across thousands of nodes in its entirety on each node. Meaning every single node has a Full "database" of information called a ledger. The second difference is that this ledger is immutable, meaning once data goes in, data cannot be changed. This will come into play later in this discussion about smart contracts.

Consensus:

The blockchain of these decentralized ledgers is synchronized by a consensus mechanism you may be familiar with called "mining" or more accurately, proof of work or optionally Proof of stake.

Proof of stake is simply staking large sums of coins which are at risk of loss if one were to perform a malicious action while helping to perform consensus of data.

Much like proof of stake, proof of work(mining) validates hashing calculations to come to a consensus but instead of loss of coins there is a loss of energy, which costs money, without reward if malicious actions were to take place.

Each block contains transactions from the transaction pool combined with a nonce that meets the difficulty requirements. Once a block is found and accepted it places them on the blockchain in which more then half of the network must reach a consensus on.

The point is that no central authority controls the nodes or can shut them down. Instead there is consensus from all nodes using either proof of work or proof of stake. They are spread across the whole world leaving a single centralized jurisdiction as an impossibility.

Things to Note:

First Note: Immutability

So, the thing to note is that our smart contracts are located on the blockchain
And the blockchain is immutable
This means an Agile development model is not going to work once a contract is deployed.
This means that updates to contracts is next to impossible
All you can really do is create a kill-switch or fail safe functions to disable and execute some actions if something goes wrong before going permanently dormant.
If you don't include a kill switch the contract is open and available and you can't remove it

Second Note: Code Is Open Source

Smart Contracts are generally open source
Which means people like ourselves are manually bug hunting smart contracts and running static analysis tools against smart contract code looking for bugs.

When issues are found the only course of action is:

Kill the current contract which stays on the blockchain
Then deploy a whole new version.
If there is no killSwitch the contract will be available forever.

Now I know what you're thinking, these things are ripe for exploitation.

And you would be correct based on the 3rd note

Third Note: Security in the development process is lacking

Many contracts and projects do not even think about and SDLC.
They rarely add penetration testing and vulnerability testing in the development stages if at all
At best there is a bug bounty before the release of their main-nets
Which usually get hacked to hell and delayed because of it.
Things are getting better but they are still behind the curve, as the technology is new and blockchain mostly developers and marketers. Not hackers or security testers.

Forth Note: Potential Data Exposure via Future Broken Crypto

If sensitive data is placed on the blockchain it is there forever
Which means that if a cryptographic algorithm is broken anything which is encrypted with that algorithm is now accessible
We all know that algorithms are eventually broken!
So its always advisable to keep sensitive data hashed for integrity on the blockchain but not actually stored on the blockchain directly

Exploitation of Re-Entrancy Vulnerabilities:

With a bit of the background out of the way let's get into the first attack in this series.

Re-Entrancy attacks allow an attacker to create a re-cursive loop within a contract by having the contract call the target function rather than a single request from a user. Instead the request comes from the attackers contract which does not let the target contracts execution complete until the tasks intended by the attacker are complete. Usually this task will be draining the money out of the contract until all of the money for every user is in the attackers account.

Example Scenario:

Let's say that you are using a bank and you have deposited 100 dollars into your bank account. Now when you withdraw your money from your bank account the bank account first sends you 100 dollars before updating your account balance.

Well what if when you received your 100 dollars, it was sent to malicious code that called the withdraw function again not letting the initial target deduct your balance ?

With this scenario you could then request 100 dollars, then request 100 again and you now have 200 dollars sent to you from the bank. But 50% of that money is not yours. It's from the whole collection of money that the bank is tasked to maintain for its accounts.

Ok that's pretty cool, but what if that was in a re-cursive loop that did not BREAK until all accounts at the bank were empty?

That is Re-Entrancy in a nutshell. So let's look at some code.

Example Target Code:

function withdraw(uint withdrawAmount) public returns (uint) {

1. require(withdrawAmount <= balances[msg.sender]);

2. require(msg.sender.call.value(withdrawAmount)());

3. balances[msg.sender] -= withdrawAmount;

4. return balances[msg.sender];

}

Line 1: Checks that you are only withdrawing the amount you have in your account or sends back an error.

Line 2: Sends your requested amount to the address the requested that withdrawal.

Line 3: Deducts the amount you withdrew from your account from your total balance.

Line 4. Simply returns your current balance.

Ok this all seems logical.. however the issue is in Line 2 - Line 3. The balance is being sent back to you before the balance is deducted. So if you were to call this from a piece of code which just accepts anything which is sent to it, but then re-calls the withdraw function you have a problem as it never gets to Line 3 which deducts the balance from your total. This means that Line 1 will always have enough money to keep withdrawing.

Let's take a look at how we would do that:

Example Attacking Code:

function attack() public payable {

1. bankAddress.withdraw(amount);

}

2. function () public payable {

3. if (address(bankAddress).balance >= amount) {

4. bankAddress.withdraw(amount);

}

Line 1: This function is calling the banks withdraw function with an amount less than the total in your account

Line 2: This second function is something called a fallback function. This function is used to accept payments that come into the contract when no function is specified. You will notice this function does not have a name but is set to payable.

Line 3: This line is checking that the target accounts balance is greater than the amount being withdrawn.

Line 4: Then again calling the withdraw function to continue the loop which will in turn be sent back to the fallback function and repeat lines over and over until the target contracts balance is less than the amount being requested.

Review the diagram above which shows the code paths between the target and attacking code. During this whole process the first code example from the withdraw function is only ever getting to lines 1-2 until the bank is drained of money. It never actually deducts your requested amount until the end when the full contract balance is lower then your withdraw amount. At this point it's too late and there is no money left in the contract.

Setting up a Lab Environment and coding your Attack:

Hopefully that all made sense. If you watch the videos associated with this blog you will see it all in action. We will now analyze code of a simple smart contract banking application. We will interface with this contract via our own smart contract we code manually and turn into an exploit to take advantage of the vulnerability.

Download the target code from the following link:

https://github.com/cclabsInc/BlockChainExploitation

Then lets open up an online ethereum development platform at the following link where we will begin analyzing and exploiting smart contracts in real time in the video below:

https://remix.ethereum.org

Coding your Exploit and Interfacing with a Contract Programmatically:

The rest of this blog will continue in the video below where we will manually code an interface to a full smart contract and write an exploit to take advantage of a Re-Entrency Vulnerability:

Conclusion:

In this smart contract exploit writing intro we showed a vulnerability that allowed for re entry to a contract in a recursive loop. We then manually created an exploit to take advantage of the vulnerability. This is just the beginning, as this series progresses you will see other types of vulnerabilities and have the ability to code and exploit them yourself. On this journey through the decentralized world you will learn how to code and craft exploits in solidity using various development environments and test nets.

Related news

J.A.C.

"Odysseus is a tool designed for testing the security of web applications. Odysseus is a proxy server, which acts as a man-in-the-middle during an HTTP session. A typical HTTP proxy will relay packets to and from a client browser and a web server. Odysseus will intercept an HTTP session's data in either direction and give the user the ability to alter the data before transmission. For example, during a normal HTTP SSL connection a typical proxy will relay the session between the server and the client and allow the two end nodes to negotiate SSL. In contrast, when in intercept mode, Odysseus will pretend to be the server and negotiate two SSL sessions, one with the client browser and another with the web server." read more...

Download: http://www.bindshell.net/tools/odysseus

More information

J.A.C.

Nmap is a free utility tool for network discovery, port scanning and security auditing, even though we can use it for more than that but in this article we will learn how to do these three things with nmap.

The original author of nmap is Gordon Lyon (Fyodor). Nmap is licensed under GPL v2 and has available ports in many different languages. Nmap is available for Linux, Windows, and Mac OS X. You can download your copy of nmap from their website.

Lets get started with nmap.

When performing pentests we always look for networks we are going to attack. We need to identify live hosts on the network so that we can attack them. There are plenty of tools available for finding live hosts on a network but nmap is one of the best tools for doing this job.

Lets start with simple host (target) discovery scans i,e scans that will tell us which ip address is up on our target network. Those ip addresses which are up on our target network are the ones that are assigned to a device connected on our target network. Every device on the network is going to have a unique ip address.
To perform a simple host discovery scan we use the following command

nmap -v -sn 10.10.10.0/24

flags we used in the above command are
-v for verbose output
-sn to disable port scan (we don't want to scan for ports right now)

Following the flags is the ip address of the target network on which we want to look for live hosts. The /24 at the end of the ip address is the CIDR that specifies the subnet of the network on which we are looking for live hosts.

After running the above command you should get a list of live hosts on your target network.
If you just want to know the list of ip addresses your command is going to scan, you can use the -sL flag of the nmap like this.

nmap -sL 10.10.10.0/24

this command will simply output the list of ip addresses to scan.

We sometimes want to do dns resolution (resolving ip addresses to domain names) when performing our network scans and sometimes we don't want dns resolution. While performing a host discovery scan with nmap if we want to perform dns resolution we use -R flag in our command like this:

nmap -v -sn -R 10.10.10.0/24

And if we don't want to perform dns resolution of hosts during our scan we add the -n flag to our command like this:

nmap -v -sn -n 10.10.10.0/24

After we have discovered the hosts that are up on our target network, we usually put the ip addresses of these hosts into a file for further enumeration.

Next step in our enumeration would be to detect which operating system and which ports are running on these live hosts, for that we run this command:

nmap -O -v 10.10.10.119

here we use -O (capital o not zero) for operating system detection and by default nmap performs SYN Scan for port discovery. However nmap scans for 1000 ports only by default of a particular host.

To make nmap go over a list of ip addresses in a file we use -iL flag like this:

nmap -O -v -iL targetlist

where targetlist is the name of the file which contains ip addresses that we want to perform port scan on.

To make nmap scan all the ports of a target we use the -p flag like this:

nmap -p- -v 10.10.10.121

We can also specify a range of ports using the -p flag like this:

nmap -p1-500 -v 10.10.10.121

here 1-500 means scan all the ports from 1 to 500.

We can use a number of scan techniques to discover open ports on our network but I will only discuss some of them for brevity.

We can perform a TCP SYN scan using nmap with -sS flag like this:

nmap -sS -v 10.10.10.150

We have also flags for TCP connect and ACK scans which are -sT -sA

nmap -sT -v 10.10.10.150

nmap -sA -v 10.10.10.150

We can also perform UDP scan as well instead of TCP scan using -sU flag

nmap -sU -v 10.10.10.150

We can perform TCP Null, FIN, and Xmas scans using the flags -sN, -sF, -sX

nmap -sN -v 10.10.10.150

nmap -sF -v 10.10.10.150

nmap -sX -v 10.10.10.150

If you don't know what these scans are then please visit Port Scanning Techniques and Algorithms for explanation.

After discovering the open ports on our target host, we want to enumerate what services are running on those open ports. To enumerate services and versions information on open ports we use the -sV flag like this:

nmap -sV -v 10.10.10.118

This should give us information about what services are running on what ports and what versions of those services are running on the target host.

nmap has an interesting feature called NSE nmap scripting engine. It allows users to write their own scripts, using the Lua programming language, to automate a wide variety of networking tasks. nmap ships with a diverse set of scripts which are very helpful to enumerate a target. To use the nmap default set of scripts while enumerating the target, we use the -sC flag like this:

nmap -sC -sV -v 10.10.10.118

We can also save the results of our nmap scans to a file using the -o flag like this

nmap -sC -sV -v -oA defaultscan 10.10.10.119

here -oA tells the nmap to output results in the three major formats at once and defaultscan is the name of the file that will be prepended to all the three output files.

This is the end of this short tutorial see you next time.

References:
https://nmap.org/book/scan-methods-null-fin-xmas-scan.html

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Sunday 28 January 2024

Blockchain Exploitation Labs - Part 1 Smart Contract Re-Entrancy