Beyond Hello World
This will be our last week of basics before we hop into actual vulnerabilities.
In the last chapter, we covered a
lot of differences between solidity and a traditional language and the keywords
it uses to differentiate functionality within functions and transactions. We
also reviewed a simple transaction on Remix.
Hopefully, creating your first transaction and reviewing it was a useful
exercise.
In this chapter, we will cover
some other key aspects of understanding before we hop into our vulnerability discovery
and exploitation. These key aspects will round off your understanding and
really benefit you when attacking smart contracts. This will enable us to look
at advanced solidity concepts with an offensive security mindset and help us to
determine how to use them to our advantage when hacking smart contracts in the
rest of this series.
I am sure you have noticed from
the simple hello world example that Solidity is very much like a traditional
program from a structural and coding standpoint. It only has some keywords and
financial transnational differences due to its use case.
We will now cover another smart
contract example where we will learn a lot more about the other key aspects of
coding in solidity that makes it different and interesting, yet still is very
easy to understand. This will be a
fuller featured contract that covers a large portion of typical functionality.
We will break down each part of this smart contract in chunks and explain what
the contract does which will provide enough context to jump into the
exploitation chapters that follow and start to do some really cool attacks.
I would suggest that you type out
this code into Remix and play around with it rather than copy paste or rely on
reading this chapter alone.
Note: On the deposit function,
just note you will need to add a value to the value field above the deploy
options. You can also check the video walk through in the references for a
functionality walk through if you get stuck.
Deploying this contract and playing
with it, will give you an understanding of how it works in order to better
understand what the code does. This is similar to a reconnaissance phase when
testing an application where a walk through of the application functionality is
the first thing you should do prior to running attacks and scans against your target.
The deeper understanding of how an application works at a functional level is
always a tremendous asset into subverting its business logic which is where the
real vulnerabilities are found that do the most damage. If you do not
understand what the application does, you will not find the best attack vectors
against it.
Hands on Lab – Type out and review contract
functionality:
Below is the full contract for
your review. Type this out in remix, play with it a bit, and try out the
following steps. Then come back for an explanation of each piece of the code.
Action Steps:
ü Type out the code below and try to understand what it does
ü Compile and deploy the code into remix
ü Deposit 1 Ether into your account using the value field and denomination drop down
ü Check your Balance
ü Withdraw your balance (note this is in a smaller denomination we will explain that)
ü Check your Balance again
ü Click the isOwner button from a few of your accounts, and click the owner button to show the owner
ü Then finally try the withdrawAll from a non-owner account followed by trying withdrawAll
from the owner account and note your balances.
1. pragma solidity 0.6.6;
2.
3. contract HelloWorld_Bank{
4. address public owner;
5. mapping (address => uint) private balances;
6.
7. constructor () public payable {
8. owner = msg.sender;
9. }
10.
11.//Setting Up authorization
12. function isOwner () public view returns(bool) {
13. return msg.sender == owner;
14. }
15.
16. modifier onlyOwner() {
17. require(isOwner());
18. _;
19. }
20.
21. function deposit () public payable {
22. require((balances[msg.sender] + msg.value) >= balances[msg.sender]);
23. balances[msg.sender] += msg.value;
24. }
25.
26. function withdraw (uint withdrawAmount) public {
27. require (withdrawAmount <= balances[msg.sender]);
28.
29. balances[msg.sender] -= withdrawAmount;
30. msg.sender.transfer(withdrawAmount);
31. }
32.
33.
34. function withdrawAll() public onlyOwner {
35. msg.sender.transfer(address(this).balance);
36. }
37.
38. function getBalance () public view returns (uint){
39. return balances[msg.sender];
40.}
41.}
Video Walk Through:
Code Level Walk Through of HelloWorld Bank
While walking through the
application in the action steps, you should have gotten a feel for what the
contract does. By typing out the code you should also have at least a
high-level understanding of the code logic.
We will now break the code into chunks and make sure that your
understanding does not hold you back from learning as we move into exploitation
in the next section.
1. pragma solidity 0.6.6;
2.
3. contract HelloWorld_Bank{
4. address public owner;
5. mapping (address => uint) private balances;
Our first chunk of code starts
off similarly with our pragma line which states the compiler version used for
execution of the smart contract as seen in the last chapter followed by the
contract name. Next, we have two
variables which are created on lines 4 and 5. Both of these variables have a
great importance to the flow of the application.
The first variable created is
“owner” on line 4. This will be the contracts administrator which is not
explicitly defined here, but instead defined in the next chunk of code in the
constructor. Defining an owner in the constructor is common convention used in
solidity to have an administrative user to limit usage of specific
functionality. Usually, authorization of functionality is handled in a security
library, for example Openzeppelin, which we will cover extensively when fixing
smart contract vulnerabilities. However, in this case, we will show a simple
implementation of authorization.
The second variable created
“balances” on line 5 is something called a mapping in solidity. Mappings are
similar to a dictionary lookup. It is a key value pair where in this case the
address is mapped to a uint value. The
key is the address of the user, while the value is the users balance within the
contract. So, if you were to perform a
dictionary lookup of a user’s address you would be provided back their bank
balance. You will also note that this is a private variable meaning that you
cannot retrieve this value directly outside of the contract by referencing it. However,
private variables as we will in later chapters are not as private as we think
on the blockchain.
1. constructor () public payable {
2. owner = msg.sender;
3. }
4.
5. function isOwner () public view returns(bool) {
6. return msg.sender == owner;
7. }
This
next section of code is called the constructor. The constructor runs one time
when the contract is deployed and will set things up for the contract. In this
case we are creating a constructor which is payable meaning that when you deploy
the contract you can send Ethereum and that Ethereum will be stored within the
contract’s balance. This is useful if the contract requires a balance for some
of its actions right out of the gate.
In line 2 we see our previously
created owner variable being set to msg.sender. This is a way for the contract
to set an administrative user when the contract is created. Since the
constructor runs only one time, it’s a good place to set an initial user. Often
you will see this paired with a change owner function that is protected by the
owner’s authorization level and allows the current owner to set a new
administrative user. The msg.sender variable in solidity is simply the users
address who called the function, or in this case the user who published the
contract initially. This is tied to the user’s public address they use for
transactions.
Each time a user interacts with a
contract, their address is known by the contract as the msg.sender value and
this address is used to associate values with their account sort of like a
session variable in a sense. You can use
this value to map functionality to that user. In the context of this contract
you will see the msg.sender value used to set the Owner, validate the Owner, map
balances on accounts and transfer value back to the user.
On line 5 you will see a function
created solely for the purpose of checking if the user interacting with a
contract is the owner of the application. It checks this by returning true if
the msg.sender value equals the current owners address. This is how the
application enforces its authorization level on administrative users. For
example, if you used require(isOwner) in the beginning of a function the
function would refuse to run the rest of its code if the user calling the
contract was not the owner:
1. modifier onlyOwner() {
2. require(isOwner());
3. _;
4. }
Above you will see an
authorization modifier using isOwner implemented in line 2. This modifier is used
to return a simple true or false based on the same require statement we
referenced using isOwner. However, with a modifier we can check within the definition of a function instead of the body of the
function as you will see further below with the withdrawAll function. For now, as an example of a modifiers usage check out the following
doesSomethingCool function definition, note onlyOwner within the definition. This is how we would use a modifier for authorization
checks.
1. function doesSomeThingCool() public onlyOwner
If
the modifier is referenced in the function definition as shown in
doesSomethingCool, the function body will not run unless the user’s msg.sender
value equals that of the owner of the contract.
After it checks for a true or false value on line 2 and the modifier
code ends, the calling function will continue running as normal following the
_; from line 3. This _; value simply
means continue running calling code as normal within the function provided the require
modifier returned true. This is a much cleaner way to handle authorization
across multiple functions with code reuse and ability to change code in one location
rather than hunting down every function that needs authorization of some sort.
These next two functions should
be pretty self-explanatory by now, but in the spirit of learning Solidity in
this chapter we will deep dive all of the code.
1. function deposit () public payable {
2. require((balances[msg.sender] + msg.value) >= balances[msg.sender]);
3. balances[msg.sender] += msg.value;
4. }
5.
6. function withdraw (uint withdrawAmount) public {
7. require (withdrawAmount <= balances[msg.sender]);
8.
9. balances[msg.sender] -= withdrawAmount;
10. msg.sender.transfer(withdrawAmount);
11.}
Above we have two functions, a
deposit function for filling your account with Ether from an external account
and a withdraw function for removing your Ether from the contract. You will
notice on line 1 that the definition of deposit has the words public and
payable. The reason being that in order to deposit value to an account the
function must be marked as a payable function. This goes for addresses as well,
when using addresses within value transfers those addresses must also be marked
as payable. This was something that was added the Solidity as of version 5,
prior to version 5 if you are auditing code you will not see this keyword
required within all portions of value transfer events.
In line 2 you will see a require
line, the require line is a conditional check that if it fails the transaction
will halt and revert back to the state before it was called. In this instance,
if the value is not a positive value, it will fail and the function will return
an error. If the value is indeed a
positive number, the next line will run and increase the account value of the
user by the value that was sent.
The withdraw function at line 6
only receives a withdraw amount that is checked on line 7 to require that
amount to be withdrawn is less than or equal to the account balance of that
user. If this check fails and the user does not have a high enough balance for
the withdraw, then transaction returns an error. If it succeeds, then on lines 9 and 10 we
decrease the balance of the user internally followed by transferring the
approved amount back to the users account address.
Checks Effects Interactions:
Also note that this code follows
the proper Solidity secure coding pattern of Checks, Effects, Interactions
(CHI). We will go through Solidity coding patterns throughout the book. These
are coding patterns which hinder attack vectors by design. In the CHI pattern,
we always want to first check that the data is valid for the transaction which
we did with the require statement. Then we want to do the effect of the
transaction which is to reduce the balance of the user internally to the
system. Finally, we want to interact with the external address we are
transferring the value to. This pattern will become clear within the Reentrancy
attack chapter.
Effectively an attacker could
re-enter the contract and perform more actions bypassing initial checks if the
value being transferred is not updating the balance prior to interacting with
an un-trusted external party. In order to prevent the attacker from continually
removing value from the contract, we always make sure to update the balance
before transferring the value out of the contract. If the transaction happens to fail, the
transfer function will revert the actions taken in the contract effectively
refilling the users account.
At this point you are probably
starting to notice that Solidity is pretty easy to understand. However, there
are a lot of Gotchas if secure coding patterns are not used or dangerous low
level functionality is handled incorrectly.
The final snippet of code should
be easy to understand. At this point we have covered all of these concepts.
1. function withdrawAll() public onlyOwner {
2. msg.sender.transfer(address(this).balance);
3. }
4.
5. function getBalance () public view returns (uint){
6. return balances[msg.sender];
7. }
8.
The first thing to note is on
line 1 which has the onlyOwner modifier created in the beginning of the
contract. If you remember from the explanation earlier, when this modifier is
added to the function definition, it will run the code within isOwner which
checks if the user is the original contract owner created in the constructor when
the contract was deployed. If this user is the owner, then the call within the
body of the function executes and transfers all of the Ethereum value out of
the contracts balance. It does this by
simply using a transfer function with the address of the contract and
this.balance.
That should all make sense if you
have been following along but what doesn’t make sense is a bit less obvious.
Can you guess what that is?
Before reading the next
paragraph, think about what’s wrong with this function?
So, did you think about it? Did
you ask yourself the question, “Why does this function even exist?” This is an immediate red flag within the code,
that the contract being used in this banking application might have nefarious purposes
by the creators of the contract. At no time should the owner of the contract
have the ability to empty the contract of all its funds. Including that of all
of the users funds who are holding their Ethereum within their personal
accounts on the contract. Often you will
see functions like this within less the reputable games which are planning an
exit scam as soon as the contract balance reaches a desired threshold.
So, while its good to look for
obvious vulnerabilities within code also think about the use case of the code
being reviewed and if something looks off it probably is.
The final getBalance function on
line 5 is simply a function that returns the balance of the user who calls the
function. You will notice that within the function definition it uses the
“view” keyword indicating that it is not modifying anything and should not
incur fees for processing. It also indicates that it is returning a uint value
which it does in line 6. The function returns the msg.sender’s balance by
querying the balances mapping with the msg.sender key.
Summary
This chapter should round out
your knowledge of solidity enough to get started looking at vulnerabilities. We
have covered a lot of common coding themes within solidity which may not be
seen in other languages. We will be covering a lot of coding patterns along
with vulnerable functionality within the following chapters on exploitation. We
will walk through each vulnerability and why it’s an issue within Solidity and
then we will walk through how to attack it with examples of how an attacker
would craft requests or additional attacking code to exploit the flaws. For
additional information on the code above and a walk through of the functionality
in real time, check out the chapters video in the references below.
Contact Info:
References:
https://www.youtube.com/watch?v=U9IWSHcfR08
Open Zeppelin
https://github.com/OpenZeppelin/openzeppelin-contracts
Checks Effects Interactions
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