A set of challenges to hack implementations of DeFi in Ethereum.

Featuring flash loans, price oracles, governance, NFTs, lending pools, smart contract wallets, timelocks, and more!

Created by @tinchoabbate Visit damnvulnerabledefi.xyz

My solutions

Code's solution are in ./test/... and if needed, in ./contracts/attacker-contracts/...

(My others Blockchain projects: https://github.com/olivier7delf/Blockchain-content-table)

1 Unstoppable

There's a lending pool with a million DVT tokens in balance, offering flash loans for free. If only there was a way to attack and stop the pool from offering flash loans ... You start with 100 DVT tokens in balance.

Solution: ./test/unstoppable/unstoppable.challenge.js

on this.token:

using transfer() instead of depositTokens()

poolBalance = poolBalance.add(amount); is not trigger

then, when calling function flashloan:

assert(poolBalance == balanceBefore); // is now False

2 Naive receiver

There's a lending pool offering quite expensive flash loans of Ether, which has 1000 ETH in balance.

You also see that a user has deployed a contract with 10 ETH in balance, capable of interacting with the lending pool and receiveing flash loans of ETH.

Drain all ETH funds from the user's contract. Doing it in a single transaction is a big plus ;)

Solution:

Anyone call call the flashloan function, and then, choose the naive receiver... Fix:

3 Truster

More and more lending pools are offering flash loans. In this case, a new pool has launched that is offering flash loans of DVT tokens for free.

Currently the pool has 1 million DVT tokens in balance. And you have nothing.

But don't worry, you might be able to take them all from the pool. In a single transaction.

Solution:

The flashloan function calls an external function we can code.

This function will approve our smart contract address to transfer token token.

Then our smart contract transfer token from the pool to attacker's address

4 Side Entrance

Solution:

The function flashloan do a flashloan that do a deposit of all the tokens (pool) to the attackercontract.

Then, the attacker contract sends all these tokens to the attacker.

5 The Rewarder

There's a pool offering rewards in tokens every 5 days for those who deposit their DVT tokens into it.

Alice, Bob, Charlie and David have already deposited some DVT tokens, and have won their rewards!

You don't have any DVT tokens. But in the upcoming round, you must claim most rewards for yourself.

Solution:

Create a contract that calls the function flashLoan:

• that calls receiveFlashLoan, it will do:
  • a deposit of the amount loaned, which generate a reward
  • withdraw the amount
  • transfer back tokens loaned
• then transfer token from rewardToken to the attacker

(take care to have approve transfer from theRewarderPool to flashLoanPool)

6 Selfie

A new cool lending pool has launched! It's now offering flash loans of DVT tokens.

Wow, and it even includes a really fancy governance mechanism to control it.

You start with no DVT tokens in balance, and the pool has 1.5 million. Your objective: take them all.

Solution:

Create a contract that calls the function flashLoan:

• thats call receiveFlashLoan, it will do:
  • have a lot of governance tokens (same as the pool tokens)
  • do a snapshot of the governance token -> it gives & remembers vote rights
  • throught queueAction function, use the function drainAllFunds to send the tokens to the attackerEOA (in 2days)
  • store the actionId to use it later
  • transfer back the tokens to the pool flashloan
• "wait" 2 days and then activate executeAction from governance smart contract
  
  

8 Puppet

There's a huge lending pool borrowing Damn Valuable Tokens (DVTs), where you first need to deposit twice the borrow amount in ETH as collateral. The pool currently has 100000 DVTs in liquidity.

There's a DVT market opened in an Uniswap v1 exchange, currently with 10 ETH and 10 DVT in liquidity.

Starting with 25 ETH and 1000 DVTs in balance, you must steal all tokens from the lending pool.

Solution:

The way how puppet calculates the number of token for N ETH can be fooled:

indeed _computeOraclePrice is just gonna div the #ETH by the #token to estimate the value, but if #ETH << #Token, as the state when we borrow:

• uniswap token: 1009
• uniswap ETH: 0.099

Then, with few ETH (20), you can borrow a lot of Tokens.

Small fixes:

• (still unsafe) a simple solution is to have a lot more token in uniswap, so it become really harder to be in this situation.
• (safer) Forbid huge variation in short time, i.e: take care about flashloan...
  
  

9 Puppet-v2

Solution: Quite similar but we need to swap token and ether before

10 Free-rider

A new marketplace of Damn Valuable NFTs has been released! There's been an initial mint of 6 NFTs, which are available for sale in the marketplace. Each one at 15 ETH.

A buyer has shared with you a secret alpha: the marketplace is vulnerable and all tokens can be taken. Yet the buyer doesn't know how to do it. So it's offering a payout of 45 ETH for whoever is willing to take the NFTs out and send them their way.

Solution:

The marketplace has a big issue, it uses the token of the smart contract instead of these of the buyer.

Even if you buy 1 or 6 NTFs, the function _buyOne check only if you have send more that 15 ETH (price for one token) and then use token from the contract.

You can buy every token with 15ETH, which are kept buy the buyer.

So, we need to:

• Do a flash loan through IUniswapV2Pair of 15 ETH
• Buy the token for free
• Pay back the flash loan, with the interest
• Transfer the NTFs and we get paid (receive function)