[CTF] Verilog CTF (Onchain)

Introduction

Source code of the challenge: https://github.com/Verilog-Solutions/CTF-2022-wMaticV2

The project consists of a buggy WMATICv2 token contract containing a function redeem allowing reentrancy attack. Additionally, there is a Bounty.sol contract, which rewards the first person who successfully calls its getBounty function.

There are two trackers of the total supply of WMATICv2 tokens and our goal is to successfully call getBounty, which requires these two trackers to be significantly deviated.

Below is the intended solution that conducts flash loan and reentrancy attacks:

0. Borrow k MATIC such that k > balance() / 10
1. depositMATIC() with k MATIC
2. redeem(k * 1e18) -- reentrancy contract --> getBounty()
3. Return k MATIC

Using ItyFuzz to Solve

Here are the contracts we are interested in:

• WMATICV2: 0x5d6c48f05ad0fde3f64bab50628637d73b1eb0bb

• Bounty: 0xbcf6e9d27bf95f3f5eddb93c38656d684317d5b4

The contracts are exploitable before block number 35690977. We'll fork the chain at block number 35690976 and let ItyFuzz find the exploit.

To conduct an ItyFuzz campaign, run the following command:

ityfuzz evm\
 -t 0x5d6c48f05ad0fde3f64bab50628637d73b1eb0bb,0xbcf6e9d27bf95f3f5eddb93c38656d684317d5b4\
 -f -c POLYGON\
 --onchain-block-number 35690976\
 --onchain-etherscan-api-key <your etherscan api key> # (Optional) specify your Polygon etherscan api key

Optionally, you can also use your RPC instead of the default public RPC by setting environment variable ETH_RPC_URL. For instance, to use your local RPC, run:

export ETH_RPC_URL=http://polygon_rpc.xxx.com

After a few seconds to minutes, you should see ItyFuzz exit with the full exploit to take the fund.