Ethereum: transaction rolled back during swapExactETHForToken call on uniswapRouter02

Ethereum Reverse Transaction: Uniswap router 0x02 SwapExactETHForToken

As an Ethereum developer, you are probably no stranger to the intricacies of interacting with decentralized exchanges and smart contracts. I recently encountered a frustrating issue with the Uniswap router (0x02) when trying to swap ETH for an ERC20 token using the swapExactETHForToken function.

Problem: Transactions are reversed

To understand what is happening, let’s analyze the scenario:

• A user pre-approves a withdrawal permission on the Uniswap router (0x02) to spend ETH on their behalf.

• Calls the swapExactETHForToken function of the smart contract, passing in the required parameters (the amount of ETH and the address of the ERC20 token).

• The transaction is sent to the Ethereum network.

However, each time this transaction occurs, it is reversed by the Ethereum Virtual Machine (EVM). This problem arises due to a common issue known as “reentrancy” or “reentrancy with callbacks”.

Uniswap Router Issue

In the case of the Uniswap router (0x02), the swapExactETHForToken function is vulnerable to reentrancy attacks. When a contract calls this function, it triggers a callback in the Uniswap router, which can lead to an infinite loop of calls and eventually result in the transaction being rolled back.

Fix: Unwrap the transaction

To fix the problem, I wrapped the swapExactETHForToken call using the unwrap function provided by the Uniswap library. This allows us to avoid triggering the callback and prevent the reentrancy attack.

Here is an example of how you can use the “unwrap” function:

pragma solidity ^ 0,8,0;
import "
MySwap contract {
SwapRouter02 public uniswapRouter;
constructor() {
uniswapRouter = new SwapRouter02();
}
function swapExactETHForToken(uint256 _amountIn, uint256[] memory _tokenA, address _to) public {
// Call the unwrap function to prevent re-entry
uniswapRouter.unwrap(_amountIn, _tokenA, _to);
}
}

By wrapping the swapExactETHForToken call with the unwrap function, we ensure that the callback is not triggered and the transaction is successfully executed.

Conclusion

While the problem of reentrancy attacks can be frustrating, it is essential to understand what is happening and take steps to resolve it. In this case, wrapping the swapExactETHForToken call with the unwrap function provides a solution to prevent reentrancy attacks on the Uniswap router (0x02).

As developers, we need to be aware of these issues and take the necessary precautions to ensure the security and integrity of our smart contracts.

Additional Tips

• Always wrap transaction calls that trigger callbacks or reentrancy with the unwrap function.

• Check your smart contract implementation for potential vulnerabilities to reentrancy attacks.

• Consider implementing additional security measures such as retry mechanisms or secure download permissions to mitigate other types of issues.