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Malicious user may frontrun GoodDollarExpansionController::mintUBIFromReserveBalance() to make protocol funds stuck

MediumMento·Sherlock · Stablecoin · 24th October 2024CodebaseM-3

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Summary

GoodDollarExpansionController::mintUBIFromReserveBalance() or GoodDollarExpansionController::mintUBIFromInterest() transfer funds to the reserve and mint $G to the distribution helper. However, GoodDollarExchangeProvider::mintFromInterest() mints 0 tokens whenever the supply is 0. An attacker can buy all $G from the exchange to trigger this.

Root Cause

In GoodDollarExpansionController::142 and GoodDollarExpansionController::161, amountMinted is not checked for a null value.

Internal pre-conditions

None.

External pre-conditions

None.

Attack Path

  1. Attacker calls Bancor::swapIn() or Bancor::swapOut(), buying all $G in the exchange, making PoolExchange.tokenSupply null.
  2. GoodDollarExpansionController::mintUBIFromReserveBalance() or GoodDollarExpansionController::mintUBIFromInterest() is called, adding reserve asset funds without minting $G.

Impact

Funds are added to the reserve without the corresponding amount of $G being minted.

PoC

GoodDollarExpansionController::mintUBIFromInterest() and GoodDollarExpansionController::mintUBIFromReserveBalance() do not check if amountToMint is null:

solidity
function mintUBIFromInterest(bytes32 exchangeId, uint256 reserveInterest) external {
  require(reserveInterest > 0, "Reserve interest must be greater than 0");
  IBancorExchangeProvider.PoolExchange memory exchange = IBancorExchangeProvider(address(goodDollarExchangeProvider))
    .getPoolExchange(exchangeId);

  uint256 amountToMint = goodDollarExchangeProvider.mintFromInterest(exchangeId, reserveInterest);

  require(IERC20(exchange.reserveAsset).transferFrom(msg.sender, reserve, reserveInterest), "Transfer failed"); //@audit safeTransferFrom.  //@audit lost if reserve asset is also a stable asset
  IGoodDollar(exchange.tokenAddress).mint(address(distributionHelper), amountToMint);

  // Ignored, because contracts only interacts with trusted contracts and tokens
  // slither-disable-next-line reentrancy-events
  emit InterestUBIMinted(exchangeId, amountToMint);
}

...

function mintUBIFromReserveBalance(bytes32 exchangeId) external returns (uint256 amountMinted) {
  IBancorExchangeProvider.PoolExchange memory exchange = IBancorExchangeProvider(address(goodDollarExchangeProvider))
    .getPoolExchange(exchangeId);

  uint256 contractReserveBalance = IERC20(exchange.reserveAsset).balanceOf(reserve);
  uint256 additionalReserveBalance = contractReserveBalance - exchange.reserveBalance;
  if (additionalReserveBalance > 0) {
    amountMinted = goodDollarExchangeProvider.mintFromInterest(exchangeId, additionalReserveBalance);
    IGoodDollar(exchange.tokenAddress).mint(address(distributionHelper), amountMinted);

    // Ignored, because contracts only interacts with trusted contracts and tokens
    // slither-disable-next-line reentrancy-events
    emit InterestUBIMinted(exchangeId, amountMinted);
  }
}

GoodDollarExchangeProvider::mintFromInterest() returns 0 if exchange.tokenSupply is 0.

solidity
function mintFromInterest(
  bytes32 exchangeId,
  uint256 reserveInterest
) external onlyExpansionController whenNotPaused returns (uint256 amountToMint) {
  PoolExchange memory exchange = getPoolExchange(exchangeId);

  uint256 reserveinterestScaled = reserveInterest * tokenPrecisionMultipliers[exchange.reserveAsset];
  uint256 amountToMintScaled = unwrap(
    wrap(reserveinterestScaled).mul(wrap(exchange.tokenSupply)).div(wrap(exchange.reserveBalance))
  );
  amountToMint = amountToMintScaled / tokenPrecisionMultipliers[exchange.tokenAddress];

  exchanges[exchangeId].tokenSupply += amountToMintScaled;
  exchanges[exchangeId].reserveBalance += reserveinterestScaled;

  return amountToMint;
}

Mitigation

Revert if the amountToMint from the GoodDollarExchangeProvider::mintFromInterest() call is null. The same should also be done for GoodDollarExpansionController::mintUBIFromExpansion() amountMinted from the GoodDollarExchangeProvider.mintFromExpansion() call.