# User to sell the last supply will make the exchange contribution forever stuck Medium · Sherlock · Stablecoin · 24th October 2024 Finding M-1 of the Mento competition. - Protocol: https://audits.sherlock.xyz/contests/598 - Report: /reports/mento - Codebase: https://github.com/0xsimao/2024-10-mento-update/tree/098b17fb32d294145a7f000d96917d13db8756cc - Source: https://github.com/sherlock-audit/2024-10-mento-update-judging/issues/17 --- ### Summary [BancorExchangeProvider::_getScaledAmountOut()](https://github.com/sherlock-audit/2024-10-mento-update/blob/main/mento-core/contracts/goodDollar/BancorExchangeProvider.sol#L345) decreases the amount out when the token in is the supply token by the exit contribution, that is, `scaledAmountOut = (scaledAmountOut * (MAX_WEIGHT - exchange.exitContribution)) / MAX_WEIGHT;`. Whenever the last supply of the token is withdrawn, it will get all the reserve and store it in `scaledAmountOut`, and then apply the exit contribution, leaving these funds forever stuck, as there is 0 supply to redeem it. ### Root Cause In `BancorExchangeProvider:345`, the exchange contribution is applied regardless of there being supply left to redeem it. ### Internal pre-conditions 1. All supply must be withdrawn from the exchange. ### External pre-conditions None. ### Attack Path 1. Users call `Broker::swapIn()`, that calls `GoodDollarExchangeProvider::swapIn()`, which is the exchange contract that holds the token and reserve balances, selling supply tokens until the supply becomes 0. ### Impact The last exit contribution will be forever stuck. This amount is unbounded and may be very significant. ### PoC Add the following test to `BancorExchangeProvider.t.sol`: ```solidity function test_POC_swapIn_whenTokenInIsToken_shouldSwapIn() public { BancorExchangeProvider bancorExchangeProvider = initializeBancorExchangeProvider(); uint256 amountIn = 300_000 * 1e18; bytes32 exchangeId = bancorExchangeProvider.createExchange(poolExchange1); vm.startPrank(brokerAddress); uint256 amountOut = bancorExchangeProvider.swapIn(exchangeId, address(token), address(reserveToken), amountIn); (, , uint256 tokenSupplyAfter, uint256 reserveBalanceAfter, , ) = bancorExchangeProvider.exchanges(exchangeId); assertEq(amountOut, 59400e18); assertEq(reserveBalanceAfter, 600e18); assertEq(tokenSupplyAfter, 0); vm.expectRevert("ERR_INVALID_SUPPLY"); bancorExchangeProvider.swapIn(exchangeId, address(token), address(reserveToken), 1e18); } ``` ### Mitigation The specific mitigation depends on the design. --- Related findings: - [Yield form LRTs are forever stuck in the protocol and cannot be withdrawn](https://0xsimao.com/findings/autonomint-yield-form-stuck-withdrawn): Autonomint - [Stuck ETH in Curve exchanges due to sending msg.value to the exchange instead of amountIn](https://0xsimao.com/findings/singularity-stuck-eth-curve-exchanges): Singularity - [Malicious user can call `borrowing::calculateCumulativeRate()` any number of times to inflate debt rate as `lastEventTime` is not updated](https://0xsimao.com/findings/autonomint-calculate-times-inflate-debt): Autonomint - [`totalCdsDepositedAmountWithOptionFees` is incorrectly reduced in `CDSLib::withdrawUser()`, leading to stuck option fees](https://0xsimao.com/findings/autonomint-fees-reduced-withdraw-stuck): Autonomint